Electron configurations

What an electron configuration tells you

• An electron configuration states which sublevels an atom's electrons occupy and how many are in each.
• It is written using s, p, d notation — for example sodium is $1s^2\,2s^2\,2p^6\,3s^1$.
• At Level 3 you need the configurations of the atoms and ions of the first 36 elements, hydrogen to krypton.

Reading the notation

Each block of the configuration has three parts:

• The number is the energy level (the shell) — 1, 2, 3 or 4.

• The letter is the sublevel — s, p or d.

• The superscript is the number of electrons in that sublevel.

• So $3p^6$ means six electrons in the p sublevel of the third energy level.

How many electrons fit

• An s sublevel holds a maximum of 2 electrons.

• A p sublevel holds a maximum of 6 electrons.

• A d sublevel holds a maximum of 10 electrons.

• The first energy level has an s sublevel only. The second has s and p. The third has s, p and d. That is why the numbers run 2, 8, 18.

The filling order

• Electrons fill the lowest available energy sublevel first.
• The order by energy is:

$$1s \quad 2s \quad 2p \quad 3s \quad 3p \quad 4s \quad 3d \quad 4p$$

• Note the 4s and 3d swap. The 4s sublevel is lower in energy than 3d, so 4s fills first even though it belongs to a higher shell.
• That single fact is what makes the transition metals work, and it is the most common place to drop a mark.

Writing a configuration

To write the configuration of an atom:

• Find the atomic number — that is the number of electrons in the neutral atom.

• Fill the sublevels in the order above until you have used them all up.

• Check by adding the superscripts: they must total the atomic number.

• Worked through for a few elements:

  • Neon (Z = 10): $1s^2\,2s^2\,2p^6$
  • Phosphorus (Z = 15): $1s^2\,2s^2\,2p^6\,3s^2\,3p^3$
  • Calcium (Z = 20): $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2$
  • Iron (Z = 26): $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^6$
  • Krypton (Z = 36): $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^{10}\,4p^6$

Configurations of ions

• A positive ion (cation) is formed by losing electrons; a negative ion (anion) by gaining them.

• Metals lose electrons from the outermost shell first.

  • For a transition metal this means the 4s electrons leave before any 3d electrons, even though 4s filled first.
  • $\text{Fe}$: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^6$
  • $\text{Fe}^{2+}$: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6$ — the two 4s electrons are gone.
  • $\text{Fe}^{3+}$: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5$ — both 4s electrons plus one 3d.

• Non-metals gain electrons into the highest partly filled sublevel.

  • $\text{S}$: $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$, so $\text{S}^{2-}$: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$.

• Check every ion by counting. The total number of electrons must equal the atomic number minus the charge.