Mass number

• Electrons have negligible mass so the entire mass of atom lies in its protons and neutrons which are present in the nucleus.

• Protons and neutrons have appox. 1 unit mass the sum of number of protons and neutrons is called the mass number of the atom.

Mass number = No. of protons + No. of neutrons

• The mass number of an element is denoted by the letter A and is written at the top of the symbol of element.
• The mass number of sodium is 23, so we can say that for sodium A = 23. The mass number of carbon is 12, so for carbon, A = 12.
• The mass number of hydrogen is 1, so for hydrogen, A = 1.
• In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

For example:

Relationship between Mass Number and Atomic Number:

• The relationship between mass number and atomic number of an element.

Mass number = No. of protons + No. of neutrons

Mass number = Atomic number + No. of neutrons