LAKSHYA CLASSES

Chapter 1-Periodic Table – Periodic Properties and Variations of

Properties

Introduction:

Periodic Table
 It is a table that classifies all the known elements in accordance with their
properties in such a way that elements with similar properties are grouped
together in the same vertical column and dissimilar elements are separated to
one another.
 The 115 known elements are arranged in the Periodic Table in order of their
increasing atomic number.
 The vertical columns are called ‘groups’.
 The horizontal rows are called ‘periods’.

Approaches to periodic classification of Elements

Dobereiner’s Triads
In 1817, Dobereiner classified elements having similar chemical properties into
groups of three called ‘triads’. He noted that the atomic mass of the middle element
in a triad is the arithmetic mean of the other two. This is called Dobereiner’s Law
of Triads.

Limitations of Dobereiner's triads .

Dobereiner could identify only three triads from the elements known at that time.
So, all the elements could not be arranged in triads.
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Newland’s Law of Octaves

In 1864, Newland arranged elements in the increasing order of atomic mass and
noticed that the eighth element was similar in physical & chemical properties to the
first element, just like the eight nodes in the music. This relationship is called Law
of Octaves. E.g. elements with atomic mass up to 40 are

H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca

Limitations of Newlands’ Law of octaves

1. This law was applicable only to lighter elements (up to atomic mass 40).
2. Newlands assumed that only 56 elements existed in nature. But, later on
many new elements were discovered whose properties did not fit into law of
octaves.
3. To fit the elements in his table, he adjusted two elements in the same slot.
He also put some unlike elements under same note.

Lother Meyer’s Law In 1869, Lother Meyer classified that atomic volume is the
periodic function of atomic mass.

Mendeleev’s Periodic Law

In 1871, Mendeleev stated the law of chemical periodicity. The physical and
chemical properties of the elements are a periodic function of their atomic masses,
i.e. when the elements are arranged in the increasing order of their atomic masses;
the elements with similar properties are repeated after certain regular intervals.
This is called Mendeleev’s Periodic Law.

Achievement of Mendeleev’s Periodic Table.

1. Mendeleev placed an element with slightly larger atomic mass before an

element with slightly lower atomic mass so that the elements with same
properties fell in the same group. E.g. Co (58.93) was placed before Ni
(58.71).
2. Mendeleev left some gaps for the elements to be discovered.
3. Correction of doubtful atomic mass.
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Limitations of Mendeleev’s Periodic Table

1. Correct position of hydrogen could not be assigned.

2. Anomalous position of isotopes.
3. Uncertainty in discovery of new elements.
4. Wrong order of atomic masses.

Modern Periodic Law

These anomalies were removed by a scientist named Henry Mosely in 1913 in the
form of generation of modern periodic table,which we are using right now. The
physical and chemical properties of elements are a periodic function of their
atomic numbers.

Long Form of the Periodic Table

It is based upon modern periodic law i.e. elements are arranged according to the
increasing order of atomic number.

Explanation of limitations of Mendeleev’s Periodic Table.

i. Position of isotopes. Since all isotopes of an element have same atomic

number, they can be placed in same group. E.g. C-12 and C-13 both have
atomic number 6.
ii. Anomalous position of some elements. Co (58.93) and Ni (58.71)Cobalt
with higher atomic mass is placed before nickel because the atomic numbers
of cobalt and nickel are 27 and 28.
iii. Uncertainty in prediction of new elements. Atomic masses do not increase
in regular manner i.e. they may be in decimals. But, atomic numbers
increase in regular manner i.e. increase by 1 in going from one element to
the next.

Groups. The vertical columns in the Modern Periodic Table are called groups.
There are 18 groups in the Modern Periodic Table.

Periods. The horizontal rows in the Modern Periodic Table are called periods.
There are 7 periods in the Modern Periodic Table.
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Period no 1 2 3 4 5 6 7
No of elements 2 8 8 18 18 32 incomplete
Name of Very Very
Short Short Long Long incomplete
period short long

 Henry Moseley showed that the atomic number of an element is a more

fundamental property than its atomic mass.

