Class 9th – ATOMS AND MOLECULES

Atoms are indivisible particles which cannot be divided further. The Indian philosopher Kanada was the first to call that matter are made up of small particles called Parmanu.

John Dalton called matter is made up of small particles called Atom.

LAWS OF CHEMICAL COMBINATION:

1. LAW OF CONSERVATION OF MASS
2. LAW OF CONSTANT PROPORTION

LAW OF CONSERVATION OF MASS:

• It was given by  Lavoisier in 1774
• According to him “matter is neither created nor destroyed in a chemical reaction”.
• EXAMPLE:

• In a chemical reaction the reactant are written on the left hand side & the product are written on the right hand side.
• As the law says, conservation of mass, it means the mass will conserved in the chemical reaction.
• The total mass of the reactant will be equal to the total mass of the product & will remain same throughout the chemical reaction.
• As there is no change in the mass during a chemical reaction this will result in total mass of the reactant equal to total mass of the product.

So, this shows that mass of reactant = mass of product.

LAW OF CONSTANT PROPORTION:

• This concept was given by Proust in 1779.
• This law is also known as law of definite proportion.
• According to law of constant proportion, the elements combine in the same ratio by mass.
• Water is made up of hydrogen and oxygen they always combine in the ratio 1:8 by mass.

DALTON’S ATOMIC THEORY

All matter is made up of small particles which cannot be further divided is called atomic theory of matter.

1. All matter is made up f small particles called Atoms.
2. Atoms are very small and cannot be further divisible into small particles.
3. Atoms can neither be created nor be destroyed in a chemical reaction.
4. Atom of given element, with virtue of anything are always equal and identical.
5. Atoms of different elements possess different properties and masses.
6. Atoms combine in fix ratio.

DRAWBACKS OF DALTONS ATOMIC THEORY:

According to him atoms are the tinniest particle which cannot be divided into further parts, but atoms are further divided into their subatomic particles.

The subatomic particles of atoms are Electron (e), Proton (p) and Neutron (n)