IMAT Chemistry · Atomic Structure

Electron Configurations of Ions and Valence Electrons

An ion's configuration comes from adding or removing electrons at the outermost shell of the atom; most main-group ions end up with eight electrons in their outer shell.

All Atomic Structure notes

Why this matters

Ion configurations were asked in 2015, 2017 and 2021, each time as five configurations to choose from. The 2026 ministry paper asked about the outer shell of Mg²⁺ and the valence electrons of Cl⁻.

Concept 1 of 2: Writing the electron configuration of an ion

First count the electrons the ion really has, then write the configuration as you would for any atom with that many electrons. For a positive ion, the electrons that leave are the outermost ones, the ones furthest from the nucleus. In a transition metal the 4s electrons are outermost, so they leave before the 3d ones, even though 4s filled first.

Definition

Negative ions: add the extra electrons into the next available orbitals. $\mathrm{N^{3-}}$ ( $7 + 3 = 10$ electrons) is $1s^2\,2s^2\,2p^6$ .
Positive main-group ions: remove electrons from the highest shell. $\mathrm{Ca^{2+}}$ loses both 4s electrons.
Transition-metal ions: remove the 4s electrons first, then 3d. $\mathrm{Fe^{2+}}$ is $[\mathrm{Ar}]\,3d^6$ , not $[\mathrm{Ar}]\,3d^4\,4s^2$ .
Always check: the superscripts must add up to $Z - q$ .

Worked example

Write the ground-state configurations of

\mathrm{Ca^{2+}}

(

Z = 20

\mathrm{N^{3-}}

(

Z = 7

) and

\mathrm{Fe^{2+}}

(

Z = 26

Practice this conceptself-check · 4 quick reps

Transition metals lose 4s electrons before 3d

4s fills before 3d, but once 3d holds electrons the 4s electrons are the outermost ones, and they leave first.

\mathrm{Fe^{2+}}

[\mathrm{Ar}]\,3d^6

and

\mathrm{Cu^{2+}}

[\mathrm{Ar}]\,3d^9

. An option that keeps 4s electrons in such an ion is wrong.

Concept 2 of 2: Valence electrons and the outer shell of an ion

Valence electrons are the ones in the outermost shell, the ones that take part in bonding. A metal atom that loses its valence electrons is left with the full shell underneath as its new outer shell. A non-metal that gains electrons fills its outer shell. Either way, a main-group ion usually ends with eight outer electrons, like a noble gas.

Definition

Valence electrons are the electrons in the shell with the highest $n$ .
For main-group atoms, the number of valence electrons is set by the group: groups 1 and 2 have 1 and 2; groups 13 to 17 have 3 to 7; group 18 has 8 (helium 2).
Main-group ions usually have a full outer shell of 8 electrons (an octet): $\mathrm{Na^+}$ , $\mathrm{Mg^{2+}}$ , $\mathrm{Cl^-}$ , $\mathrm{O^{2-}}$ .
Ions with the helium arrangement have 2: $\mathrm{H^-}$ , $\mathrm{Li^+}$ , $\mathrm{Be^{2+}}$ .
Losing a whole shell is why a cation is smaller than its atom (see the Periodic Table chapter).

Worked example

Compare the outer shell of a sulfur atom (

Z = 16

) with that of

\mathrm{S^{2-}}

, and the outer shell of a magnesium atom (

Z = 12

) with that of

\mathrm{Mg^{2+}}

Practice this conceptself-check · 3 quick reps

An anion's valence electrons are not its group number

A halogen atom has 7 valence electrons, but its 1− ion has 8: the extra electron joins the same outer shell. Options giving 7, or giving the total number of electrons, are the usual traps.

A cation with no electrons left in its old outer shell still has an outer shell

When

\mathrm{Mg}

becomes

\mathrm{Mg^{2+}}

, shell 3 is empty and shell 2 becomes the outer shell, with 8 electrons. The ion has the same protons and neutrons as the atom, two fewer electrons, and a smaller radius.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Watch out for (3)

Transition metals lose 4s electrons before 3d
→ Writing the electron configuration of an ion
An anion's valence electrons are not its group number
→ Valence electrons and the outer shell of an ion
A cation with no electrons left in its old outer shell still has an outer shell
→ Valence electrons and the outer shell of an ion

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