Lesson 2 of 7 · Atoms & bonding
Valence electrons & ions
Valence electrons are the electrons that can take part in chemical bonding. For main-group atoms, these are the electrons in the outermost occupied shell. An atom gains a positive charge when it loses electrons and a negative charge when it gains them.
How to find valence electrons
The main-group elements are in groups 1, 2 and 13–18. In groups 1 and 2, the group number gives the valence-electron count. In groups 13–18, subtract ten. Helium is the exception: it has two electrons in its filled 1s shell. Transition metals need a closer look at their electron configurations because d electrons can also take part in bonding.
| Group | Valence electrons | Examples of ions |
|---|---|---|
| 1 | 1 | Li⁺, Na⁺, K⁺ |
| 2 | 2 | Mg²⁺, Ca²⁺ |
| 13 | 3 | Al³⁺ |
| 14 | 4 | No general simple-ion rule |
| 15 | 5 | N³⁻ in ionic nitrides |
| 16 | 6 | O²⁻, S²⁻ |
| 17 | 7 | F⁻, Cl⁻ |
| 18 | 8; He has 2 | No common simple ions |
Reference: OpenStax Chemistry 2e, §6.4: Electron configurations
Count electrons, then charge
Losing an electron makes an atom one unit more positive; gaining an electron makes it one unit more negative. Na⁺ has 11 protons and 10 electrons. Cl⁻ has 17 protons and 18 electrons. The nucleus does not change during ordinary ion formation.
To write a neutral ionic formula, make the positive and negative charges add up to zero. One Mg²⁺ ion needs two Cl⁻ ions, giving MgCl₂. The subscript 2 tells you how many chloride ions are needed; each chloride still has a charge of −1.
Reference: OpenStax Chemistry 2e, §2.6: Ionic and molecular compounds
Ion charge vs oxidation state
An ion has an overall electrical charge. An oxidation state is a number assigned to an atom using rules for counting bonding electrons. For a monatomic ion, the two values agree. In a molecule, they can mean very different things: oxygen has oxidation state −2 in H₂O, but water contains covalent O–H bonds, not free oxide ions.
Reference: IUPAC Gold Book: Oxidation state
When the octet rule helps
The octet rule helps explain the formulas of many main-group compounds: atoms often form bonds that give them eight electrons in their valence shell. It has exceptions, including compounds with too few electrons for an octet and species with an unpaired electron. Whether a compound is stable depends on its overall energy.
Work through the examples
Worked example 1
How many valence electrons does sulfur have?
- Sulfur is in group 16.
- Subtract ten, or inspect [Ne] 3s² 3p⁴ and count the six electrons with n = 3.
Answer: A neutral sulfur atom has six main-group valence electrons.
Worked example 2
Write the formula formed from Al³⁺ and O²⁻.
- Find a total charge that both ions can supply: six is the smallest multiple of both three and two.
- Two aluminium ions provide +6 and three oxide ions provide −6.
Answer: Al₂O₃; the net charge is zero.
Avoid these mistakes
- Do not use the main-group shortcut for every transition metal.
- Helium has two valence electrons, not eight.
- A common ion charge does not list every possible oxidation state.
Check your understanding
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Read the complete answer key
1. B — 7. Chlorine is in group 17; its outer shell is 3s² 3p⁵.
2. A — 10. A 2+ ion has lost two electrons, so 12 − 2 = 10.
3. C — CaF₂. Two fluoride ions balance one calcium ion: +2 + 2(−1) = 0.
4. B — No. Oxidation state is formal electron bookkeeping. Water is a covalently bonded molecule.
Common questions
Why can iron form Fe²⁺ and Fe³⁺?
Transition-metal atoms can lose different numbers of electrons because their s and d electron energies depend on occupation and chemical environment. A single group-number shortcut cannot determine every stable ion.
Does carbon usually form C⁴⁺ ions?
Carbon commonly forms covalent bonds. An oxidation state of +4, as in CO₂, should not be interpreted as a free C⁴⁺ ion inside the molecule.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.