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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.

How do oxygen atoms bond covalently to form an oxygen molecule?

  1. Each oxygen atom has 8 electrons, and their electron configuration is 1s 2 2s 2 2p 4 .

  2. Each oxygen atom has 6 valence electrons, meaning that each atom has 2 unpaired electrons.

  3. 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 ( [link] ).

    A double covalent bond in an oxygen molecule
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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.

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 O 2 , 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 thesame 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.

Covalent bonding and valency

  1. Explain the difference between the valence electrons and the valency of an element.
  2. 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
    F
    Ar
    C
    N
    O
  3. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
    1. Water (H 2 O)
    2. Chlorine (Cl 2 )

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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