Atoms And Molecules Ncert class 12th science
Introduction:
☞Atoms and Molecules: An Indian Philosopher, Maharishi Kanad, said that if we go on the dividing matter, we will get smaller and smaller particles of Matter ultimately we will get the smallest particle of matter which can not be divided further.
☞Maharishi Kanad called these Particles 'Parmanu'
☞And John Dalton called these particles by the name of 'Atom'
Laws Of Chemical Combination
●Law of conservation of mass
●Law of constant proportions
Law of conservation of mass:
☞This Law was first proposed by French Chemist Antoine Lavoisier in 1789, which states that:-
Matter can neither be created nor be destroyed in a Chemical Reaction.
☞The law of conservation of mass means that is a chemical reaction.
i.e Total mass of the reactant = Total mass of product
Eg:- If A and B combine to form different products C and D, The total mass of A and B before the Chemical reaction, is equal to the total mass of C and D.
i.e mass of A + mass of B = mass of C + mass of D
i.e total mass of reactant = Total mass of products.
Explanation:-
(a)A burning of candle slowly decreases its mass and finally, nothing is left behind.
similarly, petrol or kerosene on combustion completely disappears no residue is left behind.
Law of constant proportions:-
☞This law was first proposed by J.L. Proust in 1799, Which states that:-
A chemical compound always consists of the same elements combined together in the same proportion by mass.
☞This law means that whatever be the source from which it is obtained, A pure chemical compound is always made of the same elements in the same mass percentage.
eg:- In water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8
Dalton's Atomic Theory:
☞According to Dalton's Atomic theory, all matter whether an element, a compound or a mixture is composed of small particles called atoms.
☞He gave six postulates. They are:-
(i)All matter is made of very tiny particles called Atoms.
(ii)Atoms are indivisible particles, which can not be created or destroyed in a chemical reaction.
☞Atoms of a given element are identical in mass and chemical properties.
☞Atoms of different elements have different masses and chemical properties.
☞Atoms combine in the ratio of small whole numbers to form compounds.
Atoms
☞They are the building blocks of all matter.
☞Atoms are very small and can be seen with a very powerful microscope.
☞Atomic radius is measured in nanometers which is 10 -9m.
☞Dalton used symbols for elements and he was the first one to do so.
☞Using one or two letters of the name of the element as its symbol suggests by Berzelius.
☞Nowadays, symbols of some elements are formed by the first letter of their names and other symbols are taken from the name of elements in Latin, Greek or German.
Atomic Mass:
- According to Dalton, each atom had a characteristic atomic mass.
- Atomic masses were determined by the law of chemical combination and the compound formed.
- In 1961, IUPAC has accepted 'atomic mass unit'(u) to express the atomic and molecular mass of elements and compounds.
- The atomic mass equal to 1/12 of the mass of an atom of carbon-12.
- 1 amu or u = 1/12*Mass of an atom of c 12 1 u = 1.66*10 -27 kg
ATOMS EXIST INTO TWO WAYS:
☞in the form of molecules
☞in the form of ions
Molecules:
A molecule is a group of two or more atoms that are chemically bonded together. That is, tightly held together by attractive forces.
A molecule is the smallest particles of an element or a compound that is capable of independent existence and shows all the properties of that substance.
Types of molecules
- Elementary molecule
- Compound molecule
The elementary molecule is composed of atoms of the same element. Eg:-
☞ A molecule of Hydrogen elements contains 2 Hydrogen. Atoms combined together and it is written as H2
The number of atoms present in one molecule of an element is called its ATOMICITY
Monoatomic:-
☞Atomicity is one
Eg:- Sodium(Na), Magnesium(Mg)
Diatomic:-
☞Atomicity is two. Eg:- Oxygen(O2), Hydrogen(H2)
Triatomic:-
☞Atomicity is three. Eg:- Ozone(O3)
Polyatomic:-
☞Atomicity is two or more than two.
Compound molecule:-
The molecules of a compound contain two or more different types of atoms chemically combined together.
Atoms
↳ Atoms make Electrons, protons, Neutrons
⇓ ⇓ ⇓
They carries:- (-ve)charge (+ve) charge No charge i.e neutral
☞Every atom normally contains an equal number of electrons and protons which balance the charges in the atoms and make it electrically neutral.
If one or more electron is either removed or added to an atom, then the atom has either less no. of electrons than normal, and it gets an overall electric charge.
The atom becomes an ion
☞If an atom has fewer electrons than Normal, Then it gets (+ve charge).
☞And if an atom has more electrons than Normal, then if gets (-ve charge).
Only electrons can be transferred from one atom to another in the formation of ions.
Ions
An ion is a positively or negatively charged atom.
Types of ions:
☞Cation
☞Anions
Cation:-Formed by loss of one or more electron by an atom.
Anions:-Formed by a gain of one or more electron by an atom.
Molecular mass
The molecular mass of a substance is the relative mass of its molecular mass of a carbon-12 atom taken as 12 units.
How to Calculate Molecular mass?
The molecular mass is equal to the sum of the atomic masses of all the atoms present in present in the molecule of the substance.
Eg:-
H2O(Water)
CaCO3(Calcium Carbonate)
SO2(sulphur dioxide)
C12H22O11(Sugar)
Q1. Calculate the molecular mass of H2O(Water).
Ans:-
H2O(Water)= 2*1+ 16 = 18u
CaCO3(Calcium Carbonate)=40 + 12 + 3*16= 100
SO2(sulphur dioxide)= 32 + 16*2 = 64u
C12H22O11(Sugar)= 12*12 + 1*22 + 16*11 = 342u
Chemical formula
☞The chemical formula of a compound is a symbolic representation of its composition.
☞The combining power or capacity of an element is known as valency.
☞Valency can be used to find out how the atoms of an element will combine with atoms of another element to form a chemical compound.
Rules of writing chemical formula
☞The valency of charges on the ion must balance.
☞When a compound consists of a metal and a non-metal, name of a symbol of metals is written first.
☞The main objective is to complete the octet in the outermost shell during a chemical bond in a compound.
Example:-
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