 The repetition of properties of elements after certain regular intervals is

called periodicity in elements.

 The cause of periodicity in properties of the elements is the repetition of

similar outer electronic configuration after certain regular intervals.

 The elements in the first group (Li, Na, K, …) are called alkali
metals because they all react with water to form alkalis. Water soluble bases
are called alkalis. All alkalis are the bases but all bases are not the alkalis.

 The elements in the second group (Ca, Sr, Ba, …) are called alkaline earth
metals because their oxides are alkaline in nature and exist in earth.

 The elements in the 17th group (F, Cl, Br, …) are called halogens because
they reacts with metals to form salts. (halo means salt, gene means
producer)

 Isotopes The atoms of same element having same atomic numbers but
different atomic masses are called isotopes. Isotopes of some elements are –

Hydrogen (At No Carbon (At No = Oxygen (At No= Chlorine (At

= 1) 6) 8) No= 17)
1 2 3 12 14
H ,H ,H C ,C O , O17
16
Cl35, Cl37

 Valence electrons The electrons present in the outermost shell of an atom

are called valence electrons.
 Valency The number of electrons lost or gained by an atom to complete its
octet or duplet is called its valency.
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 Valency of an element can be calculated from electronic configuration in

two ways –
i. Valency = number of valence electrons (if they are 1, 2, 3 or 4).
ii. Valency = 8 – number of valence electrons (if they are 5, 6, 7 or 8).
 Electropositive elements. The elements which lose electrons and form
positive ions are called electropositive elements. All the metals are
electropositive in nature.
 The tendency to lose electrons by metals to form positive ions is called
metallic character or electropositive character .
 Electronegative elements. The elements which gain electrons and form
negative ions are called electronegative elements. All the nonmetals are
electronegative in nature.
 The tendency to gain electrons by nonmetals to form negative ions is called
nonmetallic character or electronegative character.
 Metalloids or semimetals. The elements which show the properties of both
metals and nonmetals are called metalloids or semimetals. E.g. boron,
silicon, germanium, arsenic, antimony, tellurium and polonium.

In the Modern Periodic Table, a zigzag line separates the metals from the
nonmetals.

 Metals are found on left hand side of the periodic table while nonmetals are
found on the right hand side of the periodic table
 Oxides of metals are basic while oxides of nonmetals are acidic in nature.
 Maximum number of electrons that can be accommodated in a shell are
calculated using the formula 2n2 where n is number of shell.
 The position of an element in the periodic table tells us about its reactivity.
 Need for classification of elements. It is very difficult to study the properties
of all the elements and their compounds. So, to study the properties of
elements and their compounds easily and conveniently scientists classified
them.
 Besides gallium, germanium (Ge) of group IV A, polonium (Po) of group VI
A and astatine (At) of group VII A have been discovered that fill the gaps
left by Mendeleev in his periodic table.
 Out of all the elements known, noble gases are most unreactive. They are
present in atmosphere in very low concentration. So, they are grouped
together in separate group (zero group) in Mendeleev’s periodic table.
 Atomic number of helium is 2. Its first shell is the last shell which can
accommodate only 2 electrons. So, outermost shell of helium is completely
filled. This is why helium is placed in the 18th group of the periodic table.
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Trends in the Modern Periodic Table

Characteristics In Period (Left to Right) In Group ( Top to Bottom)

Valence Increase due to the increase
Remains the same
electrons in atomic number
First increases from 1 to 4
Valency Remains the same
then decreases to 0
Increases due to the increase in
Decreases due to increase in the new shells. This increases
Atomic size or nuclear charge which tends the distance between the nucleus
atomic radius to pull the electrons closer to and the outermost shell. So the
the nucleus atomic size increases in spite of
increase in nuclear charge.
Decreases. When the
effective nuclear charge Increases because the effective
Metallic acting on the valence nuclear charge on valence
character electrons increases, the electrons decreases due to
tendency to lose electrons increase in atomic size.
decreases.
Increases. When the effective
Decreases because the effective
nuclear charge acting on the
Nonmetallic nuclear charge on valence
valence electrons increases,
character electrons decreases due to
the tendency to gain
increase in atomic size.
electrons increases.
Reactivity of metals increases
because the tendency to lose
First decreases and then electrons increases but the
Reactivity
increases reactivity of nonmetals decrease
because the tendency to gain
electrons decreases.
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Mendeleev’s Periodic Table Modern Periodic Table

