Ionic vs covalent bonds: how to tell them apart

Key ideas

Electronegativity (EN) is how strongly an atom pulls on shared electrons. The difference between two atoms tells you how evenly the electrons are shared:

\Delta \text{EN} = \lvert \text{EN}_A - \text{EN}_B \rvert

Bond type from the electronegativity difference (OpenStax guideline)
ΔEN	Bond type	What happens to the electrons	Example
less than 0.4	Nonpolar covalent	Shared about equally	C-H (0.35), Cl-Cl (0)
0.4 to 1.8	Polar covalent	Shared, but pulled toward the more electronegative atom	O-H (1.24), H-Cl (0.96)
more than 1.8	Ionic	Transferred, making ions	Na-Cl (2.23), K-F (3.16)

These cutoffs are a guide, not a law. Bonding is a range from equal sharing to full transfer, and some textbooks use 1.7 or 2.0. The metal plus nonmetal rule usually gives the same answer.

Typical properties
Property	Ionic compounds	Covalent (molecular) compounds
Made of	Metal + nonmetal	Nonmetals
Particles	Ions in a repeating crystal lattice	Separate molecules
Melting point	High (NaCl: 801 °C)	Usually low (water: 0 °C)
State at room temperature	Solid	Gas, liquid or soft solid
Conducts electricity	When melted or dissolved in water	Usually not
Example	NaCl, MgO, $\mathrm{CaF_2}$	$\mathrm{H_2O}$ , $\mathrm{CO_2}$ , $\mathrm{CH_4}$

Worked examples

Example 1: sodium chloride

Problem Classify the bond in NaCl.

Find the difference.
$\Delta \text{EN} = 3.16 - 0.93 = 2.23$
2.23 is more than 1.8. Also, Na is a metal and Cl is a nonmetal.
Sodium gives its one outer electron to chlorine, making $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ .

Answer Ionic (ΔEN = 2.23)

Example 2: hydrogen chloride

Problem Classify the bond in HCl and say which end is partly negative.

Find the difference.
$\Delta \text{EN} = 3.16 - 2.20 = 0.96$
0.96 is between 0.4 and 1.8, so the electrons are shared unequally.
Chlorine has the higher EN, so it pulls the shared pair closer and gets a partial negative charge, $\delta^-$ . Hydrogen gets $\delta^+$ .

Answer Polar covalent (ΔEN = 0.96), with Cl partly negative

Example 3: a carbon to hydrogen bond

Problem Classify the C-H bond in methane, $\mathrm{CH_4}$ .

Find the difference.
$\Delta \text{EN} = 2.55 - 2.20 = 0.35$
0.35 is less than 0.4, so the electrons are shared almost equally.

Answer Nonpolar covalent (ΔEN = 0.35)

Example 4: write an ionic formula

Problem Write the formula of the compound formed by aluminum ions, $\mathrm{Al^{3+}}$ , and oxide ions, $\mathrm{O^{2-}}$ .

The total charge must be zero. The smallest number both 3 and 2 go into is 6.
Two $\mathrm{Al^{3+}}$ give +6. Three $\mathrm{O^{2-}}$ give $-6$ .
$2(+3) + 3(-2) = 0$
Write the metal first, with the counts as subscripts. This is the same as the crisscross shortcut.

Answer $\mathrm{Al_2O_3}$

Common mistakes and how to fix them

Calling every bond with hydrogen ionic. Hydrogen is a nonmetal. H-Cl, H-O and H-F are polar covalent. Fix: use ΔEN, not the position of H at the top of group 1.
Forgetting the absolute value. ΔEN is never negative. Fix: subtract the smaller from the larger.
Saying ionic compounds are molecules. NaCl is a lattice of ions, so chemists call the smallest unit a formula unit, not a molecule.
Not reducing ionic formulas. $\mathrm{Mg^{2+}}$ and $\mathrm{O^{2-}}$ make MgO, not $\mathrm{Mg_2O_2}$ . Fix: use the smallest whole-number ratio.

Practice questions

Which compound has ionic bonds?
1. KBr
2. $\mathrm{CO_2}$
3. $\mathrm{CH_4}$
4. $\mathrm{N_2}$
Show answer
Answer: KBr
K is a metal and Br is a nonmetal. $\Delta\text{EN} = 2.96 - 0.82 = 2.14$ , more than 1.8. The others are all nonmetals sharing electrons.
What type of bond is the O-H bond in water?
1. Nonpolar covalent
2. Polar covalent
3. Ionic
Show answer
Answer: Polar covalent
$\Delta\text{EN} = 3.44 - 2.20 = 1.24$ , between 0.4 and 1.8. Oxygen pulls the shared electrons closer and gets $\delta^-$ .
Write the formula for the ionic compound of calcium, $\mathrm{Ca^{2+}}$ , and nitride, $\mathrm{N^{3-}}$ .
Show answer
Answer: $\mathrm{Ca_3N_2}$
The smallest number both 2 and 3 go into is 6. Three $\mathrm{Ca^{2+}}$ give +6 and two $\mathrm{N^{3-}}$ give $-6$ .
Which of these bonds is the most polar?
1. C-H
2. N-H
3. O-H
4. F-H
Show answer
Answer: F-H
Polarity grows with ΔEN. C-H is 0.35, N-H is 0.84, O-H is 1.24 and F-H is 1.78.
A white solid melts at 770 °C and conducts electricity when dissolved in water. Which kind of compound is it most likely?
1. Ionic
2. Covalent (molecular)
Show answer
Answer: Ionic
A high melting point and conducting in water are signs of ions in a lattice. Dissolved ions are free to move and carry charge.

Frequently asked questions

Is there a bond that is halfway between ionic and covalent?

Yes. Polar covalent bonds are in the middle: the electrons are shared, but one atom pulls harder. Bonding is a range from perfectly equal sharing (like Cl-Cl) to nearly complete transfer (like K-F), not two separate boxes.

Why do ionic compounds conduct electricity only when melted or dissolved?

Electricity needs moving charged particles. In a solid crystal, the ions are locked in place. When the solid melts or dissolves in water, the ions can move, so the liquid or solution conducts.

What is a metallic bond?

In a metal, atoms share a sea of electrons that move freely through the whole piece. That is why metals conduct electricity as solids and can be bent and hammered. High school courses often teach it as a third bond type next to ionic and covalent.

Sources

OpenStax Chemistry 2e, 7.1 Ionic Bonding, accessed October 1, 2026
OpenStax Chemistry 2e, 7.2 Covalent Bonding, accessed October 1, 2026
PubChem Periodic Table (Pauling electronegativity), accessed October 1, 2026

Key ideas

Worked examples

Common mistakes and how to fix them

Frequently asked questions

Sources

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