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Some differences between covalent and ionic bonding have already been mentioned. Here is another one. It is possible for covalent bonding to occur between atoms of the same element. That is not possible with ionic bonding. With ionic bonding you need to have two different elements, one to lose electrons and one to gain electrons. The atoms involved in covalent bonding all need to gain electrons and they do not have to be different elements. So let's look at covalent bonding in pure elements.

Let's start with the simplest case, a hydrogen atom bonding to another hydrogen atom. Each has one electron and wants one more. By coming together, each can "gain" one electron from the other. Since neither atom lets go of its electron, the two atoms are bonded together by their mutual attraction for the shared pair of electrons.

H2 is an element because it contains only hydrogen atoms. H2 is a molecule, no additional bonding is needed. There are two atoms in the molecule so it is a diatomic molecule. Hydrogen is one of several elements that form diatomic molecules.

Atoms Molecule
H         H H:H


Nitrogen atoms have five valence electrons and need three more. They are small and have a strong attraction for electrons. Thus when a nitrogen atom bonds to another nitrogen atom, each atom can attract three of the other's valence electrons and form a triple bond. Three elecrons from each atom  get lined up between the atoms. Although that can be represented as shown in the middle diagram, triple bonds are usually not written this way. Generally they will be written with the three pairs of electrons arranged as shown in the bottom diagram. So the nitrogen molecule, which can be written as N2, has a triple bond.
: N        N :
: N   N :

: N : : : N :


Practice with Diatomic Elements

Now see what you can do with O, F, Cl, Br, and I. You can check your results below to make sure you have them correct.



Here you can see the Lewis diagrams for these elements.

O : : O
: F : F :
: Cl : Cl :
: Br : Br :
: I : I :


Elements Continued

These seven elements (H, N, O, F, Cl, Br, and I) are called the diatomic elements because, as pure elements, they form molecules containing two atoms. You should commit these seven diatomic elements to memory. However, as you have already noticed, there are not necessarily two atoms of these elements in the molecules formed when they make compounds.


Oxygen also forms a molecule called ozone that contains three atoms. If you have not already checked out the diagram shown, see if you can figure out a good Lewis diagram for O3.

: :   O :   O:

Phosphorus and sulfur are not diatomic elements. They generally form molecules of P4 and S8 although other forms of these elements also exist. In part this is because they form single bonds rather than the triple and double bonds found in N2 and O2. Phosphorus and sulfur atoms do not have a strong enough pull on electrons to form multiple bonds by themselves. Check with your instructor if you are interested in how these molecules are put together. Nonmetal portion of periodic table showing molecular formulas of P and S.


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