A calcium atom becomes a calcium ion by losing two electrons. Electrons are negatively charged, so losing two electrons creates an ion with a charge of 2+.
According to the octet rule, atoms gain or lose electrons to fill their outer energy levels. This helps them achieve noble gas configuration. Calcium is atomic number 20 on the periodic table. Its nearest noble gas is argon, which has an atomic number of 18. To achieve argon's electron configuration, calcium needs to lose two electrons. When it does this, it becomes a positively charged ion. Although calcium is in the same period as the noble gas krypton, the calcium never attempts to attain krypton's electron configuration. This is because it takes less energy to lose two electrons than it would to gain six electrons and fill the outer energy level.
Calcium ions serve several functions in the human body. Because calcium is necessary for muscle contraction, the heart would not be able to beat without the right amount of calcium in the bloodstream. The neurons also rely on calcium ions to release neurotransmitters. Some chemical reactions would not be possible without calcium ions because certain enzymes rely on calcium to function properly.