Ionic vs Covalent Bonds: What's the Difference?

The key difference in ionic vs covalent bonds is what happens to the electrons. In an ionic bond, one atom (usually a metal) gives electrons to another (usually a non-metal), and the oppositely charged ions that result attract each other. In a covalent bond, two non-metal atoms share a pair of electrons. Sodium chloride is ionic; water is covalent.

Why do atoms bond at all?

Atoms bond to reach a more stable arrangement of electrons, usually a full outer shell like the noble gases have. That's eight outer electrons for most atoms (the octet rule), or two for hydrogen. An atom can get there in two ways: by losing or gaining electrons (ionic), or by sharing them (covalent). Which way it goes depends on how strongly each atom pulls on electrons.

What is an ionic bond?

An ionic bond is the electrostatic attraction between positive and negative ions, formed when electrons are transferred from one atom to another.

Take sodium and chlorine. Sodium (2, 8, 1) has one outer electron; chlorine (2, 8, 7) has seven. Sodium gives its electron to chlorine:

• Sodium becomes Na⁺, with a full shell of 2, 8.
• Chlorine becomes Cl⁻, with a full shell of 2, 8, 8.

The ions don't pair off one to one. Each Na⁺ is surrounded by six Cl⁻ and each Cl⁻ by six Na⁺, in a giant ionic lattice that goes on in every direction. That's why we write NaCl as a ratio, not a molecule.

Metals on the left of the periodic table lose electrons to become positive ions; non-metals on the right gain them to become negative ions. Magnesium oxide (Mg²⁺ O²⁻) and calcium chloride (Ca²⁺ with two Cl⁻) are other examples.

What is a covalent bond?

A covalent bond is a shared pair of electrons between two atoms, holding them together because both nuclei attract the same pair. It forms between non-metals, which all want to gain electrons, so neither can simply take them.

In a hydrogen molecule (H₂), each atom brings one electron and they share the pair. In water (H₂O), oxygen shares one pair with each hydrogen, giving oxygen eight outer electrons and each hydrogen two. Atoms can share more than one pair:

• Single bond: one shared pair, as in H–H or C–H in methane (CH₄).
• Double bond: two shared pairs, as in O=C=O (carbon dioxide).
• Triple bond: three shared pairs, as in N≡N, which is why nitrogen gas is so unreactive.

The number of bonds an atom usually makes is its valence: hydrogen 1, oxygen 2, nitrogen 3, carbon 4.

What is the difference between ionic and covalent bonds?

Here is the difference between ionic and covalent bonds at a glance:

	Ionic	Covalent (simple molecules)
Electrons	transferred	shared in pairs
Usually between	metal + non-metal	non-metal + non-metal
Particles	positive and negative ions	neutral molecules
Structure	giant lattice	small separate molecules
Melting point	high (NaCl: 801 °C)	low (water: 0 °C; CO₂ turns to gas at −78 °C)
Conducts electricity?	when molten or dissolved, not as a solid	no
Dissolves in water?	often	some do, many don't

Ionic compounds have high melting points because you have to overcome strong attractions between millions of ions in every direction. Simple covalent substances melt easily because the covalent bonds inside each molecule aren't broken at all: only the weak forces between molecules are.

Ionic compounds conduct when melted or dissolved because the ions are then free to move and carry charge. In the solid they are locked in place.

The exception: giant covalent structures

Some covalent substances form giant networks instead of small molecules. In diamond, every carbon is bonded to four others, so it doesn't melt at all at normal pressure: it turns to vapour at around 3,600 °C. Graphite and silicon dioxide (sand) are also giant covalent. Graphite is the rare non-metal that conducts electricity, because each carbon has one electron free to move along its layers.

How electronegativity decides the bond

Electronegativity is how strongly an atom pulls on the electrons in a bond. Fluorine is the most electronegative element (3.98 on the Pauling scale); caesium and francium are the least (about 0.8).

The difference in electronegativity between two atoms tells you what kind of bond they form. As a rule of thumb:

• Less than about 0.4: a non-polar covalent bond, shared equally (H–H, C–H).
• About 0.4 to 1.8: a polar covalent bond, shared unequally. In water, oxygen (3.44) pulls harder than hydrogen (2.20), so oxygen is slightly negative (δ−) and each hydrogen slightly positive (δ+).
• More than about 1.8: mostly ionic. Sodium (0.93) and chlorine (3.16) differ by 2.2.

So ionic and covalent aren't two separate boxes: they are two ends of a scale, with polar covalent bonds in between.

Common mistakes

• Calling NaCl a molecule. It's a lattice of ions. "Molecule" belongs to covalent substances.
• Thinking covalent bonds are weak. The bond inside a molecule is strong; it's the forces between molecules that are weak.
• Saying solid salt conducts. Only molten or dissolved ionic compounds do.
• Mixing up ionic bonding with metallic bonding. Metals bond to each other with a "sea" of shared electrons, which is why they conduct as solids.

Want to know where those outer electrons sit in the first place? Watch the shells fill in the Electron Lab, and see what happens when electrons jump between them in why fireworks are different colours.

Frequently asked questions

Is water ionic or covalent?

Water is covalent. Each water molecule is one oxygen atom sharing a pair of electrons with each of two hydrogen atoms. The bonds are polar, because oxygen pulls the shared electrons harder, so the oxygen end is slightly negative and the hydrogen ends slightly positive. That polarity is why water dissolves so many ionic compounds, like salt.

Which is stronger, an ionic or a covalent bond?

Individual bonds of both kinds are strong, and their strengths overlap. What really differs is the structure. An ionic solid is held together by strong attractions in every direction, so it has a high melting point. A simple covalent substance has strong bonds inside each molecule but only weak forces between molecules, so it melts easily. Giant covalent structures like diamond are the strongest of all.

How can you tell if a compound is ionic or covalent?

Look at the elements first. A metal combined with a non-metal, like sodium chloride or magnesium oxide, is ionic. Non-metals combined with each other, like carbon dioxide or methane, are covalent. To check, compare electronegativities: a difference above about 1.8 points to an ionic bond. Properties help too: high melting points and conducting when molten suggest ionic.

Is HCl ionic or covalent?

Hydrogen chloride is covalent: it's a gas made of H–Cl molecules sharing one pair of electrons. The bond is quite polar, though, and when HCl dissolves in water it splits into H⁺ and Cl⁻ ions, making hydrochloric acid. That's why the solution conducts electricity even though the gas doesn't.

Try it yourself

Open the Molecule Builder and drag atoms together to make H₂O, CH₄ and CO₂. Count the bonds on each atom and check them against the valences above. Notice that carbon dioxide only becomes stable once you give it two double bonds. Then go to the Chemistry Lab, add table salt to water and mix in silver nitrate: the white precipitate is silver ions and chloride ions finding each other and locking into a solid lattice.