Making sense of chemical bonding for SPM

Chemical bonding is where a lot of SPM Chemistry finally starts to make sense, or falls apart, depending on whether one core idea has landed. Students often learn ionic and covalent bonding as two separate lists of facts to memorise. They are actually two answers to a single question, and once you see the question, the whole chapter becomes logical instead of fiddly. This explainer builds bonding up from that one idea.

Why atoms bond at all

Everything in bonding follows from one goal: atoms bond to achieve a stable, full outer electron shell, the octet (eight valence electrons), or a duplet (two) for the smallest atoms. Noble gases already have full outer shells, which is why they sit unreacting on the far side of the table. Every other atom is, in a sense, trying to reach that same stable arrangement, and bonding is how it gets there.

There are only two ways to complete an outer shell: give or take electrons, or share them. Those two routes are ionic bonding and covalent bonding. That is the whole fork in the road, and it maps neatly onto the Periodic Table: metals (which lose electrons easily) meeting non-metals (which gain them) tend toward transfer; non-metals meeting non-metals tend toward sharing.

Ionic bonding: transferring electrons

When a metal meets a non-metal, electrons are transferred from the metal to the non-metal. The metal atom loses its valence electrons to become a positive ion (a cation); the non-metal gains them to become a negative ion (an anion). The oppositely charged ions then attract each other strongly, and that electrostatic attraction is the ionic bond.

Take sodium chloride. Sodium (2.8.1) has one valence electron; chlorine (2.8.7) needs one. Sodium transfers its single electron to chlorine. Sodium becomes Na⁺ (now 2.8, a full outer shell) and chlorine becomes Cl⁻ (now 2.8.8). The two ions attract and you have NaCl.

Magnesium oxide works the same way but with two electrons: magnesium (2.8.2) transfers both valence electrons to oxygen (2.6, which needs two). You get Mg²⁺ and O²⁻. Counting how many electrons must move is the key skill, it also predicts the formula, because the charges must balance.

Covalent bonding: sharing electrons

When two non-metals meet, neither wants to give up electrons, so they share instead. Each shared pair of electrons is a covalent bond, and by sharing, both atoms count the shared electrons toward a full outer shell.

• Hydrogen (H₂): each hydrogen atom has one electron and needs two; they share one pair, giving each a full duplet, a single covalent bond.
• Water (H₂O): oxygen (2.6) needs two more electrons and shares a pair with each of two hydrogen atoms, forming two covalent bonds.
• Methane (CH₄): carbon (2.4) needs four and shares a pair with each of four hydrogens.

Dot-and-cross diagrams simply draw this sharing, using dots for one atom’s electrons and crosses for the other so you can see the shared pairs. Getting the outer-shell counts right before you draw is what makes them correct.

The real payoff: bonding decides properties

The reason bonding matters so much is that it explains why substances behave as they do, a favourite of SPM Paper 2. Because ionic compounds are giant lattices of strongly attracting ions, they have high melting and boiling points, and they conduct electricity when molten or dissolved (the ions are then free to move) but not when solid. Simple covalent (molecular) substances are held to each other by weak forces between molecules, so they have low melting and boiling points and generally do not conduct electricity, because there are no free-moving charged particles. If you can reason from structure to property like this, you can answer a whole family of questions without memorising each one.

Common mistakes to avoid

• Saying atoms “want” a full shell as if it were a fact rather than the reason, examiners reward the explanation, not just the label. Tie behaviour back to the octet.
• Getting the direction of transfer wrong. Metals lose electrons; non-metals gain them. Never the reverse.
• Miscounting shared pairs in dot-and-cross diagrams. Fix each atom’s valence-electron count first, then draw.
• Confusing “molten/dissolved ionic conducts” with “ionic always conducts.” Solid ionic compounds do not conduct; the ions are locked in place.

Studying bonding well

Bonding sits at the base of huge parts of Chemistry, so time spent here pays off everywhere. Work through the chemical bond chapter until you can decide ionic-versus-covalent from the elements involved, draw the dot-and-cross diagram, and then explain the resulting properties. Keep the vocabulary tight using a chemistry glossary, ion, cation, anion, covalent bond, lattice, because marks here are often about precise wording. If dot-and-cross diagrams or the transfer-versus-share decision keep tripping you up, this is a topic that a focused one-to-one session can straighten out quickly, because the whole chapter really does hang on that one idea of the full outer shell.