Chapter 9 · Electrons in Atoms and the Periodic Table
9.7Electron Configurations and the Periodic Table
12 min · two checks
Predict
Why do the alkali metals all end in s¹?
The idea
- Read a valence configuration from a position on the table.
- Write a noble-gas core abbreviation.
The periodic table is an electron-filling diagram. The two columns on the left are the s block. The six columns on the right are the p block. The ten-column dip in the middle is the d block. The detached fourteen-column rows are the f block. The period number is the n of the s or p orbital being filled. Chlorine is in period 3, group 17, five steps into the p block, so its valence configuration is 3s² 3p⁵.
A noble-gas core saves writing. Sodium is [Ne] 3s¹. Bromine is [Ar] 4s² 3d¹⁰ 4p⁵. When you use the core, do not forget d electrons that filled in the previous period if the element is past the transition series. Valence electrons for main-group elements are the outermost s and p electrons. Those are the ones that repeat down a group, which is why the group repeats chemically.
Keep these
- s block, d block, p block — the table is a filling map.
- Main-group valence electrons are the outer s and p.
- Noble-gas cores must still account for filled d subshells.