1. Elements are arranged in 1. Elements are arranged in
increasing order of their atomic increasing order of their atomic
masses. numbers.

2. There are no separate positions for 2. Separate positions are not

isotopes of an element as their required as they have same
atomic masses are different. atomic numbers.

3. Some elements of higher atomic 3. No such problems occur in this

masses have been placed before table. All elements are in proper
elements of lower atomic masses. order.

4. Some dissimilar elements are

grouped together while some 4. All similar elements are grouped
similar elements are not grouped together.
together.

5. Electronic configuration of an 5. Electronic configuration of an

element cannot be calculated from element can be easily calculated
its position. from its position.

Periodicity The properties that reappear at regular intervals, or in which there is

gradual variation at regular intervals, are called periodic properties, and the
phenomenon is known as the periodicity of elements.

Shells/Orbits Electrons in an atom revolve around the nucleus in certain selected

but fixed concentric circular paths called shells or orbits. These are associated with
a definite amount of energy, so are also called energy levels. Valency

 It denotes the the combining capacity of the atom of an element. It is equal

to the number of electrons an atom can donate or accept or share.
 On moving from left to right in a period, the number of valence electrons
increases from 1 to 8.
 Certain elements lose electrons in steps and hence show variable valency
e.g. Cu, Fe, Ag, Au etc.
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 On moving down a group, the valence electrons and valency of all the
elements in a group remains the same.

Periodic Properties;

Atomic radii ; defined as one half of the inter-nuclear distance between the two
centres of the neighbouring atoms in a covalently bonded molecule.

Variation of Atomic Radii

Variation in a period Atomic radii in general, decrease with increase in atomic

number, going from left to right in a period. This is explained on the basis of
increasing nuclear charge along a period. The nuclear charge increases
progressively by one unit while the corresponding addition of one electron takes
place in the same principal shell. As the electrons in the same shell do not screen
each other from the nucleus, the nuclear charge is not neutralized by the extra
valence electron. Consequently the electrons are pulled closer to the nucleus by the
increased effective nuclear charge resulting in the decrease in the size of the atom.
In this way the atomic size goes on decreasing across the period.

element Li Be B C N O F
Atomic radius- 135 90 82 70 70 66 64
pm

The atomic radius abruptly increases in the case of noble gas element Neon as it
does not form covalent bonds. So the value of Neon radius is Van der Waals radius
which is considerably higher than the value of other covalent radii.

Variation in a group The atomic radii of elements increases from top to bottom
in a group because the nuclear charge increases with increasing atomic number.
Although, there is an increase in the principal quantum number from one atom to
another, the number of electrons in the valence shell remain the same. The effect of
increase in the size of the electron cloud out weighs the effect of increased nuclear
charge and so the distance of the valence electron from the nucleus increases down
the group. Thus the size of the atom goes on increasing down the group in spite of
increasing nuclear charge.

element Li Na K Rb Cs
Atomic 135 154 196 211 225
radius
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Ionic Radii These are radii of ions in ionic crystals. Ionic radius may be defined as
the effective distance from the center of nucleus of an ion up to which it has an
influence on its electron cloud. In ionic compounds the inter nuclear distance may
be taken as equal to the sum of the ionic radii of the two ions. The inter nuclear
distance in ionic crystals are obtained from X-ray studies.

Radius of the cation A cation is formed by the loss of one or more electrons from
the gaseous atom. Thus, the whole of the outer most shell of electrons is removed
resulting in the smaller size in the cation. For example, in lithium atom, there is
only one electron in the outermost ’2s’ shell. As the lithium atom changes to
Li+ ion the outer most ’2s’ shell disappears completely. This disappearance results
in the decrease in size.

