Gaining an electron to become an anion: Chlorine

Ions with a negative charge due to the gain of electrons are called anions. In general, an anion is slightly larger than its corresponding atom because the protons have to attract one or more extra electrons. The attractive force is slightly reduced, so the electrons are free to move outward a little.

Chlorine, a member of the halogen family  the VIIA family on the periodic table often forms anions. It has seven valence electrons and a total of 17 electrons, and its electron configuration is 1s²2s²2p⁶3s²3p⁵. So to obtain its full octet, chlorine must lose the seven electrons in energy level 3 or gain one at that level.

Because elements don’t gain or lose more than three electrons, chlorine must gain a single electron to fill energy level 3. At this point, chlorine has 17 protons (17 positive charges) and 18 electrons (18 negative charges). So chlorine becomes an ion with a single negative charge (Cl–). The neutral chlorine atom becomes the chloride ion. The electronic configuration for the chloride anion is

Cl^–: 1s²2s²2p⁶3s²3p⁶

The chloride anion is isoelectronic with argon. The chloride anion is also slightly larger than the neutral chlorine atom. To complete the octet, the one electron gained went into energy level 3. But now there are 17 protons attracting 18 electrons, so the electrons can move outward a bit.