Final answer:
Oxygen would be smaller than sulfur, while selenium would be larger due to additional electron shells. Coulomb's Law explains how attractions between positive and negative charges and repulsions between like charges affect atomic size, with cations being smaller and anions being larger.
Step-by-step explanation:
In predicting the size of atoms in the same group as sulfur, elements higher up the group would be smaller while those lower down would be larger due to increased electron shells. Hence, oxygen (O) would be smaller than sulfur, and selenium (Se) would be larger. The fact that protons and electrons attract each other while like charges repel is an essential aspect of Coulomb's Law. This attraction and repulsion affect atomic size. An atom's electrons are attracted to the positively charged nucleus. However, additional electron shells increase the distance from the nucleus, causing a weaker attraction and larger atomic size due to electron-electron repulsion within these shells. Additionally, when electrons are lost to form a cation, there is a stronger pull from the nucleus on the reduced number of electrons, resulting in a smaller ion, while anions, having gained electrons, experience more electron-electron repulsion, leading to an increase in size.