When atoms share electrons, they form chemical bonds. The type of bond that forms depends on the electronegativity of the atoms involved. Electronegativity is the ability of an atom to attract electrons. The greater the difference in electronegativity between two atoms, the more ionic the bond will be.

In a covalent bond, the electrons are shared equally between the atoms. This type of bond is typically formed between atoms with similar electronegativities. In an ionic bond, one atom donates electrons to the other atom. This type of bond is typically formed between atoms with a large difference in electronegativity.

The sharing of electrons between atoms falls on a continuum from covalent bonds to ionic bonds. This means that there are many different types of bonds that exist between these two extremes. The type of bond that forms in a particular case depends on the electronegativities of the atoms involved.