Hydrogen has three isotopes (A), (B) and (C). If the number of neutron(s) in (A), (B) and (C) respectively, are (x), (y) and (z), the sum of (x), (y) an (z) is :

Solution

Chemistry ma’am · 2 steps

All three isotopes have one proton, so neutrons are just mass number minus 1: that gives 0, 1 and 2.

1. Same proton count. Isotopes of an element have the same atomic number. Every hydrogen isotope has exactly 1 proton, so the number of neutrons is the mass number minus 1.

   Isotope	Mass number	Protons	Neutrons
   Protium, ¹₁H	1	1	x = 0
   Deuterium, ²₁H (D)	2	1	y = 1
   Tritium, ³₁H (T)	3	1	z = 2

   x + y + z = 0 + 1 + 2 = 3

2. So. The total number of neutrons in the three isotopes is 3, which is option (a).

The idea to keep. For any isotope, neutrons = mass number minus atomic number. Hydrogen's isotopes have mass numbers 1, 2 and 3, so they carry 0, 1 and 2 neutrons.

Answer from the NTA final answer key. Solution by PrepWiser.