Worked example 1
How many valence electrons does a sulfur atom have, and how many bonds does neutral sulfur usually form?
Try it first: Find the group number first.
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Molecular StructureOctet ruleContent level: Core 18 min
Valence Electrons and the Octet Rule
What you'll be able to do: Count valence electrons from the periodic table and use the octet rule to predict how many bonds an atom forms.
Best after: Electron Configurations and Orbital Filling
Introduction
Almost every Lewis structure you will ever draw starts with one number: how many valence electrons are in play.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Determine the valence electron count of a main-group element from its group
- Draw Lewis symbols showing valence electrons as dots
- State the octet rule and the duet rule for hydrogen
- Predict the usual number of bonds formed by C, N, O, F and H
Lesson
What counts as a valence electron
Valence electrons are the electrons in the highest occupied principal energy level. For main-group elements the group number gives the count directly: group 1 has 1, group 2 has 2, group 13 has 3, and so on up to group 18 with 8. Sulfur is in group 16, so it has 6 valence electrons regardless of which period it sits in.
Only main-group elements follow the group-number shortcut. Transition metals are handled separately.
Lewis symbols
A Lewis symbol is the element symbol surrounded by one dot per valence electron. Dots are placed singly on the four sides first and then paired up, which is why nitrogen shows three single dots plus one pair and oxygen shows two single dots plus two pairs. Those unpaired dots are the electrons available for bonding.
The octet rule
Main-group atoms are most stable when they are surrounded by eight valence electrons, matching the nearest noble gas. Metals reach that count by losing electrons, nonmetals by gaining or sharing them. Hydrogen is the exception that everyone must remember: it is complete with two electrons, matching helium.
Bonding electrons are counted by both atoms in the bond, so a shared pair helps each atom toward its own octet.
Predicting bond number
For a neutral nonmetal, the usual number of covalent bonds is eight minus the group valence count. Carbon with 4 valence electrons forms 4 bonds and no lone pairs, nitrogen with 5 forms 3 bonds and 1 lone pair, oxygen with 6 forms 2 bonds and 2 lone pairs, and fluorine with 7 forms 1 bond and 3 lone pairs. Hydrogen forms exactly 1 bond.
Treat charge as part of electron bookkeeping
Count valence electrons for every atom before drawing bonds. Add one electron for each negative charge and subtract one for each positive charge on a polyatomic ion. The total determines how many electrons must appear as bonds and lone pairs in the finished structure. An octet check without an electron-total check can accept a structure with the wrong charge.
Write the target electron total above the skeleton before placing any lone pairs.
Key ideas
Definition
Valence electron
An electron in the outermost occupied shell; for main-group elements the group number gives the count.
Rule
Octet rule
Main-group atoms tend toward eight valence electrons; hydrogen tends toward two.
Rule
Bonds equals 8 minus valence
C forms 4 bonds, N forms 3, O forms 2, F forms 1, H forms 1.
Key concept
Reading a group
Phosphorus is in group 15, so it has 5 valence electrons and usually forms 3 bonds with 1 lone pair.
Equation
Total valence electrons for a molecule
total = sum of group valence counts, minus the charge
Worked examples
Worked example 2
Count the total valence electrons available for the ammonium ion NH₄⁺.
Try it first: Count the neutral atoms first, then adjust for the charge.
0 of 3 steps revealed.
Worked example 3
How many valence electrons must a Lewis structure for sulfate, SO₄²⁻, contain?
Try it first: Sum neutral-atom valence electrons, then adjust for the ion's charge.
0 of 3 steps revealed.
Common mistakes
Counting all electrons in the atom instead of only the outer shell.
Why it's wrong: Core electrons are held tightly and take no part in bonding.
Check instead: Use the main-group number, not the atomic number.
Giving hydrogen an octet.
Why it's wrong: Hydrogen has only a 1s orbital and is complete with two electrons.
Check instead: Check that every H shows exactly one bond and no lone pairs.
Forgetting to adjust the total for an ionic charge.
Why it's wrong: A cation has lost electrons and an anion has gained them, so the total changes.
Check instead: Add one electron per negative charge, remove one per positive charge.
After the lesson · Question bank
Independent practice
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What you should now know
Valence electrons are the outermost s and p electrons of a main-group atom, and the main-group number tells you how many there are. Atoms gain, lose or share electrons to reach a noble-gas count of eight (two for hydrogen), which sets the typical number of bonds and lone pairs each element carries.
- Main-group valence electron count equals the group valence number
- Lewis symbols place dots singly before pairing them
- Atoms seek eight valence electrons, hydrogen seeks two
- Usual bond count is 8 minus the valence electron count
- Adjust the total valence count for any ionic charge
Sources and further reading
- Chemistry 2e, Section 7.3: Lewis Symbols and StructuresOpenStax · Rice University · 7.3View source
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