ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

When an electropositive atom becomes an ion, specifically a cation (a positively charged ion), it typically loses one or more of its outermost electrons. This loss of electrons results in a decrease in electron-electron repulsion within the atom, which allows the remaining electrons to be pulled closer to the nucleus due to the increased effective nuclear charge experienced by these electrons.

As a result, the ionic radius of the cation is smaller than the radius of the neutral atom from which it originated. In the case of electropositive atoms, which often have low ionization energies and readily lose electrons, this shrinkage in size is pronounced. The decreased electron cloud size, combined with a relatively unchanged nuclear charge, leads to a smaller ionic radius.

In contrast, other potential answers do not accurately reflect the observed behavior of electropositive atoms upon ionization. The size does not remain the same or become larger upon forming a cation, as the processes involved lead specifically to a contraction of the effective radius of the ion compared to the original atom. While the variability among different elements can affect the degree of size change, it does not apply universally to all electropositive atoms, reinforcing that these atoms consistently become smaller as cations.