Chemistry: Periodic Table of Elements
Chemistry: Periodic Table of Elements
Chemistry: Periodic Table of Elements
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It was observed during early 1800s, that the elements could be listed in a way that elemental
properties reoccurred periodically through the list. Mendeleev, a Russian chemist (1834-1907)
formulated a table based on this by arranging the elements in order of increasing atomic weight
(with a few exceptions). He found that physical and chemical properties were similar for every
seventh element (actually the correct figure is after every eighth element, a complete group
being missing at that time). The columns or the groups formed a family of elements sharing
many properties.
The horizontal rows were called periods. There were some empty places in the table, which
Mendeleev claimed will be discovered later. Actually they were soon discovered with predicted
properties. This showed that strength of Mendeleev’s theory. The table was later improved upon
and explained by modern quantum theory.
In 1799 chemist Proust showed that a substance copper carbonate whether prepared in the lab
or obtained naturally contained same three elements – copper, carbon, and oxygen – and
always in the same proportion of weight – 5.3 parts of copper to 4 parts of oxygen to 1 part of
carbon. Soon he found similar situation in many other compounds through numerous
experiments. This prompted him generalize to a law called Law of Definite Proportions or Law
of Constant Composition. This says:
A compound always contains elements in certain definite proportions, and in no other
combinations.
Law of conservation of mass and law of definite proportions led to attempts to formulate
theories that would account for these laws. English school teacher John Dalton (1766-1844)
provided a theory on the basis of the assumption of atoms not unlike those conjectured by
ancient Greeks. Before doing that Dalton himself discovered another law – the law of multiple
proportion, that says:
Elements might combine in more than one set of proportions – each different combination
producing a different compound.
For example, Carbon combines with oxygen in a weight proportion 3:8 to form a gas carbon
dioxide, but in 3:4 proportion to form another– Carbon monoxide, a poisonous gas.
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Dalton’s atomic theory given below offered a logical explanation of all three laws-
1. All elements are made of tiny, indestructible and indivisible particles – atoms.
2. All atoms of an element are identical, but differ from any other element.
3. Atoms of different elements can form combinations to give compounds.
4. A chemical reaction involves change not in atoms themselves, but in the way atoms are
combined to form compounds.
Dalton showed that the matter must be atomistic (not continuous) to account for law for
definite proportion – otherwise it would be any proportion. Similarly law of multiple
proportion is explained by assuming different number of atoms combining. As a reaction is
only re-ordering of atoms, the law of conservation is also explained.
Dalton set up a table of relative weight of atoms as a part of his theory. This is based on
taking hydrogen the lightest material as 1. All other elements were ascribed with an atomic
weight (relative weight) in comparison with hydrogen.
Q. Hydrogen Sulfide gas can be decomposed to give sulfur and hydrogen in a weight ratio
of 16:1. If the relative weight of sulfur is 32 when hydrogen is taken to be 1, how many
hydrogen atoms are combined with each sulfur atom in the gas.
A.
16 x.32 1.32
1 y.1 2.1
Where x is the no. of sulfur atom = 1
y is the no. of hydrogen atom = 2
Hydrogen sulfide will, therefore, be H2S
Q. When we burn a 10 kg piece of wood, only 0.05 kg of ash is left. Explain this apparent
contradiction of the law of conservation of mass.
Q. Suppose you dissolved some salt in water. Is this a physical or chemical change? How
about mixing ingredients for a cake before baking? How about after baking? Show reason
in all cases.
Chemical Reactions
The practical importance of chemistry lies in understanding chemical reactions, or the way
different elements and compounds behave chemically in the presence of each other under
different conditions. Such a chemical equation is most easily represented by a chemical
equation of the general form:
A+B C+D where A and B are reactants, and C and D are products.
The arrow indicates the way reaction proceeds but may not give exact quantities on both
sides. An equal sign is used when quantities balance on both sides.
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There were some anomaly in Dalton’s results because he did not know that there are
diatomic gases such as O2 and H2. Therefore he could not see how could one volume of
oxygen make two volumes of steam (water), atom being indivisible. But that was the case,
and the same with many other compounds such as ammonia in which one volume of
nitrogen was making two volumes of ammonia. This was resolved by Avogadro in 1811.
He suggested two hypotheses known as Avogadro’s hypotheses.
1. Equal volume of gas under same pressure and temperature contains the same number of
molecules.
2. Molecules of certain gases are diatomic (consisting of two atoms of the same element).
This could give rise to balanced chemical equations such as
N2 + O2 = 2NO One volume of nitrogen and one volume of oxygen gives two
volumes of nitric oxide.
2H2 + O2 = 2H2O Two volumes of hydrogen and one volume of oxygen give
two volumes of steam.
