Question

A certain negative ion ${{\text{X}}^{ - 2}}$ has in its nucleus 18 neutrons and 18 electrons in its extra nuclear structure. What is the mass number of the most abundant isotope of ‘X’?
A)$35.46$
B)32
C)36
D)39

Answer 1

To solve this question, it is required to have knowledge about atomic mass of an element and how it can be calculated. The atomic mass of an element depends on the number of protons and neutrons present in it. The number of protons and electrons are always equal in a neutral compound. So, we shall first find out the number of electrons in the neutral atom. Then, we shall calculate its atomic mass from the number of protons and neutrons present in it.

Complete step by step answer:
As we know that, during anion formation, the atom adds new electrons to its outermost valence shell. The charge on the anion indicates the number of extra electrons present in the cell. An atom does this so as to reach a stable configuration, i.e. fill its valence shell.
In the atom given in the question, the number of electrons present in the anion is 18. The charge on the atom is $- 2$ . This means that there are two extra electrons present in the atom. So, the original number of electrons in the neutral atom will be $18 - 2 = 16$ . As we know, in a neutral atom, the number of electrons and protons are equal. So, there must be 16 protons in the given atom. The atomic number of the atom will thus be 16. In the question, the number of neutrons are 18. Thus, the atomic mass of this atom is $16 + 18 = 34$. This is an isotope of sulphur. But, according to the question, the atomic mass of its abundant isotope is 32.
$\therefore$ The correct option is option B, i.e. 32.

Note: In this question, though the atomic number according to the question will be 34. We shall always consider its atomic mass and use that to determine the atomic mass of the abundant isotope.