1.2.2 Atomic number & Mass number
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1.2.2 Atomic number & Mass number
Atomic number
Representation
It is represented by the symbol 'Z'.
Calculation
As all atoms of an element have the same number of protons in their nuclei, they have the same atomic number. Hence, each element has a specific atomic number termed its identification number. For example, all hydrogen atoms have 1 proton, their atomic number is Z=l. All atoms in carbon have 6 protons, their atomic number is Z=6. Similarly, in oxygen, all atoms have 8 protons having atomic number Z=8, and sulfur having 16 protons shows atomic number Z = 16.
Mass number
Representation
It is represented by the symbol 'A'.
Calculation
It is calculated as:
A=Z + n
where 'Z' is the atomic number and 'n' is the number of neutrons.
Each proton and neutron has a 1amu mass. For example, a hydrogen atom has one proton and no neutron in its nucleus, its mass number A= l + 0 = 1. The carbon atom has 6 protons and 6 neutrons, hence it's mass number A=12.
Example 1.1
How many protons and neutrons are there in an atom having A = 238 and Z = 92.
Solution:
First of all, develop data from the given statement of the example and then solve it with
the help of data.
Data
= 238 – 92
A=238
Z=92
Several protons?
Several neutrons?
Number of protons = Z = 92
Number of Neutrons = A - Z
= 146
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