Covalent bond as shared electron pair

Count the gap

Carbon is in Group 4 with four outer electrons. Four gaps, so it can share four pairs of electrons.

Pick an atom. Count the dashed gaps in its outer shell.

The simplest bond

Both nuclei are attracted to the pair. That attraction is what holds the atoms together.

Peel a layer off, or tap a part, to see what it does.

Chemistry · Covalent bonding

Single, double or triple?

How many pairs of electrons are shared in each of these bonds?

6 of 6 still to sort.

Decide how many pairs are shared in each bond, using the gap each atom needs to fill.

What you need to know

• Covalent bonds form between non-metal atoms. A molecule is a fixed number of two or more of them, covalently bonded together.
• Atoms gain stability by getting a full outer shell. In covalent substances, they get there by sharing electrons.
• Two atoms share a pair of outer-shell electrons, one from each. In H₂, that pair sits between the two nuclei.
• The covalent bond is the strong electrostatic attraction between the shared pair and the nuclei of both atoms. Sharing is how it forms.
• Have a goYour classmate Jo says "the covalent bond is the sharing." She is nearly right. What one thing would you tell her to change?

  Sharing is how the bond forms. The bond is the attraction between the shared pair and both nuclei.

  Sharing is the process; the attraction is the thing that holds the atoms together.

• The gap rule: for every one electron needed to complete the outer shell, one covalent bond forms.
• Have a goAn atom is three electrons short of a full outer shell. How many covalent bonds will it form?

  Three.

  One covalent bond forms for every electron needed, so the size of the gap is the number of bonds.

• In H₂O, oxygen (Group 6, six outer electrons) forms two bonds, one per hydrogen. It ends with eight outer electrons, each hydrogen with two.
• Atoms can share more than one pair. Each atom in O₂ shares two (a double bond); in N₂, three (a triple bond), one reason nitrogen gas is so stable.
• Carbon is in Group 4 with four outer electrons, so it shares four pairs. In CH₄ that is one with each hydrogen; in CO₂, two with each oxygen.
• Have a goIn CO₂ each oxygen shares two pairs with the carbon atom. How many pairs is that for carbon altogether, and does it match carbon’s four?

  Four pairs (2 + 2), which matches carbon’s four.

  Both oxygen atoms share with the same carbon, so their pairs add up to the four carbon needs.

• A dot and cross diagram shows the electrons shared by each atom, so pairs can be counted. It does not explain how a bond holds atoms together, or show 3D shape.
• A displayed formula is a 2D model with a line for each shared pair, as in H–H and O=O. Models are useful, but none is perfect.

The big picture

A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of two non-metal atoms. Count how many electrons an atom is short of a full outer shell and you know how many bonds it forms.

⚠ Watch out

Saying the covalent bond is "the sharing" or a "stick" between the atoms. Sharing is how the bond forms. The bond is the strong electrostatic attraction between the shared pair and the nuclei of both atoms.

Memory hook

Count the gap, count the bonds. And remember: the bond is the pull, not the share.

Check yourself

Cover the page. An atom is two electrons short of a full outer shell: how many bonds does it make, and what is each bond?