Covalent bonds are chemical bonds between two non-metal atoms. An example is water, where hydrogen (H) and oxygen (O) bond together to make (H2O). As they are both non-metals—which need to gain electrons—they have to share, so their outer shells cross over in order to have a full outer shell. A full outer shell has eight electrons. The electrons in this outer shell are called valence electrons.
The number of valence electrons is decided by the size of the atom. Electrons orbit an atomic nucleus in the same kind of way that planets orbit stars. There are layers of paths around an atomic nucleus. The first layer always contains only two electrons, while the layers after that usually contain up to eight.
That is to say, it wants the number of electrons in the outer-most layer to be as high as they can be.

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