Although, Dalton’s atomic theory remains till today as one of the most brilliant triumphs of scientific vision, but in the light of later developments in science, it has been modified. The modified Dalton’s atomic theory is commonly known as the modern atomic theory. The modern atomic theory is described as follows.
i) All forms of matter are made upon of atoms which cannot be created or destroyed.
ii) Atom is made upon mainly of electrons, protons and neutrons. Therefore, atom no longer remains indivisible. However, atom is still the smallest particle which takes part in the chemical reactions. During chemical reactions, atoms do not split into smaller particles.
iii) Atoms of an element can have different masses. The atoms of an element which have similar chemical properties but differ in their masses are called isotopes of that element. For example , hydrogen have the following three isotopes
Hydrogen (protium) Mass number 1
Deuterium Mass number 2
Tritium Mass number 3
iv) Atoms of different elements may sometimes have equal mass numbers. The atoms of different elements having equal mass numbers are called isobars. For example, an atom of organ (Mass no. 40), and that of Potassium (Mass no 40), are isobars.
v) Dalton’s compound atom is now called a molecule. A molecule is the smallest unit of a substance which can exist independent under ordinary conditions.