Which atom has a larger atomic radius S or S2-?
The Correct Answer and Explanation is:
The correct answer is: S²⁻ has a larger atomic radius than S.
Explanation
When comparing the atomic radius of a neutral sulfur atom (S) and its ion (S²⁻), it is important to understand how gaining or losing electrons affects the size of an atom. A neutral sulfur atom has 16 protons and 16 electrons. When it gains two extra electrons to form the S²⁻ ion, it still has 16 protons, but now it has 18 electrons.
This increase in the number of electrons without an increase in protons means there is less effective nuclear charge holding each electron closely to the nucleus. The electrons repel each other more because of the increased number, and the attraction from the nucleus is spread across more electrons. As a result, the electron cloud expands. This makes the S²⁻ ion larger in size than the neutral sulfur atom.
The atomic radius increases when electrons are added to an atom without a corresponding increase in the positive charge of the nucleus. In contrast, if an atom loses electrons and becomes a positively charged ion, the radius decreases because the remaining electrons are drawn closer to the nucleus.
In the case of sulfur:
- S (neutral): 16 protons and 16 electrons. The electron cloud is balanced and more compact.
- S²⁻ (ion): 16 protons and 18 electrons. The added electrons increase electron–electron repulsion and reduce the effective nuclear pull on each electron.
This makes S²⁻ much larger than neutral sulfur. The difference in size between atoms and their ions is a general trend seen across the periodic table. Negative ions (anions) are always larger than their neutral atoms, while positive ions (cations) are smaller.
Therefore, S²⁻ has a larger atomic radius than S because it has gained electrons, causing increased repulsion and a more spread-out electron cloud.
