When an atom donates electrons and other gains electrons, they form ions and the bond is called the ionic bond. When the atoms share electrons between them, they form a molecule, and the bond is called the covalent bond.
The atoms can share one, two or three pairs of electrons in the same bond. If one pair is shared it's called a single bond or a covalent simple; if two pairs are shared it's called a double bond or a covalent double, and if three pairs are shared it's called a triple bond or a covalent triple.
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