With the removal of electrons from an atom the magnitude of the nuclear charge
remains the same while the number of electrons decreases. As a result the nuclear
charge acts on less number of electrons. The effective nuclear charge per electron
increases and the electrons are more strongly attracted and pulled towards the
nucleus. This causes a decrease in the size of the ion.

Radius of the anion The negative atom is formed by the gain of one or more
electrons in the neutral atom. The number of electrons increases while the
magnitude of nuclear charge remains the same. The same nuclear charge acts on
larger number of electrons than were present in the neutral atom. The effective
nuclear charge per electron is reduced and the electron cloud is held less tightly by
the nucleus. This causes an increase in the size of the ion. Thus anions are larger in
size than the corresponding atom.

Variation of ionic radii in a group The ionic radii in a particular

group increases in moving from top to bottom because of the increase in the
principal quantum number though the number of electrons in the valence shell
remains the same.

Isoelectronic ions are ions with same number of electrons.

In isoelectronic series of ions, as the nuclear charge increases the electrons are
pulled more and more strongly and the size decreases.

ion Na + Mg 2+ Al 3+ P 3- S 2- Cl -
Ionic radius 95 65 50 212 184 181
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Metallic Character
Those elements which have a tendency to lose their valence electrons and form a
positive ion are considered as metals. Na − e− → Na+

The metallic character of elements dependes on :

i. Atomic Size: As the nuclear size increases the outermost orbit becomes far from
the nucleus thus experiences a lesser nuclear force . as a result of this, electrons can
be removed easily from the outermost shell, thus increasing the metallic character
of the element.

ii. Nuclear charge: The stronger the magnitude of nuclear charge, more is the
force exerted by the nucleus on the electrons of the outermost orbit. this makes it
difficult to remove the electrons from the outermost orbit, thereby dereasing the
metallic character.

In addition to this concept Metals acts as a reducing agents also. Elements which
loses electrons to complete their octet are called reducing agents. Metals having
higher tendency to loose electrons are stronger reducing agents.

Trends in Metallic Character:

Down a group

Down the group, atomic size increases and nuclear charge also increases . The
effect of increased atomic size is greater as compared to increased nuclear charge.
Therefore metallic character increases as we move down the group. They loose
electron easily.
Example: In group 1 Lithium is least metallic element while francium is the most
metallic element.
Across the period
Across the period nuclear pull increases due to increase in atomic number and thus
the size decreases. Hence elements cannot loose electrons easily.
Metallic character decreases across a period, moving from left to right.

Non-metallic Character
Those elements which have a tendency to gain electrons in order to attain an octet
in their outermost orbit are considered as non-metals.
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The non metallic character of elements depends on :

i. Atomic Size: As the nuclear size decreases the outermost orbit becomes close to
the nucleus thus experiences a greater nuclear force . As a result of this, electrons
cannot be removed easily from the outermost shell, in fact they can gain electrons
easily to make their outermost orbital complete, thus increasing the non-metallic
character of the element.

ii. Nuclear charge: The stronger the magnitude of nuclear charge, the greater is
the tendency to gain electrons , hence more non metallic is the element.

Trends in Non Metallic character

Down the group: Non-metallic character decreases down the group.

Across the period: Non-metallic character increases across a period, i.e., from left
to right.

For example in the third period,

Na<Mg<Al<Si<P<S<Ca

Non-metals are Oxidizing Agents.

Chemical Reactivity
In metals, greater the tendency to lose electrons, greater is the reactivity. In non-
metals, greater the tendency to gain electrons, greater is the reactivity.

Ionization Energy ;The amount of energy required to remove the most loosely
bound electron from an isolated gaseous atom is called ionization energy (IE).

