Every substance in our surrounding is made up of unique elements. There are 118 elements identified worldwide so far. Out of these elements, 92 elements occur in the nature and the remaining elements are synthesised in the laboratories. Copper, iron, gold and silver are some of the elements found in the nature. Elements like technetium, promethium, neptunium and plutonium are synthesised in the laboratories, each element is made up of similar, minute particles called atoms. For example, the element gold is made up of gold atoms which determine its characteristics. The word atom is derived from tha Greek word atoms. Tomos means smallest divisible particles and atomas means smallest indivible particles. Ancient Greek philosophers like Democritus, have spot about atoms. Even our tamil poet avvaiyar has mentioned about atoms in her poem while describing thirukkural. But, none of them have scientific base. The first scientific theory about atom was given by John Dalton. Followed by him J. J. Thomson and Rutherford have given their theory about atom. In this lesson, we will study how atomic theories evolved at different times. We will also study about valency, molecular, formula, rules for naming chemical compounds and balancing chemical equations. John Dalton provided a basic theory about the nature of matter . He proposed a model of atom known as Dalton atomic theory in1808 based on his experiments. The main postulatesof Dalton atomic theory are :

Atomic the matters are made up of extremely small particles called atoms ( Greek philosopher democratus used the same name for tha smallest indivisible particles). 

Atoms of the same element are identical in all aspects ( size, shapi, mass and properties). 

Atoms of different elements have different sizes and masses. And posses different properties. 

Atoms can neither be created not be destroyed. I. E., atom is indestructible. 

Atoms of different elements may combine with each other in a fixed simple ratio to form molecules. 

An atom is the smallest particles of mattt that takes part in a chemical reaction. 

John Dalton son of a poor weaves, began his career as a village school teacher at the age of 12 he became the principal of the school seven years later. In 1793 , he moved to Manchester to teach physics, chemistry and mathematics in a college. He proposed his atomic theory in 1803 . He carefully recorded each day, the temperature, pressure and amount of rainfall from his youth till the end. He was a meticulous meteorologist.