Answer:

 

Let's look at hydrogen and oxygen. Do you predict H and O will form a covalent bond? How many hydrogens will combine with the oxygen, O?

First thing to do is check the group numbers. Hydrogen is still in group 1A. It can share only one electron. Oxygen is in group 6A. It has six valence electrons, 2s22p4. Some math tells us that the oxygen can accept two electrons to complete the valence shell octet. This means that the H and O are predicted to form a covalent bond. The oxygen "needs" two electrons so it must form two bonds with two different hydrogen atoms to complete an octet. The formula for the combination will be H2O.

Now you see why water is H2O and not H5O.

The periodic table gives us the power to predict correctly that all the elements in group 6A will combine with hydrogen to form covalent bonds. The formulas for all of these combinations will be in the same proportions. There typically will be two hydrogens for every atom from group 6A; H2O, H2S, H2Se, H2As. Guess what happens when atoms from group 5A combine with hydrogen.

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