A covalent bond formed by the mutual sharing of one electron pair between two atoms is called single bond. It is denoted by the single short line (-) In single bond formation each atom provide one electron. Usually single bond is sigma bond. But in diboron is pi bond .
Single bond between the oxygen and each hydrogen in water molecule ( H2O ). Each of the covalent bond contains two electrons. One from hydrogen atom and one from oxygen atom. Both atoms share the electrons.