The sum of the number of protons and neutrons present in the nucleus of an atom is called the mass number. The mass number is denoted by A . Thus, mass number A = No. of protons + No. of neutrons. The mass number of an element is nearly the same as its atomic mass but the latter has certain units of measurements. However, whereas the mass number is always a whole number, the atomic mass of the element may not be a whole number always.

How many protons, electrons and neutrons are there in the following atoms? N 7 14 , C 6 12 , C 6 14 , O 8 14 , O 8 16 , K 19 40 , Ar 18 40

The relationship between atomic number Z , mass number A and the number of neutrons in an atom is: Mass number A = No. of protons p or atomic number Z + No. of neutrons n Hence, A = Z + n So, n = A - Z Z = p = e (the number of electrons), for a neutral atom. By using the above relationship, the number of protons, electrons and neutrons in the given atoms can be calculated as follows: Symbols protons p electrons e neutrons n = A - Z N 7 14 7 7 7 C 6 12 6 6 6 C 6 14 6 6 8 O 8 14 8 8 6 O 8 16 8 8 8 K 19 40 19 19 21 Ar 18 40 18 18 22