Understanding the structure of the H2O molecule can help us to understand not only its chemical properties, but also how it can be split using electricity to form Hydrogen fuel and Oxygen gas through the process of H2O electrolysis. By looking at how the water molecule is built we can also improve our basic understanding of the bonding of elements and become more familiar with the uses of electron dot diagrams.
A water molecule is constructed from two Hydrogen atoms and one Oxygen atom. Both these elements are in the first 20 of the periodic table. If we are familiar with middle high school chemistry then we know that electrons fill shells according to the pattern 2, 8, 8, 2. Hydrogen has only one electron so it has the electron configuration of just 1. It needs one extra electron to have a full outer shell. Oxygen has eight electrons and so has an electron configuration of 2, 6. Oxygen needs a further two electrons to get to 2, 8 and thus have a full outer shell.
Taking into account only the electrons in the outer shells, we get the electron dot diagrams for oxygen and hydrogen as seen in the top right image.
As with all electron dot bonding exercises, all we need to do is move the diagrams around until we have each atom with a full outer shell. In this case we just slot one Hydrogen into each gap in the oxygen atom, as seen in stages below:
We now replace the pairs of electrons with lines (they mean the same thing but are easier to draw) and we see the classic structure of water. This is a flat representation of water. In reality water molecules are three dimensional, and water follows the VSEPR rules.
Chemical Properties of the H2O Molecule
The bonds between oxygen and the hydrogens in the H2O molecule are not even; the oxygen has a larger share of the electrons due to its nucleus containing more protons, which is also called its electronegativity. This leads each of those bonds to be a polar covalent bond, resulting in water having a slight positive charge on its Hydrogen atoms, as seen to the right.
This then makes water a polar molecule, as it has two negatively charged parts (the electron pairs that are unbound) and two positive parts (the Hydrogen atoms). These opposite charges are attracted to each other. This holds water molecules together which explains why water is a liquid at room temperature while almost all other similar sized molecules are gases. This also accounts for water’s excellent dissolving capacity for other charged substances such as salt, and why uncharged substances (non polar) such as oil do not mix readily with water.