What property increases when an atom gains electrons?

When an atom gains electrons, it becomes negatively charged due to the increase in the number of negatively charged electrons relative to the positively charged protons in the nucleus. This gain of electrons results in the atom having more negative charges than positive ones, leading to a net negative charge. This process is fundamental to understanding ionic bonding and the behavior of atoms in various chemical reactions.

Other properties, such as mass, might increase slightly because of the addition of electrons; however, this change is negligible compared to the property of charge. Ionization energy, which refers to the energy required to remove electrons from an atom, actually decreases for negatively charged ions compared to neutral atoms. Valence electrons increase in a different context, particularly when discussing electron configurations, but in this case, referring to the gain of electrons leading to a charge gives a more direct and relevant answer.