When you look at the matter, or physical substances, around you, you will realise that atoms seldom exist on their own. More often, the things around us are made up of different atoms that have been joined together. This is called chemical bonding. Chemical bonding is one of the most important processes in chemistry because it allows all sorts of different molecules and combinations of atoms to form, which then make up the objects in the complex world around us.
What happens when atoms bond?
A chemical bond is formed when atoms are held together by attractive forces. This attraction occurs when electrons are shared between atoms, or when electrons are exchanged between the atoms that are involved in the bond. The sharing or exchange of electrons takes place so that the outer energy levels of the atoms involved are filled and the atoms are more stable. If an electron is shared, it means that it will spend its time moving in the electron orbitals around both atoms. If an electron is exchanged it means that it is transferred from one atom to another, in other words one atom gains an electron while the other loses an electron.
DEFINITION 1: Chemical bond
A chemical bond is the physical process that causes atoms and molecules to be attracted to each other, and held together in more stable chemical compounds.
The type of bond that is formed depends on the elements that are involved. In this chapter, we will be looking at three types of chemical bonding: covalent, ionic and metallic bonding.
You need to remember that it is the valence electrons that are involved in bonding and that atoms will try to fill their outer energy levels so that they are more stable (or are more like the noble gases which are very stable).
The nature of the covalent bond
Covalent bonding occurs between the atoms of non-metals. 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, and this force holds the atoms together in a covalent bond.
DEFINITION 2: Covalent bond
Covalent bonding is a form of chemical bonding where pairs of electrons are shared between atoms.
You will have noticed in the above examples that the number of electrons that are involved in bonding varies between atoms. We say that the valency of the atoms is different.
DEFINITION 3: Valency
The number of electrons in the outer shell of an atom which are able to be used to form bonds with other atoms.
In the first example, the valency of both hydrogen and chlorine is one, therefore there is a single covalent bond between these two atoms. In the second example, nitrogen has a valency of three and hydrogen has a valency of one. This means that three hydrogen atoms will need to bond with a single nitrogen atom. There are three single covalent bonds in a molecule of ammonia. In the third example, the valency of oxygen is two. This means that each oxygen atom will form two bonds with another atom. Since there is only one other atom in a molecule of O2, a double covalent bond is formed between these two atoms.
There is a relationship between the valency of an element and its position on the Periodic Table. For the elements in groups 1 to 4, the valency is the same as the group number. For elements in groups 5 to 7, the valency is calculated by subtracting the group number from 8. For example, the valency of fluorine (group 7) is 8−7=1, while the valency of calcium (group 2) is 2. Some elements have more than one possible valency, so you always need to be careful when you are writing a chemical formula. Often, if there is more than one possibility in terms of valency, the valency will be written in a bracket after the element symbol e.g. carbon (IV) oxide, means that in this molecule carbon has a valency of 4.
Below are a few examples. Remember that it is only the valence electrons that are involved in bonding, and so when diagrams are drawn to show what is happening during bonding, it is only these electrons that are shown. Circles and crosses are used to represent electrons in different atoms.
EXERCISE 1: Covalent bonding
How do hydrogen and chlorine atoms bond covalently in a molecule of hydrogen chloride?
- Step 1. Determine the electron configuration of each of the bonding atoms. :
A chlorine atom has 17 electrons, and an electron configuration of 1s22s22p63s23p5. A hydrogen atom has only 1 electron, and an electron configuration of 1s1.
- Step 2. Determine the number of valence electrons for each atom, and how many of the electrons are paired or unpaired. :
Chlorine has 7 valence electrons. One of these electrons is unpaired. Hydrogen has 1 valence electron and it is unpaired.
- Step 3. Look to see how the electrons can be shared between the atoms so that the outermost energy levels of both atoms are full. :
The hydrogen atom needs one more electron to complete its valence shell. The chlorine atom also needs one more electron to complete its shell. Therefore one pair of electrons must be shared between the two atoms. In other words, one electron from the chlorine atom will spend some of its time orbiting the hydrogen atom so that hydrogen's valence shell is full. The hydrogen electron will spend some of its time orbiting the chlorine atom so that chlorine's valence shell is also full. A molecule of hydrogen chloride is formed (Figure 1). Notice the shared electron pair in the overlapping orbitals.
EXERCISE 2: Covalent bonding involving multiple bonds
How do nitrogen and hydrogen atoms bond to form a molecule of ammonia (NH3)?
Step 1. Determine the electron configuration of each of the bonding atoms. :
A nitrogen atom has 7 electrons, and an electron configuration of 1s22s22p3. A hydrogen atom has only 1 electron, and an electron configuration of 1s1.
Step 2. Determine the number of valence electrons for each atom, and how many of the electrons are paired or unpaired. :
Nitrogen has 5 valence electrons meaning that 3 electrons are unpaired. Hydrogen has 1 valence electron and it is unpaired.
Step 3. Look to see how the electrons can be shared between the atoms so that the outer energy shells of all atoms are full. :
Each hydrogen atom needs one more electron to complete its valence energy shell. The nitrogen atom needs three more electrons to complete its valence energy shell. Therefore three pairs of electrons must be shared between the four atoms involved. The nitrogen atom will share three of its electrons so that each of the hydrogen atoms now have a complete valence shell. Each of the hydrogen atoms will share its electron with the nitrogen atom to complete its valence shell (Figure 2).
The above examples all show single covalent bonds, where only one pair of electrons is shared between the same two atoms. If two pairs of electrons are shared between the same two atoms, this is called a double bond. A triple bond is formed if three pairs of electrons are shared.
EXERCISE 3: Covalent bonding involving a double bond
How do oxygen atoms bond covalently to form an oxygen molecule?
Step 1. Determine the electron configuration of the bonding atoms. :
Each oxygen atom has 8 electrons, and their electron configuration is 1s22s22p4.
Step 2. Determine the number of valence electrons for each atom and how many of these electrons are paired and unpaired. :
Each oxygen atom has 6 valence electrons, meaning that each atom has 2 unpaired electrons.
Step 3. Look to see how the electrons can be shared between atoms so that the outer energy shells of all the atoms are full. :
Each oxygen atom needs two more electrons to complete its valence energy shell. Therefore two pairs of electrons must be shared between the two oxygen atoms so that both valence shells are full. Notice that the two electron pairs are being shared between the same two atoms, and so we call this a double bond (Figure 3).
Covalent bonding and valency
- Explain the difference between the valence electrons and the valency of an element.
Click here for the solution.
- Complete the table below by filling in the number of valence electrons and the valency for each of the elements shown:
Element No. of valence electrons No. of electrons needed to fill outer shell Valency
3. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
- Water (H2O)
- Chlorine (Cl2)
Properties of covalent compounds
Covalent compounds have several properties that distinguish them from ionic compounds and metals. These properties are:
- Melting and boiling points: The melting and boiling points of covalent compounds is generally lower than that for ionic compounds.
- Flexibility: 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 (as is the case with graphite, this is why the lead in your pencil feels slightly slippery). In ionic compounds all the ions are tightly held in place.
- Solubility in water: Covalent compounds generally are not very soluble in water.
- Electrical conductivity: Covalent compounds generally do not conduct electricity when dissolved in water. This is because they do not dissociate as ionic compounds do.