Question

For any anion $X^{−3}$, the mass number is 14. If anion has 10 electrons, then the number of neutrons in the $X_2$ nucleus.
A: 10
B: 14
C: 7
D: 5

Answer 1

Hint: An atom consists of a nucleus having neutrons and protons which are surrounded by electrons in orbits. Number of protons (atomic number) can be considered to be equal to the total number of electrons in a neutral atom (denoted as Z). Whereas, the summation of the number of neutrons and number of protons equals the mass number of an atom (denoted as M). Thus, the number of neutrons can be calculated by subtracting the atomic number (Z) from the mass number of an atom (M).

Complete step by step answer:
We already know that:
\[Mass{\text{ }}number = number{\text{ }}of{\text{ }}protons + number{\text{ }}of{\text{ }}neutrons\]
We are given that the mass number of anion X−3 = 14
This means that:
\[number{\text{ }}of{\text{ }}protons + number{\text{ }}of{\text{ }}neutrons = 14\]
We are also given that the anion has 10 electrons
The symbol of anion = $X^{−3}$ (given) which indicates that the atom X might have gained 3 electrons from some other atom in order to become an anion i.e. $X^{−3}$.
Thus, the original neutral atom (without any charge) i.e. X must contain $10 - 3 = 7electrons$
We also know that \[Number{\text{ }}of{\text{ }}protons = Number{\text{ }}of{\text{ }}electrons\] in case of neutral atom
Therefore, we can now say that number of protons in neutral atom X = 7
So, the number of neutrons in neutral atom X and the anion $X^{−3}$ is:
\[mass{\text{ }}number{\text{ }}of\;X - number{\text{ }}of{\text{ }}protons{\text{ }}of\;X\; = 14 - 7 = 7{\text{ }}neutrons\]
If an atom of X comprises of 7 neutrons then, $X_2$ nucleus will contain number of neutrons:
\[7 + 7 = 14{\text{ }}neutrons\]

Hence option B is correct.

Note:
Each atom of a given element has the similar number of protons while atoms of distinct elements possess distinct numbers of protons. An atom consists of the same number of protons as well as electrons. As protons as well as electrons possess equal and opposite electrical charges, atoms have no overall electrical charge (i.e. neutral).