Valence electrons

Valence electrons are the electrons in the highest occupied energy level of an atom, and they are the ones involved in bonding. Sulfur has 16 total electrons, with the configuration 1s2 2s2 2p6 3s2 3p4. The first 10 electrons are core electrons, filling levels n = 1 and n = 2 completely. Only the two 3s electrons and the four 3p electrons sit in the outer level, and those are the electrons the chemistry of sulfur depends on.

That makes 6 valence electrons in total. The common mistake is reporting the atomic number as the valence count, saying sulfur has 16. All electrons are not equal: the 2s2 2p6 pile in the second level is full, stable, and tightly held, while the six electrons in level n = 3 are the chemically active ones. Only the outer electrons form bonds and take part in reactions.

There is a fast check you can use. For main-group elements, the group number is the valence electron count. Group 1 alkali metals have 1, group 2 has 2, group 16 has 6, and group 17 halogens have 7. Sulfur sits in group 16, so it has 6 valence electrons. That shortcut works only for the main-group columns, not for the transition metals in the middle of the table.

The question matters because bonding depends on these electrons. Sodium has 1 valence electron, gives it up easily, and becomes Na+. Chlorine has 7 and wants exactly one more, so it grabs the electron sodium releases. Counting the right level keeps your predictions honest, and it explains why the elements in one group behave so much alike in their reactions.