The valency of chlorine is 1. A chlorine atom (atomic number 17, electron configuration 2, 8, 7) has seven electrons in its outer shell, so it needs just one more to complete an octet. It gains or shares that one electron, which is why chlorine forms HCl, NaCl and Cl₂ with a single bond each. The chloride ion, Cl⁻, therefore also has a valency of 1.
In some compounds with oxygen and fluorine, chlorine shows higher valencies of 3, 5 and 7. That is called variable valency, and it is explained further down.
Why is the valency of chlorine 1?
Valency is the combining capacity of an atom: the number of electrons it gains, loses or shares to reach a stable arrangement. Chlorine’s electrons are arranged as:
- K shell: 2
- L shell: 8
- M shell: 7
With 7 valence electrons, chlorine is one electron short of the stable 8. Gaining one electron is far easier than losing seven, so chlorine behaves as a monovalent element. You can also get the answer with the quick rule used in school chemistry: for elements with 4 or more valence electrons, valency = 8 − valence electrons = 8 − 7 = 1.
Note the difference: chlorine has 7 valence electrons but a valency of 1. Mixing up those two numbers is the most common mistake students make with this question.
Valency of chloride (Cl⁻)
When chlorine accepts one electron it becomes the chloride ion, Cl⁻, with configuration 2, 8, 8, the same as argon. A chloride ion carries a charge of −1 and has a valency of 1. That is why one chloride combines with one sodium ion (NaCl), two chloride ions combine with one calcium ion (CaCl₂) and three combine with one aluminium ion (AlCl₃).
| Compound | Formula | How chlorine combines | Valency of chlorine |
|---|---|---|---|
| Hydrogen chloride | HCl | Shares one electron pair | 1 |
| Sodium chloride | NaCl | Gains one electron (Cl⁻) | 1 |
| Magnesium chloride | MgCl₂ | Each Cl gains one electron | 1 |
| Chlorine gas | Cl₂ | Single covalent bond Cl–Cl | 1 |
| Carbon tetrachloride | CCl₄ | Each Cl forms one bond with C | 1 |
Variable valency of chlorine: 1, 3, 5 and 7
Chlorine sits in period 3, so it has empty 3d orbitals available. When it bonds with very electronegative atoms such as oxygen or fluorine, some of its paired electrons can be promoted into those orbitals, giving 3, 5 or 7 unpaired electrons for bonding. This is why chlorine forms a family of oxyacids:
| Compound | Formula | Oxidation state of Cl | Valency shown |
|---|---|---|---|
| Hydrochloric acid | HCl | −1 | 1 |
| Hypochlorous acid | HOCl | +1 | 1 |
| Chlorous acid | HClO₂ | +3 | 3 |
| Chloric acid | HClO₃ | +5 | 5 |
| Perchloric acid | HClO₄ | +7 | 7 |
| Chlorine trifluoride | ClF₃ | +3 | 3 |
For board exams and most school questions, the expected answer to “valency of chlorine” is simply 1. Mention variable valency only when the question asks about oxyacids or higher compounds.
Valency vs oxidation state
Valency is always a plain number with no sign; it counts bonds or electrons exchanged. Oxidation state has a sign and tells you whether the atom has gained or lost electron density. In HCl chlorine has valency 1 and oxidation state −1. In HClO₄ it has valency 7 and oxidation state +7.
Worked example: writing formulas with chlorine
Use the criss-cross method with valencies. For aluminium chloride: Al has valency 3 and Cl has valency 1. Cross them over and you get Al₁Cl₃, written AlCl₃. For iron(III) chloride the same logic gives FeCl₃, and for copper(II) chloride it gives CuCl₂.
Frequently asked questions
What is the valency of chlorine?
The valency of chlorine is 1, because its outer shell has 7 electrons and it needs only one more to complete an octet.
What is the valency of chloride?
The chloride ion Cl⁻ has a valency of 1 and a charge of −1. It combines with one hydrogen or sodium ion, as in HCl and NaCl.
How many valence electrons does chlorine have?
Chlorine has 7 valence electrons. Its valency is 8 − 7 = 1.
Why does chlorine show variable valency?
Chlorine has vacant 3d orbitals, so with oxygen or fluorine it can unpair electrons and form 3, 5 or 7 bonds, as in HClO₂, HClO₃ and HClO₄.
What is the valency of chlorine in HClO₄?
In perchloric acid, HClO₄, chlorine shows a valency of 7 and an oxidation state of +7.
References
- PubChem – Chlorine (element data)
- NCERT Class 9 Science, Chapter 3 “Atoms and Molecules” and Class 12 Chemistry, “The p-Block Elements”.