READ: Covalent Bonds
READ: Covalent Bonds
A covalent bond is the force of attraction that holds together two atoms that share a pair of electrons. In covalent bonds, the outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared. By overlapping orbitals, the outer energy shells of all the bonding atoms are filled. The shared electrons move in the orbitals around both atoms. As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei. This attractive force holds the atoms together in a covalent bond.
Covalent bonds form only between atoms of nonmetals. The sharing occurs because both atoms have a strong electronegativity and do not want to lose any electrons. Because neither want to lose any electrons, they share instead. This is why metals do not form covalent bonds as they easily will lose their electrons.
The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates. These are compounds in living things.
The two atoms may be the same or different elements. Water is an example of a covalent compound. A water molecule is modeled in the picture below. A molecule is the smallest particle of a covalent compound that still has the properties of the compound.
Oxygen normally occurs in diatomic ("two-atom") molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). To help you remember which atoms are naturally diatomic molecules, they are often called the "Magnificent 7." This is because there are 7 molecules, and starting at atomic number 7 (nitrogen) they form a 7 on the Periodic Table. The picture below shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen.
In covalent bonding, the atoms acquire a stable octet of electrons by sharing electrons. The covalent bonding process produces molecular substances, as opposed to the lattice structures produced by ionic bonding. When only one pair of electrons are shared between two bonded atoms a single bond is formed. However, it is possible for atoms to share more than one pair of electrons. When two or three pairs of electrons are shared by two bonded atoms, they are referred to as double bonds ortriple bonds, respectively.
In an Electron Dot structure, each valence electron is represented by a dot, and bonds are shown by placing electrons in between the symbols for the two bonded atoms and circling the shared pair. Often, a pair of bonding electrons is further abbreviated by a dash. For example, we can represent the covalent bond in the F2 molecule by either of the Electron Dot structures shown below.
Double bonds (4 electrons shared between two atoms) can be represented either with 4 dots or 2 dashes. The Electron Dot structure for an oxygen molecule (O2 ) is shown below. You can see that this method of showing the covalent bond is much easier than drawing the entire atom as we have shown of the Oxygen molecule above on the page.
Similarly, triple bonds can be written as 6 dots or 3 dashes. An example of a molecule with triple bonds is the nitrogen molecule, N2. The Electron Dot structure for a nitrogen molecule can be represented by either of the two ways shown below. It is important to keep in mind that a dash always represents a pair of electrons.
Properties of Covalent Compounds
Covalent compounds have several properties that distinguish them from ionic compounds. These properties are:
- The melting and boiling points of covalent compounds are generally lower than those of ionic compounds. This is because it takes less energy for individual molecules than ions in a crystal to pull apart.
- Covalent compounds are generally more flexible than ionic compounds. The molecules in covalent compounds are able to move around to some extent and can sometimes slide over each other. This results in many covalent compounds being gases or liquids at room temperature. In ionic compounds, all the ions are tightly held in place and most are crystalline solids.
- Covalent compounds generally are not very soluble in water, for example plastics are covalent compounds and many plastics are water resistant.
- Covalent compounds generally do not conduct electricity when dissolved in water, for example iodine dissolved in pure water does not conduct electricity. This is because covalent compounds have shared electrons. Since these are not free to move like the transferred electrons of ionic compounds, they are poor conductors.
Polar and Nonpolar Covalent Bonds
In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. The picture below shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. As you can see this creates two poles on the molecule, a positive pole (side) and a negative pole (side).
In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral and there are no poles (+ or - sides).
Georgia Virtual, Bonding and Chemical Reactions, CC BY-NC-SA 3.0
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