Valence Electrons
Read an atom's bonding electrons straight off its group, with dot diagrams for every family.
Why it matters
are the only electrons that do chemistry. Every bond an atom forms and every dot you draw comes from this one count, so if you get it wrong, everything after it is wrong too.
When you use it
It is the very first step of every , and how you predict the charge an atom will take when it becomes an .
Where it leads
The , Lewis structures, and , all of them start from this number.
The rule
For a main-group element, the number of valence electrons equals the units digit of its group number: group 1 has 1, group 2 has 2, and groups 13 through 18 have 3 through 8. These outer-shell electrons are the ones that form bonds, so counting them is the first step of every .
Lewis dot diagram for each main group
Place one dot on each side of the symbol before pairing any up. Carbon, for instance, gets four single dots, one per side, which is exactly why it goes on to form four bonds.
Adjusting for charge
5 (from N) + 4 × 1 (from four H) − 1 (for the + charge)
= 8 valence electrons
6 (from S) + 4 × 6 (from four O) + 2 (for the 2− charge)
= 32 valence electrons
Add one electron for each unit of negative charge, and subtract one for each unit of positive charge. A positive ion has given electrons away, so it has fewer to place.
Why the count matters
Memory trick
Common mistake
Practice