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 can contain up to two electrons, while the layers after that usually contain up to eight.
For example, if an atom contained eight total electrons, the first two would orbit very close to the nucleus, the next 6 would orbit a little farther away. Every atom wants a "full" outer shell. That is to say, it wants the number of electrons in the outer-most layer to be as high as they can be. Since most atoms' outer shells can support up to eight electrons, this is the number of electrons most atoms want in their outer shell. In this example, the atom had eight total electrons, and six in the outer shell. Since there are fewer than eight electrons in the outer shell, the atom will want to "fill" the places for two electrons with electrons borrowed from another atom. When an atom "borrows" electrons with another atom, a covalent bond is formed.
With water the oxygen atom shares one electron with each hydrogen atom and the hydrogens also share one: this means that the hydrogen atoms have two each and the oxygen atom has eight.
Types of Covalent bond Covalent bonds can be split into two groups. Simple and Giant. Although they are both similar, there are some quite large distinguishing factors. For example, Giant Covalent bonds have relatively high melting and boiling points, whereas Simple Covalent bonds have a relatively low melting and boiling point. This is because of the inter-molecular forces of attraction. With giant Covalent bonds, the bonds between layers are very strong, Diamond is an example of Giant Covalent bond of carbon. However, with a simple Covalent bond, the bonds between the atoms have a strong inter-molecular bond, but there are no bonds between the molecules, which is why it has a relatively low melting and boiling point.