Ionization energy is also called as ionization potential because it is measured as

the minimum potential required to remove the most loosely held electron from
the rest of the atom. It is measured in the units of electron volts (eV) per atom
or kilo joules per mole of atoms (kJ mol-1). 1 eV per atom = 96.64 kJ mol-1 =
23.05 k cal mol-1 Thus, the ionization energy gives the ease with which the
electron can be removed from an atom. The smaller the value of the ionization
energy, the easier it is to remove the electron from the atom.
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Factors affecting Ionization Energy;

1. Size of the atom

2.Charge on the nucleus
3.Screening effect
4.Electronic arrangement

Variation along a period ;

The ionization energy increases with increasing atomic number in a period. This is
because 1) The nuclear charge increases on moving across a period from left to
right. 2) The atomic size decreases along a period though the main energy level
remains the same. Due to the increased nuclear charge and simultaneous decrease
in atomic size, the valence electrons are more tightly held by the nucleus.
Therefore more energy is needed to remove the electron and hence ionization
energy keeps increasing.

However some irregularities have been noticed due to the extra stability of the half
filled and completely filled configurations. For example, the nuclear charge on
Boron is more than Beryllium, yet there is slight decrease in ionization energy
from Be to B. This is because, in boron the last electron goes to ’2p’ orbital which
is at a slightly higher energy than ’2s’ orbital. Also, the electronic configuration of
B is less stable than that of Be (has completely filled orbitals). Hence the ionization
energy is less than that of Be. Similarly, nitrogen, which has half filled ’2p’
orbitals, is more stable than oxygen. Therefore the ionization energy of nitrogen is
more than that of oxygen.

element Li Be B C N O F Ne
Ionization 520 899 801 1056 1402 1314 1651 2050
energy

Variation down a group ;

The ionization energy gradually decreases in moving from top to bottom in a

group. This is due to the fact that: 1)The nuclear charge increases in going from
top to bottom in a group. 2)An increase in the atomic size due to an additional
energy shell (level) ‘n’. 3)Due to the increase in the number of inner electrons
there is an increase in the shielding effect on the outer most electron.
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The effect of increase in atomic size and the shielding effect is much more than the
effect of increased nuclear charge. As a result , the electron becomes less firmly
held to the nucleus and so the ionization energy decreases as we move down the
group.

element Li Na K Rb Cs
Ionization 520 496 419 403 376
energy

Electron Affinity:

Electron affinity is the amount of energy released when an electron is added to an

isolated gaseous atom.

Electron affinity is the ability of an atom to hold an additional electron. If the atom
has more tendency to accept an electron then the energy released will be large and
consequently the electron affinity will be high. Electron affinities can be positive
or negative. It is taken as positive when an electron is added to an atom. It is
expressed as electron volts per atom (eV per atom) or kilo joules per mole.

Variation along a period - The size of an atom decreases and the nuclear charge
increases on moving across a period. This results in greater attraction for the
incoming electron. Hence the electron affinity increases in a period from left to
right.

Variation down a group – As we move down a group the atomic size and nuclear
size increases. As the effect of increase in atomic size is more pronounced the
additional electron feels less attracted by the large atom. Consequently the electron
affinity decreases.

Electronegativity

The relative tendency of an atom in a molecule to attract a shared pair of electrons

towards itself is termed as Electronegativity.

The value of electronegativity of an element describes the ability of its atom to

compete for electrons with the other atom to which it is bonded. Electronegativity
is however not the property of an isolated atom. Electronegativity is measured on a
number of scale levels, the most commonly used are of Pauling or Mulliken.
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Factors Affecting Electronegativity

1)Atomic size

2)Ionisation energy and electron affinity

3)Number and nature of atoms

4)Type of hybridization

5)Charge on the ion

Variation along a period - As the nuclear charge increases from going left to
right in a period because the electrons enter the same shell, the shielding is less
effective. Thus the increased nuclear charge attract the shared pair of electrons
more strongly resulting in higher electronegativity from going left to right in a
period.

element Li Be B C N O F
electronegetivity 1.0 1.5 2 2.5 3 3.5 4

Variation down the group - Electronegativity decreases down the group because
the atomic size increases. The larger the size of the atom the lesser the tendency to
attract the shared pair of electrons.

element Li Na K Rb Cs
electronegetivity 1 0.9 0.8 0.8 0.7

Atomic Number (Z)

The atomic number of an element is equal to the number of protons in the nucleus.
The atomic number is a unique property of an element, because no two elements
have the same atomic number.

