In its ground, or lowest-energy, state, carbon has two unpaired electrons. However, there are four total outer, or valence, electrons, meaning carbon atoms have four possible bonding sites.
Because of its small size and its four bonding sites, carbon forms more different types of compounds than any other element. It is also the only element capable of forming many bonds with other of the same type of atom. Carbon's bonds can be single, double, triple or even quadruple, meaning that carbon creates long chains as well as rings.
Most carbon bonds are covalent, meaning that all involved atoms share electrons either equally or unequally. However, the bond between carbon and calcium is ionic.