TL;DR:
In an ionic bond, one atom transfers electrons to another, and the oppositely charged ions attract. In a covalent bond, atoms share electrons.
The electronegativity difference is a guide: Na and Cl differ by 2.23 (ionic), O and H by 1.24 (polar covalent).
Bond type explains properties such as melting point and whether a substance conducts electricity.
Table salt and water both look simple, yet they behave very differently. Salt is a hard crystal that melts only at a very high temperature. Water is a liquid at room temperature. The reason lies in how their atoms are held together.
Two ways to reach a full outer shell
Atoms tend to be stable with a full outer shell of electrons, most often eight. They get there by transferring or sharing electrons.
In an ionic bond, one atom gives one or more electrons to another. The giver becomes a positive ion and the receiver a negative ion, and the opposite charges attract. In a covalent bond, two atoms share one or more pairs of electrons, and the shared pair is attracted to both nuclei.
Worked example: sodium chloride versus water
Sodium chloride: transfer
Sodium (Na) has 1 electron in its outer shell. Losing it leaves a full shell underneath, giving
Na⁺.Chlorine (Cl) has 7 outer electrons. Gaining 1 makes 8, giving
Cl⁻.Check the charges:
(+1) + (−1) = 0, so one Na⁺ pairs with one Cl⁻ and the formula is NaCl.The ions pack into a repeating 3D lattice, each Na⁺ surrounded by Cl⁻ ions and the reverse.
There are no separate NaCl molecules in the crystal. The formula gives the ratio of ions, 1 to 1.
Water: sharing
Oxygen has 6 outer electrons and needs 2 more. Each hydrogen has 1 and needs 1 more.
Oxygen shares one pair with each of two hydrogens. Count for oxygen:
6 + 2 = 8. For each hydrogen:1 + 1 = 2, a full first shell.The result is a separate H₂O molecule with two O-H bonds.
Electronegativity: a way to predict the type
Electronegativity measures how strongly an atom pulls shared electrons toward itself. On the Pauling scale, Na is 0.93, Cl is 3.16, O is 3.44, H is 2.20 and C is 2.55. The difference between two atoms hints at the bond type.
Pair | Difference | Likely bond |
|---|---|---|
Na and Cl |
| Ionic |
O and H |
| Polar covalent |
C and H |
| Nearly nonpolar covalent |
A common rule of thumb says a difference above about 1.7 is ionic, between about 0.4 and 1.7 is polar covalent, and below about 0.4 is nonpolar covalent. The cut-offs are rough, and real bonds form a continuum from fully shared to fully transferred.
In a polar covalent bond like O-H, the shared pair sits closer to the oxygen. The oxygen end becomes slightly negative and the hydrogen end slightly positive. This is why water molecules attract each other so strongly for their size.
How bonding explains properties
Melting point. Melting NaCl means pulling apart a lattice held by strong attractions between ions: it melts at 801 °C. Water molecules are held by much weaker attractions between molecules, so ice melts at 0 °C.
Conductivity. Solid NaCl does not conduct, because its ions are locked in place. Melted or dissolved NaCl conducts, because the ions can move. Pure water conducts very poorly, though dissolved salts make it conduct.
Brittleness. Hit an ionic crystal and layers can slide so that like charges line up and repel, so it cracks.
Dissolving. Water's partial charges pull ions out of a salt crystal, which is why salt dissolves in it.
Keep chemicals out of your mouth: In a lab, never taste or smell chemicals to find out what they are. White crystals in a lab jar can look just like table salt and still be unsafe, so only food from the kitchen should be eaten.
A third kind of bond is worth a mention. In a metal, the outer electrons are shared across the whole lattice, free to move between the atoms. That is why metals conduct electricity when solid, which ionic crystals cannot.
How to check your answer
Charge balance. An ionic formula must have total charge zero.
Octets. Count outer electrons around each atom in a covalent molecule.
Metal and nonmetal. A metal with a nonmetal is usually ionic, and two nonmetals are usually covalent.
Property test. If it melts below 100 °C, it is very likely molecular, not an ionic lattice.
Mistakes that cost marks
Describing NaCl as molecules.
Saying the difference gives a sharp cut between ionic and covalent.
Writing the charges wrongly, such as Cl⁺.
Thinking the electron is lost for no reason. The attraction between the final ions releases energy.
Mixing up bonds inside a molecule (strong) with attractions between molecules (weaker).
Where you will use it next
Covalent molecules are drawn as dot diagrams in drawing Lewis structures step by step. Reaction speed and why particles collide is in what speeds up a chemical reaction.
Frequently asked questions
Is a polar covalent bond halfway between ionic and covalent?
In a sense. The electrons are shared unequally, so the bond has some ionic character, but they still belong to both atoms.
Why does salt dissolve in water but not in oil?
Water's polar molecules can surround and stabilise the ions. Oil molecules are nonpolar, so they cannot.
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