# Ionic vs covalent bonds

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Updated: 2026-10-01

In an ionic bond, one atom gives electrons to another, making a positive ion and a negative ion that attract each other. It usually forms between a metal and a nonmetal, like NaCl. In a covalent bond, two atoms share electrons, usually two nonmetals, like the O-H bonds in water. A large electronegativity difference (more than about 1.8) points to ionic, and a smaller one points to covalent.

## 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, \(\mathrm{CaF_2}\) | \(\mathrm{H_2O}\), \(\mathrm{CO_2}\), \(\mathrm{CH_4}\) |

> **Note: Electronegativity values used on this page (Pauling scale)**
>
> H 2.20, C 2.55, N 3.04, O 3.44, F 3.98, Na 0.93, Mg 1.31, Cl 3.16, K 0.82, Ca 1.00, Br 2.96.

## Worked examples

**Example 1: sodium chloride**

Problem: Classify the bond in NaCl.

1. Find the difference.

   $$
   \Delta \text{EN} = 3.16 - 0.93 = 2.23
   $$
2. 2.23 is more than 1.8. Also, Na is a metal and Cl is a nonmetal.
3. 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.

1. Find the difference.

   $$
   \Delta \text{EN} = 3.16 - 2.20 = 0.96
   $$
2. 0.96 is between 0.4 and 1.8, so the electrons are shared unequally.
3. 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, \(\mathrm{CH_4}\).

1. Find the difference.

   $$
   \Delta \text{EN} = 2.55 - 2.20 = 0.35
   $$
2. 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, \(\mathrm{O^{2-}}\).

1. The total charge must be zero. The smallest number both 3 and 2 go into is 6.
2. Two \(\mathrm{Al^{3+}}\) give +6. Three \(\mathrm{O^{2-}}\) give \(-6\).

   $$
   2(+3) + 3(-2) = 0
   $$
3. 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**

1. Which compound has ionic bonds?
   A. KBr
   B. \(\mathrm{CO_2}\)
   C. \(\mathrm{CH_4}\)
   D. \(\mathrm{N_2}\)

   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.

2. What type of bond is the O-H bond in water?
   A. Nonpolar covalent
   B. Polar covalent
   C. Ionic

   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 \(\delta^-\).

3. Write the formula for the ionic compound of calcium, \(\mathrm{Ca^{2+}}\), and nitride, \(\mathrm{N^{3-}}\).

   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 \(-6\).

4. Which of these bonds is the most polar?
   A. C-H
   B. N-H
   C. O-H
   D. F-H

   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.

5. A white solid melts at 770 °C and conducts electricity when dissolved in water. Which kind of compound is it most likely?
   A. Ionic
   B. Covalent (molecular)

   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](https://openstax.org/books/chemistry-2e/pages/7-1-ionic-bonding), accessed 2026-10-01
- [OpenStax Chemistry 2e, 7.2 Covalent Bonding](https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding), accessed 2026-10-01
- [PubChem Periodic Table (Pauling electronegativity)](https://pubchem.ncbi.nlm.nih.gov/periodic-table/), accessed 2026-10-01

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## Try asking Ducky

- "Look at my table of compounds and tell me if I classified each bond right."
- "Why is H-F covalent if fluorine pulls so hard? Explain with the electronegativity numbers."
- "Quiz me on writing ionic formulas from charges, five in a row."

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