The effective atomic number (EAN) of a molecule is the sum of the atomic numbers of the atoms in the molecule, minus the number of electrons in the non-bonding orbitals.

For water (H2O), the atomic numbers of hydrogen and oxygen are 1 and 8, respectively. Each hydrogen atom contributes one electron to the covalent bond with oxygen, while oxygen contributes two electrons. The non-bonding electrons are the lone pairs on oxygen.

So, the EAN of water is:

$$EAN = (2 \times 1) + 8 - 4 = 10$$

Therefore, the effective atomic number of water is 10.