What is the proper chemical formula and model for water?

The chemical formula of water is H2O. One molecule of water contains 2 atoms of hydrogen and one atom of oxygen. It is a covalent molecule and has a definite geometry. The central atom is oxygen.   The electronic configuration of Oxygen in the ground state is

1s2 2s2 2p4. (atomic number of oxygen is 8).

The 2s orbital containing 2 electrons and the three 2p orbitals containing a total of 4 electrons of oxygen undergo  sp3 hybridization. Hybridization is the process of mixing up of orbitals of slightly different energies of an atom to give same number of equivalent orbitals. The hybrid orbitals will have the same energy and shape. In this case 4  sp3 hybrid orbitals are formed. According to VSEPR theory these hybrid orbitals will get themselves arranged around the central atom in such a way that repulsion between them is minimum. Here the orbitals are arranged tetrahedrally around the central oxygen atom. The angle between the orbitals in the tetrahedral arrangement is 109028’. As an orbital can accommodate only two electrons, the 6 electrons are distributed among the 4 hybrid orbitals such that two orbitals contain two electrons each (paired electrons) and the remaining two contain one electron each (unpaired electron). Only half filled orbitals containing one electron (unpaired electron) can form covalent bond. Hence the two half-filled sp3 hybrid orbitals of oxygen form covalent bonds with two hydrogen atoms.

The electronic c configuration of Hydrogen atom is   1s1. (atomic number of hydrogen is 1).

The 1s orbital of hydrogen is half-filled (contains only 1 electron).

According to Valence Bond theory, a covalent bond is formed by the overlap of two half-filled orbitals containing electrons of opposite spin of two atoms. Thus in water, the two half-filled sp3 hybrid orbitals of oxygen  overlap with the half-filled 1s orbitals of 2 hydrogen atoms to form covalent bonds. The overlap of the orbitals takes place along the inter-nuclear axis resulting in the formation of sigma type covalent bonds. (if the overlap takes place sideways, a pi bond is formed). In water the two O-H bonds are of sigma type.

In water, the central oxygen atom has two bond pair electrons (from two covalent bonds) and two lone-pair of electrons (from two completely filled orbitals not involved in bond formation) .According to VSEPR theory the repulsion between the orbitals are in the order

Lone pair-lone pair> lone pair- bond pair > bond pair-bond pair.

As there are 2 lone pair electrons, the bond pairs get repelled and the H-O-H bond angle decreases from the normal tetrahedral angle of 109028’ to 104.50. Water is thus a bent molecule. It is a V-shaped molecule.

The atomic radius of Oxygen is greater than that of hydrogen. Hence oxygen is bigger in size than hydrogen.

Oxygen is more electronegative than hydrogen.Electronegativity is a measure of the tendency of an atom to attract the shared pair of electron s (bonded pair of electrons) of a covalent bond towards itself in a molecule. Since oxygen is more electronegative than hydrogen the shared pair of electrons in the O-H bond is displaced towards oxygen atom. As a result a partial negative charge develops at the oxygen atom and a partial positive charge at each hydrogen atom. The O-H bond is thus polar. Each O-H bond has a dipole moment. Dipole moment is a vector quantity, having both magnitude and direction. Since H2O is a bent molecule, the bond moments do not cancel each other and the molecule has a net dipole moment. Water is thus a polar molecule.

The image at ‘b’ represents the water molecule.