When atoms react, only their outermost electrons take part; the inner ones are held too tightly. Groups, Periods and Families groups atoms by those outer electrons, so the table tells you how many each main-group atom has.
Key idea
Valence electrons are the electrons in the outermost shell. For main-group elements: groups 1 and 2 have 1 and 2; groups 13 to 18 have the group number minus 10 (3 to 8). Helium has 2. A Lewis dot symbol shows them as dots around the element's symbol, one per side before pairing.
Why it works
Look at the configurations. Sulfur is $\mathrm{[Ne]\,3s^2\,3p^4}$: its outer shell, shell 3, holds $2 + 4 = 6$ electrons. Selenium, below it, is $\mathrm{[Ar]\,4s^2\,3d^{10}\,4p^4}$: the 3d electrons are tucked inside, in a lower shell, so its outer shell also holds 6. The 10 columns of the d block are the "minus 10". For transition metals the d electrons take part too, so the simple rule does not apply to them.
Worked examples
Worked example: Group 16
How many valence electrons does sulfur (group 16) have? Draw its dot symbol.
- Groups 13–18: subtract 10. $16 - 10 = 6$.
- Place one dot on each of four sides, then pair up: two pairs and two single dots around S.
Answer: 6 valence electrons
Worked example: From the shells
How many valence electrons does aluminum ($Z = 13$) have?
- Shells: 2, 8, 3.
- The outer shell holds 3. (Group 13: $13 - 10 = 3$, the same answer.)
Answer: 3
Common mistake
Counting all of an atom's electrons instead of the outer ones, or giving sulfur 16 valence electrons because it is in group 16. No main-group atom has more than 8 valence electrons. For groups 13–18, always subtract 10; for transition metals, don't use the group number at all.