Mass Number (A)

The mass number of an element is the sum of the number of protons and neutrons
in the nucleus of the atom of the element.
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At a glance

Variation of Periodic In Group In Period

Properties in the
Periodic Table
Periodic Properties
Atomic size The atomic radii of The atomic radii of
elements increase in a elements decrease in a
group on moving from period from left to right
top to bottom. with an increase in
atomic number.
Ionization energy The ionization potential The ionisation potential
decreases in a group on increases in a period in
going from top to going from left to right.
bottom.
Electron affinity On going down a group, The electron affinity
electron affinity increases in going from
decreases. left to right in a period.
Electronegativity in Electronegativity Electronegativity
going gradually decreases increases in going from
from top to bottom. left to right across the
period.
Metallic character Metallic character The metallic character
increases as one moves of elements gradually
down a group. decreases on moving
from left to right.
Non-metallic Non-metallic character Non-metallic character
character decreases down the increases across a
group. period.
Basic nature of oxides The basic nature of the The basic nature of the
oxides of the metals oxides decreases across
increases down a group. the period and finally
become acidic.
Chemical Reactivity The chemical reactivity The chemical reactivity
of the metal increases of elements first
and of non-metal decreases and then
decreases on going increases from left to
down the group. right in a period.
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Solved Examples:

1. 1. State the number of elements in Period 1, Period 2 and Period 3 of the

Periodic Table.

Ans : - There are 2 elements in Period 1 and 8 elements in each of Period 2 and
Period 3.

1. 2. Name the elements in Period 1.

Ans : - Hydrogen and Helium.

1. 3. What happens to atomic size of elements on moving from left to right in

a period?

Ans : - The atomic size of elements on moving from left to right in a period
decreases.

1. 4. What is the common feature of the electronic configuration of the elements

at the end of Period 2 and Period 3?

Ans : - The atoms of the elements at the end of Period 2 and Period 3 has 8
electrons in their outermost shell.

1. 5. If an element is in group 7 (or group 7A), is it likely to be metallic or non-

metallic in character?

Ans.: - Non-metallic.

1.6. Supply the missing word from those in brackets:

If an element has one electron in its outermost energy level, then it is likely to be
________ (metallic/non-metallic).

Ans : - Metallic.

1. 7. Complete the following sentences using the correct word.

(i) The properties of the elements are a periodic function of their
___________ (atomic number, mass number, relative atomic mass).
(ii) Moving across a ___________ of the Periodic Table, the elements
show increasing ___________ character (group, period, metallic, non-metallic).
(iii) The elements at the bottom of a group would be expected to show
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__________ metallic character than the element at the top (less, more).
(iv) The similarities in the properties of a group of elements is because they have
the same __________ (atomic number, electronic configuration, number outer
electrons).

Ans : - (i) atomic number (ii) period, non-metallic (iii) more (iv) number of outer
electrons.

1. 8. What is meant by a Group in the Periodic Table?

Ans : - The vertical column in a periodic table are called group.

1. 9. Within a group where would you expect to find the element with:
(a) the greatest metallic character? (b) the largest atomic size?

Ans : - (a) At the bottom of the group. (b) At the bottom of the group.

1.10. State whether the ionization potential increases or decreases on going down a
Group.

Ans : - Decreases.

1.11. How many elements are there in Period 2?

Ans.: - Eight.

