How to work out valency from the configuration
Valency is the number of electrons an atom gives, takes or shares to reach a full outer shell (eight electrons, or two for hydrogen and helium).
- 1 to 3 outer electrons: valency equals the number of outer electrons. Sodium (2, 8, 1) has valency 1; aluminium (2, 8, 3) has 3.
- 5 to 7 outer electrons: valency is 8 minus the outer electrons. Oxygen (2, 6) has 8 − 6 = 2; chlorine (2, 8, 7) has 1.
- 4 outer electrons: valency 4, as in carbon and silicon.
- Full outer shell: valency 0, the noble gases.
Learn them in fives
H He Li Be B · C N O F Ne · Na Mg Al Si P · S Cl Ar K Ca · Sc Ti V Cr Mn · Fe Co Ni Cu Zn. Say each group of five out loud until it's automatic, then test yourself with Element Hunt. For all 118 elements, see the periodic table or the full electronic configuration list.
Questions students ask
What are the first 20 elements?
Hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium and calcium.
What is the electronic configuration of potassium?
2, 8, 8, 1 by shells, or 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. The 19th electron goes into 4s, not 3d, because 4s is lower in energy.
Why does iron have two valencies?
Iron can lose its two 4s electrons (valency 2, ferrous) or those plus one 3d electron (valency 3, ferric). Fe³⁺ is 3d⁵, a stable half-filled subshell.
What is the atomic mass of chlorine?
35.45 u, usually rounded to 35.5 in exams. It's an average of Cl-35 and Cl-37.