Atomic Number & Mass Number

What is 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.

What is 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

Example:
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

Atomic number, mass number, and isotopes
How to use the atomic number and the mass number to represent different isotopes

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

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