1.12. The following table represent The first three periods of the modern periodic
table. Study the table and answer the questions that follow:

IA 2A 1B 2B 3B 4B 5B 6B 7B 8 8 8 3A 4A 5A 6A 7A 0
l 2
H He
3 4 5 6 7 8 9 10
LI Be B C N O F Ne
II 12 13 14 15 16 17 18
Na Mg Al SI P S Cl Ar
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
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(i) Write the formula of the sulphate of the element with atomic number 13.
(ii) What type of bonding will be present in the oxide of the element with atomic
number 1?
(iii) Which features of the atomic structure accounts for the similarities in the
chemical properties of the elements in group 7A of the periodic table?
(iv) Name the elements which has the highest ionization potential.
(v) How many electrons are present in the valency shell of the element with the
atomic number 18?
(vi) What is the name given to the energy released when an atom in its isolated
gaseous state accepts an electron to form an anion?
(vii) What is the electronic configuration of the element in the third period which
gains one electron to change into an anion?
(viii) Fill in the blanks :
The atomic size ________ as we move from left to right across the period, because
the __________ increases but the __________ remains the same.

Ans : - (i) Al2(SO4)3

(ii) Covalent
(iii) All the elements of the group 7A contain 7 valence electrons in their outermost
shell. All of them require one electron to complete their octet.
(iv) Helium (v) 8 electron (vi) Electron affinity (vii) 2, 8, 8
(viii) decreases, nuclear charge, number of shells.

1.13. The electro-negativities (according to Pauling) of the elements in period 3 of

the Periodic Table are as follows with the elements arranged in alphabetical order:

Al Cl Mg Na P S Si
1.5 3.0 1.2 0.9 2.1 2.5 1.8

(i) Arrange the elements in the order in which they occur in the Periodic Table
from left to right. (The group 1 element first, followed by the group 2 element and
so on, up to group 7.)
(ii) Choose the word or phrase from the brackets which correctly completes each of
the following statements:
(a) The element below sodium in the same group would be expected to have a
______ (lower/higher) electro-negativity than sodium and the element above
chlorine would be expected to have a _________ (lower/higher) ionization
potential than chlorine.
(b) On moving from left to right in a given period, the number of shells (remains
 LAKSHYA CLASSES

the same/increases/decreases).
(c) On moving down a group, the number of valence electrons (remains the
same/increases/decreases).

Ans : - (i) Na, Mg, Al, Si, P ,S, Cl (ii) (a) lower, higher (b) remains the same (c)
remains the same.

1. 14. Parts (i) to (v) refer to changes in the properties of elements on moving left
to right across a period of the Periodic Table. For each property, choose the letter
corresponding to the correct answer from the choices a, b, c and d.
(i) The non-metallic character of the elements:
(a) decreases (b) increases (c) remains the same (d) depends on the period.
(ii) The electro-negativity:
(a) depends on the number of valence electrons (b) decreases
(c) remains the same (d) increases.
(iii) The ionization potential:
(a) goes up and down (b) decreases (c) increases (d) remains the same.
(iv) The atomic size:
(a) decreases (b) increases
(c) remains the same (d) sometimes increases and sometimes decreases.
(v) The electron affinity of the elements in group 1 to 7:
(a) goes up and then down (b) decreases and then increases
(c) increases (d) decreases.

Ans : - (i) (b) increases (ii) (d) increases (iii) (c) increases (iv)(a) decreases (v) (a)
goes up and then down.

1. 15. The elements of one short period of the Periodic Table are given below in
order from left to right.
Li Be B C O F Ne
(i) To which period do these elements belong?
(ii) One element of this period is missing. Which is the missing element and
where should it be placed?
(iii) Which one of the elements in this period shows the property of catenation?
(iv) Place the three elements fluorine, beryllium and nitrogen in the order
of increasing electro-negativity.
(v) Which one of the above elements belongs to the halogen series?

Ans : - (i) second period

(ii) nitrogen (N) is missing. This element should be placed between carbon and
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oxygen
(iii) carbon (C) shows the property of catenation
(iv) beryllium, nitrogen and fluorine
(v) fluorine (F).

