Question

An element is represented by ${}_Z{X}^A$, where Z is atomic number which is also equal to number of proton, A is mass number and X is symbol of the element.
Mass number (A) = Number of protons (Z) + Number of neutrons.
Isotopes: Atoms having same number of proton
Isobar: Elements having same mass number
Isotones: Elements having same number of neutron (A-Z)

The sum of the number of electrons, protons and neutrons in the three isotopes ${(}^{16}O{,}^{17}O{,}^{18}O)$ of oxygen is:

• A. 36
• B. 57
• C. 75
• D. 68

Answer 1

The correct option is C 75

Oxygen has an atomic number of 8.
Isotopes of oxygen are: ${}^{16}O{,}^{17}O{,}^{18}O$

$\begin{array}{cccc}\text{S}ymbol& \text{N}eutrons& \text{P}rotons& \text{E}lectrons\\ {}^{16}O& 8& 8& 8\\ {}^{17}O& 9& 8& 8\\ {}^{18}O& 10& 8& 8\\ Sum& 27& 24& 24\end{array}$
$⟹27+24+24=75$