Class 9 Science

# Atomic Number

• Numerical problems on atomic number
• Numerical problems on mass number

Atomic number is the fundamental properties of an atom. Every atom is identified by its unique atomic number. Atomic number is denoted by āzā.

Atomic number is equal to the number of protons present in an atom.

Since an atom is electrically neutral, thus number of protons and number of electrons are equal to make an atom electrically neutral.

Atomic number = Number of protons = Number of electrons

Example:

The atomic number of Hydrogen is 1, helium is 2, lithium is 3, beryllium is 4, boron is 5, carbon is 6, nitrogen is 7, oxygen is 8, etc.

Sample exercise:

(1) Atomic number of calcium is 20. Calculate the number of electrons and protons in calcium.

Solution:

Since, Atomic number = Number of protons = Number of electrons
Therefore,
Number of electrons in calcium = 20
Number of protons in calcium = 20

(2) Number of protons in sodium atom is 11, find the atomic number and number of electrons in a sodium atom.

Solution:

Since, Atomic number = Number of protons = Number of electrons
Therefore,
Atomic number of sodium = 11
Number of electrons in sodium = 11

## Mass Number or Atomic Mass

Mass number of an atom is defined as the sum of the number of protons and number of neutrons. Mass number is nearly equal to the atomic mass of an atom. Since, protons and neutrons reside in the nucleus, thus they are also known as nucleons.

This means

Mass number of an atom = Number of protons + Number of neutrons

Example

(1) Atomic mass of aluminium is 27 u and atomic number is 13, find the number of protons and number of neutrons in aluminium.

Solution:
Since,
Atomic number = 13
Therefore, number of proton = 13

We know that; Atomic mass (Mass number) = Number of protons + Number of neutrons
Therefore,
27 u = 13 + n
Or, n = 27 ā 13 = 14
Therefore, number of proton = 13 and number of neutron = 14

(2) The atomic number of carbon is 6 and number of neutron is equal to 6. Find the atomic mass or mass number of carbon.

Solution:
Since atomic number of carbon = 6
Therefore, number of proton = 6
Now, Atomic mass = number of proton + number of neutron
Or, Atomic mass or mass number = 6 + 6 = 12 u
Thus, mass number or atomic mass of carbon = 12u

(3) The atomic number of oxygen is 8 and number of neutron is equal to 8. Find the atomic mass or mass number of oxygen.

Solution:
Since atomic number of oxygen = 8
Therefore, number of proton = 8
Now, Atomic mass = number of proton + number of neutron
Or, Atomic mass or mass number = 8 + 8 = 16 u
Thus, mass number or atomic mass of oxygen = 16u