A covalent bond forms when two non-metal atoms share one or more pairs of electrons so that each atom achieves a stable noble-gas arrangement. One shared pair is a single bond, two shared pairs a double bond, and three shared pairs a triple bond.
This page covers a single Form 4 content standard: the covalent bond. It is the second route to stability in the Chemical Bond chapter. Where the ionic bond transfers electrons, the covalent bond shares them. You need to be able to define the covalent bond exactly, work out how many electron pairs are shared, and draw the sharing in a dot-and-cross diagram, the skills examiners test again and again.
What a covalent bond is
A covalent bond is formed when two atoms share a pair (or pairs) of electrons. Each shared pair is counted by both atoms towards a full outer shell, so both atoms reach a stable octet (or duplet for hydrogen). Covalent bonding happens between non-metal atoms, because non-metals have many valence electrons and it is easier to share than to gain or lose several electrons.
The number of shared pairs gives the bond its name:
- Single covalent bond, one shared pair of electrons (for example H–H in H₂).
- Double covalent bond, two shared pairs (for example O=O in O₂).
- Triple covalent bond, three shared pairs (for example N≡N in N₂).
How the sharing works
Take hydrogen, H₂. Each hydrogen atom has 1 electron and needs 2 (a duplet, like helium). The two atoms each contribute one electron to a single shared pair, which both atoms now count, so each hydrogen has a stable duplet. In a dot-and-cross diagram you draw the shared pair in the overlap between the two atoms, using a dot for one atom’s electron and a cross for the other’s, so the sharing is visible.
For larger molecules the same idea repeats. In methane, CH₄, carbon (2.4) has 4 valence electrons and shares one pair with each of four hydrogen atoms, forming four single bonds; carbon reaches an octet and each hydrogen a duplet. In carbon dioxide, CO₂, carbon shares two pairs with each oxygen, forming two double bonds (O=C=O).
Worked example
Question. Water is formed from oxygen (electron arrangement 2.6) and hydrogen (arrangement 1). Explain how the covalent bonds form, state the type and number of bonds, and give the molecular formula.
Step 1, Count what each atom needs. Oxygen has 6 valence electrons and needs 2 more for an octet. Each hydrogen has 1 electron and needs 1 more for a duplet.
Step 2, Match the sharing. Because each hydrogen can share only one electron, oxygen must share with two hydrogen atoms. Oxygen contributes one electron to each of two shared pairs, and each hydrogen contributes one electron to its pair.
Step 3, Check the outer shells. Through the two shared pairs, oxygen now counts 8 outer electrons (a stable octet) and each hydrogen counts 2 (a stable duplet).
Step 4, Name the bonds and formula. There are two single covalent bonds (H–O–H). One oxygen atom is bonded to two hydrogen atoms, so the molecular formula is H₂O.
Answer. Oxygen shares one electron pair with each of two hydrogen atoms, forming two single covalent bonds. Oxygen reaches an octet and each hydrogen a duplet, giving the molecule H₂O.
Practice question
Question. Carbon (electron arrangement 2.4) and oxygen (2.6) form carbon dioxide. Explain how the covalent bonds form, state the type of bonds, and give the molecular formula.
Answer. Carbon has 4 valence electrons and needs 4 more for an octet; each oxygen has 6 and needs 2 more. Carbon shares two electron pairs with each oxygen atom, forming two double covalent bonds (O=C=O). Through this sharing carbon reaches an octet and each oxygen reaches an octet. One carbon atom bonds to two oxygen atoms, so the molecular formula is CO₂.
Exam tip
Learn the definition precisely: a covalent bond is the sharing of a pair (or pairs) of electrons between atoms, the word is sharing, not transfer, and it happens between non-metals. When you draw a dot-and-cross diagram, show only the outer electrons, put the shared pair(s) in the overlap region, and use dots for one atom and crosses for the other so the sharing is obvious. Count carefully: one shared pair is a single bond, two is a double bond, three is a triple bond. A very common slip is to draw an ionic-style diagram (electrons transferred, ions in brackets) for a covalent molecule, for a covalent bond there are no ions and no charges.
Where this fits
The covalent bond sits alongside the ionic bond as the two core bond types of the Chemical Bond chapter, and both exist for the same reason, to give each atom a stable noble-gas arrangement. Understanding covalent sharing prepares you for the dative bond (a special covalent bond) and for the properties of covalent compounds later in the chapter. Practise the dot-and-cross diagrams with the worked examples and test yourself on molecules such as NH₃, CH₄ and N₂ with the practice questions. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you never confuse sharing with transfer and that your diagrams place the shared pairs correctly, which is exactly what earns full marks on this standard across SPM Chemistry.
Molecules and covalent structures
Most covalent substances you meet at this level are simple molecules, small groups of covalently bonded atoms such as H₂O, CO₂, NH₃ and CH₄. Inside each molecule the covalent bonds are strong, but the forces of attraction between separate molecules are weak. That difference matters: it explains why simple covalent substances usually have low melting and boiling points, an idea you will reuse in the properties standard. Note too that sharing happens only between non-metals, because both atoms have a tendency to gain electrons and neither gives its electrons away completely as in ionic bonding. When you answer, always state the number of shared pairs and the outer arrangement each atom reaches, that detail separates a full-mark answer from a half one.
Quick recap
- A covalent bond is the sharing of a pair (or pairs) of electrons between non-metal atoms.
- Each shared pair helps both atoms reach a stable octet (or duplet for hydrogen).
- One shared pair = single bond; two = double bond; three = triple bond.
- In a dot-and-cross diagram, put the shared pair in the overlap; there are no ions or charges.
- Examples: H₂ and CH₄ (single), O₂ and CO₂ (double), N₂ (triple).
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