The English used in this article may not be easy for everybody to understand. (November 2017)
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). A full outer shell usually has eight electrons, or two in the case of hydrogen or helium. Valence electrons are the electrons held comparatively loosely in the outer shell of the atom. The structure of electron shells is determined by “Quantum Mechanics”.)
The number of valence electrons is decided by the size of the atom. Electrons orbit atomic nuclei in a way that is a very rough analogy of the way that planets orbit stars. They are like fuzzy orbital paths around an atomic nucleus. The first layer can contain up to two electrons, while the layers after that usually contain up to eight. Covalent bonds are formed by atoms sharing valence electrons.
If for example, an atom contained nine electrons, the first two would orbit very close to the nucleus, the next seven would orbit a little farther away. The outer seven electrons would be less tightly held than the inter two electrons because they are further away from the positively charged nucleus. If this atom gets close to another atom, with a loosely held electron in its outer shell, a new orbital will become available to the loosely held electron. This new electron orbital is bound to both atomic nuclei and has a lower energy level than the original electron orbital. The electron can spontaneously jump to it and emit a photon with the excess energy. Now we have an electron orbiting both atoms which results in the atom that provided the electron having a small net positive charge and the other atom having a small net negative charge. The two atoms are now held together by the electromagnetic force of attraction between positive and negative charges. This is called a covalent bond. To break this bond requires the supply of the same amount of energy as was released when it was formed.
A water molecule consists of one oxygen atom and two hydrogen atoms held together by a covalent bond. In this case the oxygen atom shares one electron with each hydrogen atom. This means that the oxygen atoms has a small net positive charge and the hydrogen atom has a small net negative charge. As a result, the oxygen and hydrogen atoms are attracted to each other due to the electromagnetic force. (Also note that because of this the water molecule is a polar molecule. i.e. it’s charge is not evenly distributed.)
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.