Losing an electron to become a cation: Sodium

Ions that have a positive charge due to the loss of electrons are called cations. In general, a cation is smaller than its corresponding atom. Why? The filled energy level determines the size of an atom or ion, and a cation gives up enough electrons to lose an entire energy level.

Consider sodium, an alkali metal and a member of the IA family on the periodic table. Sodium has 1 valence electron and 11 total electrons, because its atomic number is 11. It has an electron configuration of 1s²2s²2p⁶3s¹.

By the octet rule, sodium becomes stable when it has eight valence electrons. Two possibilities exist for sodium to become stable: It can gain seven more electrons to fill energy level 3, or it can lose the one 3s electron so that energy level 2 (which is already filled at eight electrons) becomes the valence energy level.

So to gain stability, sodium loses its 3s electron. At this point, it has 11 protons (11 positive charges) and 10 electrons (10 negative charges). The once-neutral sodium atom now has a single positive charge [11 (+) plus 10 (–) equals 1+]. It’s now an ion, an atom that has a charge due to the loss or gain of electrons. You can write an electron configuration for the sodium cation:

Na+: 1s²2s²2p⁶

Note that if an ion simply has 1 unit of charge, positive or negative, you normally don’t write the 1; you just use the plus or minus symbol, with the 1 being understood.

Atoms that have matching electron configurations are isoelectronic with each other. The positively charged sodium

ion (cation) has the same electron configuration as neon, so it’s isoelectronic with neon. So does sodium become neon by losing an electron? No. Sodium still has 11 protons, and the number of protons determines the identity of the element.

There’s a difference between the neutral sodium atom and the sodium cation: one electron. As a result, their chemical reactivities are different and their sizes are different. Because sodium loses an entire energy level to change from a neutral atom to a cation, the cation is smaller.