A nitrogen atom contains 7 protons, 7 electrons and 7 neutrons. why is it that the atomic mass of nitrogen is 14 and not 21?

Mass Number (A):

• The mass number is the sum of the number of protons and neutrons present in the atom of an element. It is denoted by “A”.
• The mass number is represented either on the left-hand side (LHS) or on the right-hand side (RHS) of the symbol of the element as a superscript.
• Mass number = Number of protons + Number of neutrons.

If a nitrogen atom contains 7 protons, 7 electrons, and 7 neutrons.

Then its mass number =Number of protons + Number of neutrons.=7+7=14.

Hence, The atomic mass of nitrogen is 14 and not 21.