• Atoms are building blocks of all matter.

• According to modern atomic theory, an atom is the smallest particle of an element which taking

part in chemical reaction.

• Atoms are very small and which can’t be seen even through very powerful microscope.

• Atomic radius is measured in nanometres. Nanometres = 10-9 m.

• Modern day symbols of Elements

→ Dalton was the first scientist to use the symbols for elements.

→ Berzilius suggested that the symbols of elements should be made from one or two letters of

name of the element.

→ The name copper was taken from Cyprus, a place from where it was found for first time.

→ Now, IUPAC (International Union of Pure and Applied Chemistry) approves names of element

→ The first letter of a symbol is always written as a capital letter (uppercase) and the second letter.

as a small letter (lowercase). For example: hydrogen (H), aluminium (Al), cobalt (Co).

→ Some other symbols have been taken from the names of elements in Latin, German or Greek.For

example: Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.

• Atomic Mass

→ Dalton’s atomic theory proposed the idea of atomic mass which explained the law of constant

proportions so well.

→ The mass of an atom of an element is called its atomic mass.

→ In 1961, IUPAC have accepted ‘atomic mass unit’ (u) to express atomic and molecular mass of

elements and compounds.

→ The atomic mass unit is defined as the quantity of mass equal to 1/12 of mass of an atom of


1 amu or u = 1/12 × Mass of an atom of C -12

1 u = 1.66 × 10-27 kg

• Atom existence

→ Atoms of most of the elements are very reactive and does not exist in free state.

→ Only the atoms of noble gases (such as He, Ne, Ar, Kr, Xe and Rn) are chemically unreactive and

can exist in the free state as single atom.

→ Atoms of all other elements combine together to form molecules or ions.