In ionic bonding, one atom hands electrons over to another. Covalent bonding is different — nobody gives anything away. Two non-metal atoms share electrons, and that shared pair is what glues them together.
📚 What you need to know
Covalent bonding happens between two non-metals.
A covalent bond is the electrostatic attraction between a shared pair of electrons and the two nuclei.
No electrons are transferred — they are shared.
By sharing, both atoms end up with a full outer shell (8 electrons, or 2 for hydrogen).
One shared pair = one single bond, drawn as a line: H–H.
A few atoms are happy with less than 8 electrons — H, Be, B and Al.
Why atoms share instead of transfer
Think about what a non-metal atom is like. It is already close to a full outer shell and it holds onto its electrons tightly. So when two non-metals meet, neither one is willing to hand electrons over. Transferring is off the table.
The compromise is to share. Each atom puts one electron into the middle, and that pair now counts towards both atoms’ outer shells. Both get to feel full without either one losing anything.
Ionic bonding moves electrons from one atom to another. Covalent bonding keeps them in the middle, where both nuclei can pull on them.
The definition to learn
Examiners are fussy about this one. The bond is not “two atoms sharing electrons” — that describes what happens, not what the bond is. The bond is the attraction:
Definition — covalent bond
the electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms
Both nuclei are positive. The shared pair sitting between them is negative. Each nucleus pulls on that pair, and because they are both pulling on the same pair, the atoms are held together.
Picture two people holding opposite ends of the same rope. Neither owns the rope, but as long as they both keep pulling, they cannot walk away from each other. That is a covalent bond.
Each hydrogen atom brings one electron. The pair in the middle belongs to both atoms at once, so both now feel like they have a full first shell.
Bonds are clouds, not sticks
We draw a covalent bond as a neat straight line, and that is fine for exams. But the line is a shortcut. Electrons are always moving, so a better picture is a charge cloud — a fuzzy region where the shared pair is most likely to be found, densest in the space between the two nuclei.
The two atomic orbitals overlap to make one shared region. That build-up of negative charge between the nuclei is what holds the molecule together.
How many bonds does an atom make?
An atom keeps making bonds until its outer shell is full. Count how many electrons it is short by, and that is how many bonds it forms. This one table saves you a lot of guessing:
Atom
Outer electrons
Electrons needed
Bonds formed
Example
Hydrogen, H
1
1 (only needs 2 in total)
1
H–Cl
Halogens (F, Cl, Br, I)
7
1
1
Cl–Cl
Oxygen, O
6
2
2
H–O–H
Nitrogen, N
5
3
3
NH3
Carbon, C
4
4
4
CH4
WORKED EXAMPLE
Explain why a nitrogen atom forms three covalent bonds, and predict the formula of the compound it makes with hydrogen.
Nitrogen is in group 15, so it has 5 outer electronsA full outer shell needs 8.8 − 5 = 3 electrons shortSo it shares 3 electrons → 3 covalent bondsHydrogen makes 1 bond each, so 3 hydrogens are neededNH₃
The octet rule — and when it breaks
The octet rule is the tendency of atoms to end up with 8 electrons in their outer shell. It works for most of the molecules you will meet. But there are two ways it can be broken:
Electron deficient — the central atom ends up with fewer than 8 and is still perfectly stable. This happens with Be (4 electrons) and B or Al (6 electrons). Hydrogen is a special case: it is full with just 2, called a duet.
Expanded octet — the central atom fits more than 8, which is possible for atoms in period 3 and below, such as sulfur in SO2.
Don’t panic when you see these. An “incomplete octet” does not mean you made a mistake. BF3 genuinely has only 6 electrons around the boron and it is a real, stable molecule. You will draw both of these in the next set of notes.
💡 Exam tip
If a question says “define”, give the full sentence: electrostatic attraction between a shared pair and the two nuclei.
Covalent = two non-metals. If you see a metal in the formula, think ionic instead.
Hydrogen only ever needs 2 electrons, never 8. Do not draw lone pairs on a hydrogen.
⚠️ Common mix-up
Sharing is not transferring. In a covalent bond no ions are formed, so there are no charges to write.
The bond is the attraction, not the electrons. The pair of electrons is what is shared; the bond is the pull between that pair and the nuclei.
Do not say the atoms “want” a full shell. Say sharing makes the arrangement more stable (lower in energy).
Up next: Lewis (Electron Dot) Formulas — a step-by-step method for drawing every electron in a molecule, so you never miss a lone pair again.
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