Question

An atom of X forms an ion according to the equation ( X + 2e^- X^2- ). The atomic number of the atom X is:

• A. 16
• B. 10
• C. 12
• D. 14

Hint:

Atoms in Group 16 (Chalcogens) always form (2-) ions, while atoms in Group 2 (Alkaline Earth Metals) always form (2+) ions.

Answer 1

The Correct Option is A

To determine the atomic number of the atom X in the equation \( X + 2e^- \rightarrow X^{2-} \), we need to understand what this equation represents. The equation shows that the neutral atom X is gaining 2 electrons to form the X^2- ion. This implies that X initially has two fewer electrons than X^2-.

The atomic number of an element equals the number of protons in a neutral atom of that element. Since the elemental state of X is neutral, the number of electrons must match the number of protons (i.e., the atomic number).

1. First, identify how many electrons the X^2- ion has. The ion X^2- is formed by adding 2 electrons to the neutral atom.
2. Since X^2- has 2 extra electrons compared to the neutral atom, X^2- must have an electron configuration total of 18 electrons.
3. Thus, the neutral atom X, which gains 2 electrons to become X^2-, must originally have 16 electrons.
4. Therefore, the neutral atom X has 16 electrons, which equals its number of protons due to its neutrality. Hence, the atomic number of element X is 16.

Thus, the correct answer to the question, "The atomic number of the atom X is:" is 16.

To confirm, consider the periodic table where sulfur (S) has an atomic number of 16, which commonly forms a sulfide ion \( S^{2-} \) by gaining two electrons, aligning with the process described in the equation.