When H forms a bond with H2O to form the hydronium ion H3O , this bond is called a coordinate covalent bond because

both bonding electrons come from the oxygen atom

Explanation:

A coordinate covalent bond is said to be formed when an electron pair is donated into a empty orbital of an atom from a filled orbital of another atom.

In this type of covalent bond, the shared electron pair is furnished by only one of the bonding species.

The bonding between H^+ and H2O to form H3O^+ involves donation of a lone pair of electrons on oxygen into the empty 1s orbital of  H^+ hence the two bonding electrons come from oxygen. This is why the bond is called a coordinate covalent bond.

astronomers such as edwin hubble.

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