Ancient Greek philosophers – Democritus and Leucippus suggested
that if we go on dividing matter, a stage will come when particles obtained
cannot be divided further. Democritus called these indivisible particles atoms
(meaning indivisible).
How and why elements combine and what happens when they
combine. Antoine L. Lavoisier laid the foundation of chemical
sciences by establishing two important laws of chemical combination.
(a) The Law of conservation of mass was stated by Antoine L.
Lavoisier as” Mass can neither be created nor destroyed in a chemical reaction”
Verification of “Law of Conservation of mass”
A solution of sodium chloride and silver nitrate are taken
separately in the two limbs of an 'H' shaped tube. The tube is sealed and
weighed precisely. The two reactants are made to react by inverting
the tube.
The following reaction takes place.
AgNO3(aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)
The whole tube is kept undisturbed for sometime so that
the reaction is complete. When the tube is weighed again it is observed
that: Weight before the reaction = Weight after the reaction
(b) The law of constant proportions which is also known as the
law of definite proportions was stated by Proust as “In a chemical substance
the elements are always present in definite proportions by mass”.
E.g. In a compound such as water, the ratio of the mass of
hydrogen to the mass of oxygen
is always 1:8, whatever the source of water. Thus, if 9 g of
water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.
According to Dalton’s atomic theory, all matter, whether an
element, a compound or a mixture is composed of small particles called atoms.
The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or
destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and
chemical properties.
(iv) Atoms of different elements have different masses and
chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form
compounds.
(vi) The relative number and kinds of atoms are constant in a
given compound
Atom: It is the smallest particle of an element which may or may not
have independent existence. The atoms of certain elements such as hydrogen,
oxygen, nitrogen, etc .do not have independent existence whereas atoms of
helium, neon, argon, etc. do have independent existence. Thus we can say that
all elements are composed of atoms.
IUPAC (International Union of Pure and Applied Chemistry)
approves names of elements. Many of the symbols are the first one
or two letters of the element’s name in English. The first letter of a symbol
is always written as a capital letter (uppercase) and the second letter as a
small letter (lowercase)
For example : (i) hydrogen, H (ii) aluminium, Al and not AL (iii) cobalt, Co
and not CO.
Symbols of some elements are formed from the first letter of the
name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl,
(ii) zinc, Zn etc.
Other symbols have been taken from the names of elements in
Latin, German or Greek. For example, the symbol of iron is Fe from its Latin
name ferrum, sodium is Na from natrium, potassium is K from kalium. Therefore,
each element has a name and a unique chemical symbol.
Molecule: A molecule is the smallest or the simplest structural unit of an
element (or) a compound which contains one (or) more atoms. It retains the
characteristics of an element. A molecule can exist freely and it is a combined
form of bonded units whereas an atom is a singular smallest form of non bonded
unit.
Molecules are of two types, namely homo atomic molecules and
hetero atomic molecules.
Homo atomic molecules: These are the molecules which are made up of
atoms of the same element. For example hydrogen gas consists of two atoms of
hydrogen (H2).Similarly oxygen gas consists of two atoms of oxygen
(O2).
HETERO ATOMIC MOLECULES : The hetero atomic
molecules are made up of atoms of different elements. They are also classified
as diatomic, triatomic, or polyatomic molecules depending upon the number of
atoms present. H2O, NH3, CH4, etc., are the
examples for hetero atomic molecules.
Atomicity: The number of atoms present in one molecule of
an element is called the atomicity of an element. Depending upon the number of
atoms in one molecule of an element, molecules are classified into monoatomic,
diatomic, triatomic or poly atomic molecules containing one, two, three, or
more than three atoms respectively.
Mon atomic molecules: Helium (He) Neon (Ne) Metals
Di atomic molecules: Hydrogen H2 Chlorine
Cl2
Tri atomic molecules: Ozone (O3)
Poly atomic molecules: phosphorous P4 Sulphur S8
Atomicity = Molecular Mass/Atomic mass
MORE TO KNOW :
Isotopes ⇒ These are the
atoms of same element with same atomic number (Z) but different mass number
(A). Example (17Cl35,17Cl37 )
Isobars ⇒ These are the
Atoms of the different element with same mass number but different atomic
number. Example (18Ar40, 20Ca 40 )
Isotones ⇒ These are the
atoms of different elements with same number of neutrons Example : (6C13, 7N14 )
AVOGADRO’S HYPOTHESIS : Amedeo Avogadro put
forward hypothesis and is based on the relation between number of molecules and
volume of gases that is “volume of a gas at a given temperature and pressure is
proportional to the number of particles”.
Avogadro’s Law: Equal volumes of all gases under the same conditions of
temperature and pressure. contain the equal number of molecules.
TEST YOUR UNDERSTANDING SKILL
(a) Find the atomicity of chlorine if its atomic mass is 35.5
and its molecular mass is 71
(b) Find the atomicity of ozone if its atomic mass is 16
and its molecular mass is 48
WHAT IS AN ION?
An ion is a charged particle and can be negatively or positively
charged.
A negatively charged ion is called an ‘anion’ and the positively
charged ion, a ‘cation’. For example, sodium chloride (NaCl). Its
constituent particles are positively charged sodium ions (Na+) and negatively
charged chloride ions (Cl–).
Ions may consist of a single charged atom or a group of atoms
that have a net charge on them.
A group of atoms carrying a charge is known as a polyatomic ion
e.g. Calcium
oxide (Ca+2 O-2)
Ionic compound A compound which consist of ions is called ionic compound
like NaCl (Na+1 Cl-1 Read more of this post Read more »
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