Class 9 Early history of atoms, Laws of chemical combination, Dalton's atomic theory

### Topics to be covered

=> Early history of atoms
=> Laws of chemical combination
=> Dalton's atomic theory

### 𝐄𝐀𝐑𝐋𝐘 𝐇𝐈𝐒𝐓𝐎𝐑𝐘 𝐎𝐅 𝐀𝐓𝐎𝐌𝐒

color{green}(•) The idea of divisibility of matter was considered long back in India, around 500 BC.

color{green}(•) An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get smaller and smaller particles. Ultimately, a stage will come beyond which further division will not be possible and these particles were named as Parmanu.

color{green}(•) Another Indian philosopher, Pakudha Katyayama, elaborated this doctrine and said that these particles normally exist in a combined form which gives us various forms of matter.

color{green}(•) Around the same era, 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).

color{green}(•) All this was based on philosophical considerations and not much experimental work to validate these ideas could be done till the eighteenth century.

### 𝐋𝐚𝐰𝐬 𝐨𝐟 𝐂𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐂𝐨𝐦𝐛𝐢𝐧𝐚𝐭𝐢𝐨𝐧

The following two laws of chemical combination were established after much experimentations by Lavoisier and Joseph L. Proust.

color{green}("𝐋𝐚𝐰 𝐨𝐟 𝐜𝐨𝐧𝐬𝐞𝐫𝐯𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐦𝐚𝐬𝐬")

Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

color{green}("𝐋𝐚𝐰 𝐨𝐟 𝐜𝐨𝐧𝐬𝐭𝐚𝐧𝐭 𝐩𝐫𝐨𝐩𝐨𝐫𝐭𝐢𝐨𝐧𝐬")

color{green}(•) This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.

color{green}(•) For eg: In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever be the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever be the method of preparation.

### 𝐃𝐚𝐥𝐭𝐨𝐧’𝐬 𝐚𝐭𝐨𝐦𝐢𝐜 𝐭𝐡𝐞𝐨𝐫𝐲

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.