7. Atomic number and Mass number

We know that,

 

Atomic Number:

The number of electrons and the number of protons is equal in a neutral atom.

 

 

The atomic number of carbon is \(6\); hence, the number of protons in the nucleus is \(6\). Similarly, the atomic number of nitrogen is \(7\); hence, the number of protons in the nucleus is \(7\).

 

We can easily calculate the number of electrons or protons in an atom if we know its atomic number. Let us recall the points for that.

Example: For \(O\), calculate the number of (i). protons and (ii). electrons.

 

Atomic number of oxygen = 8.

 

(i). Protons

 

We know, the atomic number is equal to the number of protons.

 

Hence, the number of protons = \(8\)

 

(ii). Electrons

 

In a neutral atom, the number of protons are equal to the number of electrons.

 

Hence, the number of electrons = \(8\)

We have studied that the mass of an atom is concentrated in its nucleus because the size of the electron is negligible.

The mass number or atomic mass is represented by the symbol "A".

 

By rearranging the atomic mass formula, we can calculate the number of protons, neutrons, or atomic number.

 

$\mathit{Number}\mathit{of}\mathit{neutrons}=\mathit{Mass}\mathit{number}-\mathit{Number}\mathit{of}\mathit{protons}$ (or) $\mathit{Number}\mathit{of}\mathit{neutrons}=\mathit{Mass}\mathit{number}-\mathit{Atomic}\mathit{number}$

 

$\mathit{Number}\mathit{of}\mathit{protons}=\mathit{Mass}\mathit{number}-\mathit{Number}\mathit{of}\mathit{neutrons}$ (or) $\mathit{Atomic}\mathit{number}=\mathit{Mass}\mathit{number}-\mathit{Number}\mathit{of}\mathit{neutrons}$

 

Example: Carbon

 

Number of protons = \(6\)

 

Number of neutrons = \(6\)

 

Hence, the mass number = \(6\) + \(6\) = \(12\ u\)

 

The atomic number (Z), mass number (A) and symbol of an element are written as follows in atomic notation:

 

 

Where, X is the symbol of element.

 

A = Protons + Neutrons

 

Z = Protons or electrons