Atom Concept for class 9 What is Atom for class 9

DALTON’S ATOMIC THEORY John Dalton, a British Chemists and scientists gave the Atomic Theory in 1808. This theory is popularly known as Dalton’s Atomic Theory in the honour of John Dalton. He gave the theory on the basis of Laws of Chemical Combination and explains them properly. In his theory he explains about atom.

postulates of Dalton’s atomic theory

  • Elements are made of extremely small particles called atoms.
  • Atoms of a given element are identical in size, mass, and other properties;
  • Atoms of different elements differ in size, mass, and other properties.
  • Atoms cannot be subdivided, created, or destroyed.
  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  • In chemical reactions, atoms are combined, separated, or rearranged.On the basis of Dalton’s Atomic

On the basis of Dalton’s Atomic Theory atom can be defined as the smallest particles of matter are called atoms.

Characteristics of atoms:

  • Atom is the smallest particle of matter.
  • All elements are made of tiny particles called atom.
  • Atoms are very small in size and cannot be seen through naked eyes.
  • Atom does not exist in free-state in nature. But atom takes part in a chemical reaction.

The properties of a matter depend upon the characteristics of atoms.

Atoms are the building block of an element similar to a brick which combine together to make a building.

The size of atoms is indicated by its radius.

In ancient time atoms was considered indivisible.

Symbols of Atom

Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.

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 convenience elements are represented by unique symbols. For example: Hydrogen is represented by ‘H’. Oxygen is represented ‘O’. Nitrogen is represented by ‘N’. Iron isrepresented by ‘H’. Oxygen is represented ‘O’. Nitrogen is represented by ‘N’. Iron is represented by ‘Fe’.

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