Atomic Number & Mass Number

Atomic number

• The number of protons in the nucleus of an atom is called the atomic number.
• It is represented by the symbol Z

Example:

Helium has an atomic number of 2 since it has two protons in its nucleus.

Lithium has an atomic number of 3 since it has three protons in its nucleus.

Mass number

• The mass number is the total number of neutrons and protons in the nucleus of the atom.
• It is also called the nucleon number.
• It is represented by the symbol A
• The atomic number and the mass number of an element is usually written in the following way:

The helium atom has a mass number of 4 and atomic number of 2

What is the mass number and atomic number of the ?

Solution:

Mass number = 31, atomic number = 15

• Number of neutrons = mass number – atomic number

Example:

How many neutrons are there in one atom of ?

Solution:

Mass number = 24

Atomic number = 12

Number of neutrons = mass number – atomic number

= 24 – 12 = 12

Example:

How many neutrons are there in one atom of ?

Solution:

Mass number = 307

Atomic number = 82

Number of neutrons = mass number – atomic number

= 207 – 82 = 125

The following video shows an example of calculating the number of neutrons.

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