Difference Between Ionic, Covalent and Metallic Bonding

Quick answer: Ionic bonding transfers electrons from a metal to a non-metal, making oppositely charged ions held by strong electrostatic attraction. Covalent bonding is a shared pair of electrons between two non-metals. Metallic bonding is a lattice of positive metal ions in a sea of delocalised electrons. Structure, not the bond name, explains the properties.

I am doing IGCSE and A-Level right now, and bonding is where I learned that a memorised definition is worth almost nothing. I could recite "electrostatic attraction between oppositely charged ions" perfectly and still lose three marks on "explain why magnesium oxide has a higher melting point than sodium chloride". The marks sit in the explanation chain, not the label, so this article is mostly the chain.

Work out the bond type from the periodic table first

The fastest reliable test is where the elements sit in the periodic table. Metal plus non-metal gives ionic. Non-metal plus non-metal gives covalent. A metal on its own, or a mixture of metals in an alloy, gives metallic.

That one rule covers nearly every substance you get asked about at Cambridge IGCSE, GCSE, CBSE Class 10 and WAEC level. Sodium chloride: metal plus non-metal, ionic. Carbon dioxide: two non-metals, covalent. Copper: metal, metallic.

Two exceptions matter on Cambridge International A-Level or AP Chemistry. Some compounds mix bond types: ammonium chloride is covalently bonded inside the ammonium ion, and that whole ion is then attracted ionically to a chloride ion. And bonding is a spectrum, not three boxes. At A-Level you use electronegativity difference: above about 1.8 behaves ionic, below 0.4 is non-polar covalent, in between is polar covalent.

Ionic bonding: magnesium oxide, worked step by step

Ionic bonding works because both atoms end up with a full outer shell once electrons move across. Here is the method I write out every time, using magnesium oxide.

1. Write the electron arrangements. Magnesium is 2,8,2. Oxygen is 2,6.
2. Decide the direction. Magnesium is the metal, so it loses electrons. It loses 2 and becomes Mg2+ with arrangement 2,8.
3. Do the non-metal. Oxygen needs 2 more, so it gains those same 2 electrons and becomes O2- with arrangement 2,8.
4. Balance the charges. One 2+ and one 2- cancel, so the formula is MgO.
5. Describe the structure. Not a molecule: a giant ionic lattice, with each ion surrounded by ions of opposite charge in a regular repeating pattern.

Change one detail and the formula changes. In magnesium chloride, magnesium still loses 2 electrons but each chlorine (2,8,7) needs only 1, so you need two chlorides per magnesium: MgCl2.

That charge number answers the melting point question I kept getting wrong. Sodium chloride melts at 801 degrees C; magnesium oxide melts at 2852 degrees C. Both are giant ionic lattices, so structure is not the difference. Magnesium oxide has 2+ and 2- ions instead of 1+ and 1-, so the electrostatic attraction is much stronger and far more energy is needed to overcome it. The chemistry hub groups more of these by structure type.

Covalent bonding: simple molecular versus giant covalent

A covalent bond is one shared pair of electrons, held in place because both nuclei attract that pair. Hydrogen (H2) has one shared pair, oxygen (O2) has two (a double bond), nitrogen (N2) has three. Water has two bonding pairs plus two lone pairs on the oxygen.

Here is the split that earns marks: covalent substances come in two very different structures.

Simple molecular substances are small separate molecules. Iodine melts at 114 degrees C, carbon dioxide sublimes near minus 78 degrees C. The covalent bonds inside those molecules are strong, but the intermolecular forces between molecules are weak, and melting only overcomes those weak forces.

Giant covalent structures are one continuous network of covalent bonds. Diamond stays solid above 3550 degrees C, silicon dioxide melts near 1710 degrees C. To melt them you must break actual covalent bonds throughout the lattice.

Diamond and graphite are both pure carbon and both giant covalent, so any difference must come from how the atoms are joined. In diamond each carbon forms 4 bonds, so no electrons are free and it does not conduct. In graphite each carbon forms 3, leaving one delocalised electron per atom to move along the layers, so it conducts. Those layers slide, which is why graphite is soft.