3H2 + N2 = 2NH3 Three volumes of hydrogen and one volume of nitrogen
gives two volumes of ammonia.
Molecular weight of a molecule is the sum of all atomic weights in the molecule. If we
express it in gm this is called Gram Molecular Weight (GMW). Thus gram molecular
weight of hydrogen gas is 2gm and that of nitrogen gas is 28gm (its atomic weight being
14).
Thus the last equation above shows that 6 gm of hydrogen and 28 gm of nitrogen will give
34gm of ammonia.
A chemical equation is somewhat like a recipe which tells us how much of each ingredient
a chemical reaction requires, and how much of each product can be obtained from the
reaction.
Following Avogadro’s hypothesis it was found that under Standard Temperature and
Pressure (STP), a particular volume of any gas called Gram Molecular Volume (GMV)
equal to 22.4 litre, contains the gram molecular weight of the gas. As equal volume
contains equal number of molecules, this volume also contains the same number of
molecules of any gas, which is called Avogadro Number, found to be 6 X 1023. Any gram
molecule weight of substance – solid, liquid or gas, will have this number of molecules.
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Example of a reaction reorganizing the molecule:
3 Fe + 4 H2O heat FeO4 + 4H2
Where three gram molecular weight of Fe is heated with 4 gram molecular weight of
water the result is 1 gram molecular weight of iron oxide and 4 gram molecular weight of
hydrogen.
In some instances adding another substance, which may or may not change during the
reaction will speed up (or slow down) the reaction. Such substance is called a Catalyst. An
example of using a catalyst occurs in the decomposition of potassium chlorate (KClO3)
heat
2 KClO3 2KCl 3O 2
MnO2
This is a very common method of producing small amount of oxygen and it is done by
heating potassium chlorate at 4000C. Normally the process is very slow. If a small amount
of MnO2 (manganese dioxide) as a catalyst, the reaction will take place at 2500C very
quickly. Catalysts are used extensively in industry, and thousands of proteins know as
enzymes act as catalysts to do the various physiological chemical reactions in our body.
Strong acids and bases cause damage on contact with living cells. These corrosive poisons
produce what are known as chemical burns. Both acids and bases, even in dilute solutions
break down the protein molecules in living cells.
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Usually a scale known as pH scale is used to determine the acidity or alkalinity (the strength
of base). A pH 7 represents neutral solution. A pH lower than 7 means the substance is acidic,
the less the more acidic. Similarly a pH higher than 7 is alkaline (basic), the higher the more
basic.
Looking across the periodic table with the exception of the first period we find that a
maximum of eight electrons can be in the outer orbital (2 in s orbital and 6 in p orbital
according the quantum mechanical picture of atom we saw in the earlier chapter). Group IA
(alkali metals such as sodium, potassium etc.) are very reactive because they tend to give up
electrons very easily from the outermost orbital. Group VIII A, (halogens such as chlorine,
fluorine etc.), are also very reactive but because they have incomplete outermost orbital and
are extremely ready to accept electron from other element.
Once such giving and taking of electron takes place the atoms become ions, the giver
becomes positive ion and taker the negative. This creates an electrostatic attractive bond
between the giver and taker called ionic bond, which is quite strong. Sodium and chlorine,
for example from the two groups mentioned above, can form an ionic bond to form NaCl
(sodium chloride, the common salt).
There is another major bonding process which is called covalent bond. Here atoms share
electrons from the outermost orbital to keep their outermost orbital filled – which is allowed
by quantum mechanical considerations. There are huge number of compounds which combine
using the covalent bond starting from simple combination of two hydrogen atom to form the
molecule of hydrogen gas to many complex carbon compounds.
These and other general observations, along with knowledge of atomic electronic
configurations, lead to two general rules concerning compound formation:
1. Only the electrons in the outer orbitals of an atom take part in compound formation.
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2. A stable compound is formed when each atom of the molecale achieves a closed electron
configuration (filled outer orbitals s2 and p6).
Organic Chemistry
The substances found in the living things (organic compounds) posed a different kind of
challenge to the chemistry, and comprised organic chemistry. The analysis of organic
compounds such as sugars, fats, oils etc. showed that most of these contain only a few
elements– carbon, hydrogen, oxygen, though they had more elaborate composition.
Organic molecules can be extremely complex. Sometimes they can have repeating units of
smaller molecules (called monomers), chemically bonded together in chain-like structures
(called polymer). Polymers occur in nature, but they can also be chemically synthesized–
then called synthetic.
One group of organic molecules is hydrocarbon chain of monomers consisted of hydrogen
and carbon. From simple to complex such straight chains give methane (CH4), ethane
(C2H6), propane (C3H8)