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

Which of these are isotopes? N 7 14 , C 6 12 , C 6 14 , O 8 14 , O 8 16 , K 19 40 , Ar 18 40

The atoms of the same element having the same atomic number, but different mass numbers are called isotopes of the element. They have the same number of protons but different numbers of neutrons. Among the given elements there are two pairs of isotopes. They are: (i) C 6 12 , C 6 14 (Here, C atoms have the same atomic number 6 , but different mass numbers 12 and 14 ) (ii) O 8 14 , O 8 16 (Here, O atoms have the same atomic number 8 , but different mass numbers 14 and 16 ).