Both the chemical compounds as chemical elements are made up of molecules, and these in turn are made up of atoms. The atoms remain united thanks to the formation of the so-called chemical links.
The chemical bonds are not all the same: basically they depend on the electronic characteristics of the atoms involved. There are two most common types of link: ionic bonds and the covalent bonds.
Typically, covalent bonds are those that hold non-metallic atoms together. It happens that the atoms of these elements have many electrons in their outermost shell and have a tendency to retain or gain electrons, instead of giving them up.
That is why the way in which these substances or chemical compoundsicos achieve stability is by sharing a pair of electrons, unot from every atom. In this way the shared pair of electrons is common to the two atoms and at the same time holds them together. In the gases nobles, for example, this happens. Also in halogen elements.
When the covalent bond occurs between elements of similar electronegativity, as between hydrogen and carbon, a bond is generated apolar covalent. This happens, for example, in hydrocarbons.
Likewise, homonuclear molecules (made up of the same atom) always form apolar bonds. But if the bond occurs between elements of different electronegativity, a higher electron density is produced in one atom than in another, as a result of this a pole is formed.
A third possibility is that two atoms share a pair of electrons, but that these shared electrons are contributed by only one atom of them. In that case we speak of dative or coordinate covalent bond.
For a dative link you need an element with a free electron pair (like nitrogen) and another that is electron-deficient (like hydrogen). It is also necessary that the one with the electronic pair is electronegative enough not to lose the electrons to share. This situation occurs, for example, in ammonium (NH4+).
The substances containing covalent compounds can occur in any state of matter (solid, liquid or gaseous), and in general they are poor conductors of heat and electricity.
They often show relatively low melting and boiling points Y are usually soluble in polar solvents, like benzene or carbon tetrachloride, but they have poor solubility in water. They are extremely stable.
Numerous examples of compounds or substances containing covalent bonds can be given:
- Fluorine
- Bromine
- Iodine
- Chlorine
- Oxygen
- Water
- Carbon dioxide
- Ammonia
- Methane
- Propane
- Silica
- Diamond
- Graphite
- Quartz
- Glucose
- Paraffin
- Diesel
- Nitrogen
- Helium
- Freon
