Anions

Anions are formed from nonmetals, which are located on the upper right side of the periodic table, excluding noble gases. Noble gases are inert and do not form ions because they already have a full octet of eight valence electrons.

Nonmetals are close to achieving a full octet, making them eager to gain electrons. Unlike metals that give away electrons, nonmetals steal electrons from metals to achieve a noble gas configuration. This addition of negatively charged electrons results in a negative charge for the anion, making it more negative than its neutral atom.

A neutral fluorine atom has 9 protons, 9 electrons, and 10 neutrons. When fluorine (a halogen) gains one electron, it becomes a fluoride anion with a -1 charge, now having 9 protons, 10 electrons, and 10 neutrons. The nucleus remains unchanged. The extra electron adds to the existing outer shell, causing the anion to be larger than its neutral counterpart, as electrons spread out to accommodate the new addition. Fluoride then resembles neon in electron configuration.

A neutral oxygen atom has 8 protons, 8 electrons, and 8 neutrons. Being in the negative two column, oxygen gains two electrons to become an oxide anion with a -2 charge, resulting in 8 protons, 10 electrons, and 8 neutrons. Similar to fluoride, the oxide anion is larger than the neutral oxygen atom due to the additional electrons occupying the outer shell. Oxide also achieves an electron configuration like neon.

In summary, cations come from metals, are positive, and lose electrons. Anions come from nonmetals, are negative, and gain electrons. Noble gases are ignored because they do not form ions.