Classification of Elements and Periodicty
Classification of Elements and Periodicty
Classification of Elements and Periodicty
and periodicity in
properties
Chapter 3
Lets recall and revise…
Law of triads:-
According to law of triads elements were placed in a group of 3 and
they were called triads. These were placed in the increasing order of
atomic masses
the elements were placed in increasing order of their atomic weights and it
was noted that every eighth element had properties similar to the first
element .
The relationship was just like every eighth note that resembles the first in
octaves of music. Newlands’s Law of Octaves seemed to be true to elements
only upto calcium.
Mendeleev’s periodic law
“The properties of the elements
are a periodic function of their
atomic weights.”
They are all reactive metals with low ionization enthalpies. They lose the
outermost electron(s) readily to form 1+ ion in the case of alkali metals or
2+ ion in the case of alkaline earth metals.
All the orbitals in the valence shell of the noble gases are completely
filled by electrons and it is very difficult to alter this stable arrangement
by the addition or removal of electrons.
The noble gases thus exhibit very low chemical reactivity. Preceding the
noble gas family are two chemically important groups of non-metals.
They are the halogens (Group 17) and the chalcogens (Group 16).
These two groups of elements have highly negative electron gain enthalpies
and readily add one or two electrons respectively to attain the stable noble
gas configuration.
The non-metallic character increases as we move from left to right across
a period and metallic character increases as we go down the group
D-block:
Elements in which the d-orbitals are completely filled are called the
d-block elements.
Properties:-
-forms complexes
Atomic radius
Ionic radius
Ionisation enthalpy
Electronegativity
Oxidation states
Chemical reactivity
Atomic radius
The atomic size generally
decreases across a period as for
the elements of the second period.
It is because within the period the
outer electrons are in the same
valence shell and the effective
nuclear increases as the atomic
number increases.
This results in increased
attraction between nucleus and
the electron
Atomic radius decreases from left
to right as principal quantum
number increases.
It also increases down a group as
the number of shells increase.
Variation of atomic size
to that of alkali metals
Ionic radius
The removal of an electron from an atom
results in the formation of a cation,
whereas gain of an electron leads to an
A cation is always smaller than its parent
anion.
atom while a anion is always greater to its
parent atom.
(a) For convenience and systematic study of elements having similar properties.
Electron gain enthalpy provides a measure of the ease with which an atom adds an
electron to form anion
X(g)+e– → X–(g)
Depending on the element, the process of adding an electron to the atom can be
either endothermic or exothermic
Added to the atom and the electron gain enthalpy is negative. For example, group
17 elements (the halogens) have very high negative electron gain enthalpies
because they can attain stable noble gas electronic configurations by picking up an
electron
On the other hand, noble gases have large positive electron gain enthalpies
because the electron has to enter the next higher principal quantum level leading
to a very unstable electronic configuration. It may be noted that electron gain
enthalpies have large negative values toward the upper right of the periodic table
As a general rule, electron gain
enthalpy becomes more negative
with increase in the atomic
number across a period. The
effective nuclear charge
increases from left to right across
a period and consequently it will
be easier to add an electron to a
smaller atom since the added
electron on an average would be
closer to the positively charged
nucleus. We should also expect
electron gain enthalpy to become
less negative as we go down a
group because the size of the
atom increases and the added
electron would be farther from
the nucleus
electronegativity
A qualitative measure
of the ability of an
atom in a chemical
compound to attract
shared electrons to
itself is called
electronegativity.
Flourine has the
highest
electronegativity value
of 4.
It is the most
electronegative
element.
Non-metallic elements have strong tendency to gain electrons.
Therefore, electronegativity is directly related to that non-metallic
properties of elements.