Metallic bonding and why metals behave the way they do

Metallic bonding is the attraction between a lattice of positive metal ions and the delocalised electrons flowing between them. The outer electrons are not owned by any one atom.

That single picture explains two properties at once. Metals conduct as solids and as liquids, because the electrons move either way. That is the cleanest contrast with ionic compounds, which conduct only when molten or dissolved, when the ions are free to move. Metals are malleable because layers of ions slide and the bonding still works in the new position.

Strength varies with how much charge is involved. Sodium gives one delocalised electron per atom and melts at 98 degrees C. Magnesium gives two and melts at 650 degrees C. Aluminium gives three and melts at 660 degrees C.

The four-line method for any "explain the property" question

Every property question on these syllabuses wants the same four moves, in order.

1. Name the structure and the bonding.
2. Name the exact force being overcome, or the exact particle that moves.
3. Say whether that force is strong or weak, and why.
4. Link it back to the property asked about.

Worked example. "Explain why sodium chloride has a high melting point but only conducts electricity when molten or dissolved."

Sodium chloride is a giant ionic lattice. To melt it you must overcome the strong electrostatic attraction between the oppositely charged sodium ions and chloride ions throughout the lattice, which needs a lot of energy, so the melting point is high. In the solid the ions are held in fixed positions and cannot move, so there is no conduction. When molten or dissolved the ions are free to move and carry charge, so it conducts.

Four sentences, every mark point hit. Practise the shape rather than the facts and you can handle substances you have never met. Paste a question into step-by-step explanations and check your answer has all four moves.

The mistakes that cost me marks

• Writing that "the covalent bonds break" when a simple molecular substance melts. They do not. Say the weak intermolecular forces between molecules are overcome. Mark schemes reject the first version outright.
• Leaving out the word electrostatic. Examiners want "strong electrostatic attraction between oppositely charged ions", not just "strong attraction".
• Saying metals conduct because "the ions are free to move". In metals it is the delocalised electrons. Moving ions is the ionic explanation.
• Calling NaCl a molecule. There is no such thing as a molecule of sodium chloride, only a lattice.
• Forgetting square brackets and charges on ions in dot-and-cross diagrams, or drawing inner shells when the question said outer shell only.

Test yourself

1. Magnesium oxide and sodium chloride are both giant ionic lattices. Explain in one sentence why magnesium oxide melts at a much higher temperature.
2. Iodine and diamond are both covalent. Why does iodine melt at 114 degrees C while diamond stays solid above 3000 degrees C?
3. Explain why aluminium has a higher melting point than sodium, referring to delocalised electrons.

Check your wording with practice questions, or turn the four-line method into flashcards until the phrasing is automatic.

FAQ

Is a covalent bond stronger than an ionic bond?

Individually they are comparable, and the comparison is usually the wrong one to make. What matters in exam questions is structure. A single covalent bond is strong, but simple molecular substances still melt easily, because melting only breaks the weak forces between molecules.

Why does graphite conduct electricity but diamond does not?

Each carbon in graphite forms three covalent bonds instead of four, so one electron per atom is delocalised and free to move through the layers. In diamond every outer electron is locked into a covalent bond, so nothing carries charge.

Do I need electronegativity at IGCSE and GCSE level?

No. At Cambridge IGCSE, GCSE with AQA, Edexcel or OCR, CBSE Class 10 and WAEC, the metal plus non-metal rule is enough. Electronegativity and polar bonds arrive at Cambridge International A-Level and AP Chemistry.

What is the difference between an ionic bond and an ionic lattice?

The bond is the attraction between one pair of oppositely charged ions. The lattice is the whole giant three-dimensional arrangement of millions of those ions. Property questions almost always want the lattice.

In short: get the bond type from the periodic table, then name the structure, because the properties come from the structure. Ionic means a giant lattice of ions, covalent means small molecules or a giant network, metallic means positive ions in delocalised electrons. Then answer in four lines: structure, force or particle, strength, property.