1. 16. A group of elements in the Periodic Table are given below (Boron is the
first member of the group and Thallium is the last):
Boron, Aluminium, Gallium, Indium, Thallium.
Answer the following questions in relation to the above group of elements :-
(i) Which element has the most metallic character?
(ii) Which element would be expected to have the highest electro-negativity?
(iii) If the electronic configuration of Aluminium is 2, 8, 3 how many electrons are
there in the outer shell of Thallium?
(iv) The atomic number of boron is 5. Write the chemical formula of the compound
formed when Boron reacts with Chlorine.
(v) Will the elements in the group to the right of this Boron group be more metallic
or less metallic in character? Justify your answer.

Ans : - (i) Thallium (ii) Boron (iii) 3 (iv) BCl3

(v) Less metallic due to increase in ionization energy.

1. 17. With reference to the variation of properties in the Periodic Table, which of
the following is generally true.
A Atomic size increases from left to right across a period.
B Ionization potential increases from left to right across a period.
C Electron affinity increases going down a group.
D. Electronegativity increases going down a group.

Ans : - (A) F (B) T (C) F (D) F.

1. 18. (i) Name the first and last element in period 2.

(ii) What happens to the atomic size of elements on moving from top to bottom of
a group.
(iii) Which of the elements has the greatest electron affinity among the halogens.
(iv) What is the common feature of the electronic configurations of the elements in
group 7.

Ans : - (i) Lithium, Neon.

(ii) Increases.
(iii) Fluorine.
(iv) Outermost shell has 7 electrons.
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1.19. (i) If an element has a low ionization energy then it is likely to be -----
[metallic/non-metallic].
(ii)If an element has seven electrons in its outermost shell then it is likely to have
the ------------ [largest/smallest] atomic size among all the elements in the same
period.

Ans : - (i) Metallic.

(ii) Smallest.

1.20. (i) The metal of Group 2 from top to bottom are : Be, Mg, Ca, Sr, Ba. Which
of these metals will form ions most readily and why.
(ii) What property of an element is measured by electronegativity.

Ans : - (i) Mg.

(ii) Electronegativity indicates the net result of the tendency of elements to attract
the bond forming electron pair.

1.21 Which three groups of the Periodic Table contain the most elements classified
as metalloids (semimetals)?

1, 2, and 13

1. 2, 13, and 14
2. 14, 15, and 16
3. 16, 17, and 18

Correct Answer Number: 3 Explanation: Go to the Periodic Table and note the
location of the stepped line dividing metals and nonmetals. Elements next to the
left or right of this line are metalloids. Choice three includes 5 metalloids.

1.22. Which element has the highest first ionization energy?

1. sodium
2. aluminum
3. calcium
4. phosphorus

Correct Answer Number: 4 Explanation: See Ref. Table S.

Phosphorus (P) has a first ionization energy of 1012 kJ/mol.
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Unsolved Exercise 1

1. How many valence electrons are there in an atom of silicon?

2. How many protons , electrons and neutrons does the ion contain?
3. What is the symbol for the element manganese?
4. What is the name of the element whose symbol is K?
5. Which of the following atoms is largest: Na, K, Mg, Ca?
6. Which of the following ions is largest: Na+, K+, Mg2+, Ca2+?
7. Which of the following elements is most likely to form a -2 ion? Ca, Fe, C, S, Br
8. Which of the following elements is/are diatomic gases at ordinary temperature
and pressure?
neon, sodium, oxygen, bromine, carbon
9. What is the period number and the group number for the element iodine?
10. Which element is easiest to ionize (has the lowest first ionization energy)?
sodium, potassium magnesium, calcium, fluorine

11. Arrange the following elements as per the guidelines in the bracket

Na, Cl, Mg, P [in decreasing order of atomic size]

Ca, Li, f, N [in increasing order of number of electronegativity]

Cl, Al, Na, S [in increasing order of number of ionisation potential]

Li, F, C, O [in increasing order of electron affinity]

Ar, He, Ne, Kr [in increasing order of number of shells]

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12. Give reason for the following:

Atomic radii decreases across the period

Properties of elements are periodic function of their atomic number and not atomic
weight

Atomic size of an element depends on the nuclear charge of that element

Across a period ionisation potential should increase.