Valence electrons are the electrons in an atom’s outer shell. For main-group elements, read them from the group number.
3 steps, 1 worked example, 4 practice problems. About 10 minutes. Free.
The route · 3 steps
Valence electrons come from the group number. Tap any element.
Tap an element.
Swipe sideways for the full table.
Asked for “valence electrons” or “outer-shell electrons” of a main-group element.
Clue words marked:
How many valence electrons does an oxygen atom have?
Not this type?
The idea
| Group | Valence e⁻ | Example |
|---|---|---|
| 1 | 1 | Na |
| 2 | 2 | Mg |
| 13 | 3 | Al |
| 14 | 4 | C |
| 15 | 5 | N |
| 16 | 6 | O |
| 17 | 7 | Cl |
| 18 | 8 (He: 2) | Ne |
Valence electrons
Outer shell only.
Oxygen: 6
Total electrons
Every electron.
Oxygen: 8
On the periodic table: groups: Na 1, Mg 2, Al 13, C 14, N 15, O 16, Cl 17, Ne 18.
Worked example · 3 steps
How many valence electrons does an oxygen atom have?
8O16.00On the periodic table: oxygen (O) is in group 16.
16 is in groups 13–18, so subtract 10.
Oxygen has 6 valence electrons.
Try first, then check. Hints if you need them.
Problem 1 of 4 · warm-up
How many valence electrons does a sodium atom have?
Find sodium’s group on the periodic table.
Group 1: valence = group number.
Your path:
Answer: 1
What you know: sodium.
11Na22.99On the periodic table: sodium (Na) is in group 1.
Answer: 1
Problem 2 of 4
How many valence electrons does a phosphorus atom have?
Phosphorus is in groups 13–18 on the periodic table.
Group 15: subtract 10.
Your path:
Answer: 5
What you know: phosphorus.
15P30.97On the periodic table: phosphorus (P) is in group 15.
Answer: 5
Problem 3 of 4 · watch for the trap
How many valence electrons does a bromine atom have?
Not 35. That’s every electron. Only the outer shell counts.
Answer: 7
What you know: bromine.
Watch out: the 3d¹⁰ electrons are inner electrons, not valence: [Ar] 3d¹⁰ 4s² 4p⁵ → 2 + 5 = 7.
35Br79.90On the periodic table: bromine (Br) is in group 17.
Answer: 7
Problem 4 of 4 · exam-style wording
An element in period 3 has 3 valence electrons. What is its atomic number?
Period = row (3 shells). 3 valence electrons = group 13.
Find row 3, group 13 on the periodic table.
Your path:
Answer: 13 (aluminum)
What you know: period 3, 3 valence electrons.
13Al26.98On the periodic table: aluminum (Al) sits in period 3, group 13, atomic number 13.
Answer: 13 (aluminum)
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