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) and (z) is:

Isotopes: Isotopes are chemical elements that have the same number of protons and electrons, but a different number of neutrons.

• For example, Carbon-14, Carbon-13 and Carbon-12 are all isotopes of carbon.
• Carbon-14 has 8 neutrons, Carbon-13 has 7 neutrons, and Carbon-12 has 6 neutrons.

There are 3 isotopes of hydrogen:

• ${\mathrm{H}}_1^1=\mathrm{A}$
• ${\mathrm{H}}_1^2=\mathrm{B}$
• ${\mathrm{H}}_1^3=\mathrm{C}$

Number of neutrons$=\mathrm{Z}-\mathrm{A}$

In ${\mathrm{H}}_1^1$, Number of neutrons $=1-1=0$

In ${\mathrm{H}}_1^2\left(\mathrm{D}\right)$, Number of neutrons $=2-1=1$

In ${\mathrm{H}}_1^3\left(\mathrm{T}\right)$, Number of neutrons $=3-1=2$

Total number of neutrons$=\mathrm{X}+\mathrm{Y}+\mathrm{Z}$

$$\begin{gathered} =\left(1-1\right)+\left(2-1\right)+\left(3-1\right) \\ =0+1+2 \\ =3 \end{gathered}$$

Hence, the sum of (x), (y) and (z) is 3.