Almost every atom in Chemistry wants the same thing: a full outer shell of electrons, like the nearest noble gas. Bonding is simply how atoms get there. There are two main routes, transferring electrons (ionic bonding) and sharing electrons (covalent bonding), and once you can explain both with a clear worked example, a large slice of the chemical bond chapter becomes straightforward. This guide takes each in turn.
The one idea behind all bonding
Atoms bond to achieve a stable electron arrangement, meaning a full outermost shell: two electrons for the smallest atoms (the duplet, like helium) and eight for most others (the octet). A metal atom has just a few outer electrons and finds it easier to lose them; a non-metal atom is only a few short of eight and finds it easier to gain or share. Which route an atom takes is decided by that difference, and it produces the two bond types.
Ionic bonding, transferring electrons
An ionic bond forms when a metal atom transfers one or more electrons to a non-metal atom. The metal becomes a positively charged ion (cation) and the non-metal becomes a negatively charged ion (anion), and the strong electrostatic force of attraction between these oppositely charged ions is the ionic bond. You can revise the definition at our ionic bond glossary entry.
Worked example, sodium chloride, NaCl. A sodium atom has the electron arrangement 2.8.1; a chlorine atom has 2.8.7. Sodium loses its single outer electron to become Na⁺ with the stable arrangement 2.8. Chlorine gains that electron to become Cl⁻ with 2.8.8. One atom gives exactly what the other needs, so the ratio is 1 : 1 and the formula is NaCl.
Worked example, magnesium oxide, MgO. Magnesium (2.8.2) loses two electrons to form Mg²⁺ (2.8); oxygen (2.6) gains two to form O²⁻ (2.8). Again the numbers match one-to-one, giving MgO.
When the numbers do not match, magnesium chloride, MgCl₂. Magnesium must lose two electrons, but each chlorine can accept only one. So two chlorine atoms are needed for one magnesium, giving Mg²⁺ and two Cl⁻, and the formula MgCl₂. This is the moment students most often trip, so count the electrons carefully. You can see the full electron-transfer picture on our formation of an ionic compound page.
Covalent bonding, sharing electrons
A covalent bond forms when two non-metal atoms share a pair of electrons, so that both atoms count the shared pair towards a full outer shell. Neither atom can afford to simply give electrons away, so they share instead. Each shared pair is one covalent bond; two shared pairs make a double bond. The definition is at covalent bond.
Worked example, a chlorine molecule, Cl₂. Each chlorine atom (2.8.7) needs one more electron. The two atoms share one pair of electrons, one from each atom, forming a single covalent bond. Both now count eight outer electrons, so both are stable, and we write Cl–Cl.
Worked example, water, H₂O. Oxygen (2.6) needs two more electrons; each hydrogen (1) needs one more. Oxygen shares one pair with each of two hydrogen atoms, forming two single bonds. Oxygen reaches the octet, each hydrogen reaches the duplet, and the molecule is H₂O with two lone pairs left on the oxygen.
Worked example, carbon dioxide, CO₂. Carbon (2.4) needs four more electrons; each oxygen (2.6) needs two. Carbon shares two pairs with each oxygen, forming two double bonds, written O=C=O. Everyone reaches eight.
How to structure the explanation in the exam
When a question says “explain how the bond in X is formed,” a clean answer names the atoms and their electron arrangements, states whether electrons are transferred or shared, gives the ions or shared pairs formed, and notes that each atom reaches a stable octet or duplet. For ionic compounds, add the phrase “strong electrostatic force of attraction between oppositely charged ions.” For covalent compounds, say “a shared pair of electrons.” Dot-and-cross diagrams score well here; use crosses for one atom’s electrons and dots for the other so the transfer or sharing is visible.
The common confusions
Two mistakes cost the most marks. First, mixing the language: ionic bonds involve transfer and ions, covalent bonds involve sharing and molecules, never say a covalent bond “transfers” electrons. Second, forgetting to balance the electrons, as in MgCl₂, where one magnesium needs two chlorines. Keep the electron count central and both bond types stay clear.
Bonding is the backbone of Form 4, and it explains the properties you meet later, so it rewards careful study now. You can revise the whole topic on our chemical bond chapter page. If you would like a teacher to check your dot-and-cross diagrams and exam wording, our online one-to-one lessons with our experienced SPM Chemistry teachers give that feedback, from RM50 an hour with a paid one-hour trial.
Ready for one-to-one help?
An experienced teacher can help your child put this into practice.
from RM50/hr · One-hour paid trial · Same-day reply