Revision: Structure of Atom Chemistry Science (English Medium) Class 11 CBSE

The number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table. Atomic number is also equal to numbers of electrons in an atom.

For example:

• Atomic number of carbon = 6
• Atomic number of nitrogen = 7
• Atomic number of oxygen = 8
• Atomic number of magnesium = 12

Atomic number is the number of protons in the atom of an element.

Mass number of an element is equal to the sum of protons and neutrons in the nucleus of the atom.

It is the sum of total number of protons and neutrons present in a nucleus.

1. atomic mass number = number of protons + number of neutrons.
2. Atomic number = number of protons = number of electrons. It is denoted by Z.
3. Mass number = number of protons + number of neutrons. It is denoted by A.
4. For example: Carbon atom its,
   1. Number of proton = 6
   2. Number of neutrons = 6
   3. Number of electron = 6
5. Atomic number (Z) = number of protons = number of electrons = 6.
6. Mass number (A) = number of proton + number of neutrons = 6 + 6 =12.

A mass unit equal to exactly one-twelfth `(1/12^(th))` the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

The atoms of the same element, having same atomic number Z, but different mass number A, are called isotopes.

OR

Atoms having the same atomic number (Z) but different mass numbers (A).

Electronic configuration of an atom is defined as the distribution of its electrons in orbitals.