Covalent Bond

The Covalent Bond: Sharing Electrons

Langmuir (1919) took Lewis’s ideas and made them more practical. He dropped the rigid, cube-shaped picture of the octet and gave us the term covalent bond. The core idea is simple: atoms share electrons to achieve a noble gas configuration.

How a Covalent Bond Forms: The Chlorine Molecule

Consider a single chlorine atom. Its electronic configuration is $[\text{Ne}]3s^2 3p^5$. It has seven valence electrons and is just one electron short of the argon configuration ($[\text{Ne}]3s^2 3p^6$).

When two chlorine atoms come together, they cannot simply transfer an electron — that would create ions, which is not what happens here. Instead, each chlorine atom contributes one electron to form a shared pair. This shared pair belongs to both atoms simultaneously. Now, each chlorine atom “sees” eight electrons in its valence shell: its own six non-bonding electrons plus the two shared electrons. Both atoms have attained the octet of argon.

We represent this sharing with a Lewis dot structure. For $\text{Cl}_2$, we write:

The dots represent valence electrons. The single line ($-$) between the two Cl symbols represents the shared pair — that is the covalent bond.

The Three Conditions for a Covalent Bond

For any molecule formed by covalent bonding, three conditions must hold:

1. Each bond forms from the sharing of one electron pair between the two atoms.
2. Each combining atom contributes at least one electron to that shared pair. (This is the normal case; coordinate bonds are a special exception we will see later.)
3. The combining atoms attain the outer-shell noble gas configuration (usually an octet, or a duet for hydrogen) as a result of this sharing.

Examples: Water and Carbon Tetrachloride

Let us apply these conditions.

Water ($\text{H}_2\text{O}$): Oxygen has six valence electrons ($2s^2 2p^4$). It needs two more to complete its octet. Each hydrogen atom has one electron and needs one more to complete its duet (the helium configuration). So, oxygen shares one electron pair with each hydrogen atom. The Lewis structure is:

Carbon tetrachloride ($\text{CCl}_4$): Carbon has four valence electrons ($2s^2 2p^2$). It needs four more to complete its octet. Each chlorine atom has seven valence electrons and needs one more. So, carbon shares one electron pair with each of the four chlorine atoms. The Lewis structure is:

In both cases, every atom (except hydrogen, which has a duet) is surrounded by eight electrons.

Single, Double, and Triple Bonds

When two atoms share one electron pair, the bond is a single covalent bond (e.g., $\text{Cl} - \text{Cl}$, $\text{H} - \text{O} - \text{H}$).

But atoms can share more than one pair. When they share two pairs of electrons, the bond is a double bond. When they share three pairs, it is a triple bond.

Double bond example: Carbon dioxide ($\text{CO}_2$)

Carbon has four valence electrons. Each oxygen has six. To give every atom an octet, carbon must share two pairs with each oxygen atom. The Lewis structure is:

The two lines ($=$) between C and each O represent a double bond — four shared electrons.

Double bond example: Ethene ($\text{C}_2\text{H}_4$)

In ethene, the two carbon atoms are joined by a double bond. Each carbon also shares one pair with two hydrogen atoms. The Lewis structure is:

Triple bond example: Nitrogen ($\text{N}_2$)

Each nitrogen atom has five valence electrons ($2s^2 2p^3$). It needs three more to complete its octet. So, the two nitrogen atoms share three electron pairs. The Lewis structure is:

$\text{N} \equiv \text{N}$ …