Electronic configurations of ions are predicted by starting with the neutral atom and then adding or removing electrons to match the charge. The key skill is deciding exactly which electrons are gained or lost.

This skill applies to individual ions such as $O^{2-}$ or $Ca^{2+}$, not to whole compounds.

Starting from the neutral atom

To find the electronic configuration of an ion, first work out the neutral atom from its atomic number. A neutral atom has the same number of electrons as its atomic number. Only after the neutral atom has been written should the ionic charge be applied.

For elements up to atomic number 36, electrons fill in the order:

Aufbau (diagonal rule) filling-order diagram showing the sequence of subshells (e.g., $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$). This provides a visual method for remembering why $4s$ is written before $3d$ in full $1s$ notation for elements up to $Z=36$. Source

• $1s$

• $2s$

• $2p$

• $3s$

• $3p$

• $4s$

• $3d$

• $4p$

The maximum numbers of electrons you need here are:

• s subshell: 2 electrons

• p subshell: 6 electrons

• d subshell: 10 electrons

Using full 1s notation

In 1s notation, each occupied subshell is written with a superscript showing how many electrons it contains. For ion questions on this topic, full notation is especially useful because it makes electron gain or loss easy to track.

A full configuration must:

• follow the correct filling order

• show the correct number of electrons

• end at the correct subshell for that species

For example, a full configuration might look like $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6$. For ions involving period 4 s- and p-block elements, the configuration may also include $3d^{10}$ and $4p$ electrons.

Why period 4 ions need extra care

For elements from potassium to krypton, the full configuration no longer ends at $3p$. Period 4 s- and p-block ions may contain a filled $3d^{10}$ subshell even though the ion itself is not a d-block species. This means the written answer can be longer than students expect.

A correct ion configuration for these elements may include $4s$ and $4p$ electrons around a filled $3d^{10}$ subshell. If $3d^{10}$ is missing from a full 1s answer, the electron total will be wrong even if the final shell looks correct.

Turning the atom into an ion

The ionic charge tells you whether electrons have been lost or gained.

A positive charge means electrons have been removed. A negative charge means electrons have been added. The number in the charge tells you how many electrons have changed.

A reliable method

When a question gives an atomic number and a charge, follow the same routine every time:

• identify the neutral atom

• write the full 1s configuration

• change the electron total using the charge

• alter only the electrons in the outer region of the configuration

• recount electrons in the final ion

A $3+$ ion has three fewer electrons than the atom. A $2-$ ion has two more electrons than the atom. Do not change the written subshell order just because the species is charged.

Positive ions

A positive ion, or cation, forms when electrons are removed from the neutral atom. For s- and p-block ions, remove electrons from the outermost occupied shell first. This is one of the most important rules in this topic.

Key points:

• subtract the number of electrons shown by the positive charge

• remove electrons from the highest principal energy level first

• do not remove electrons from an inner shell while outer-shell electrons are still present

This matters especially in period 4 p-block elements. Their neutral atoms contain electrons in $4s$, $3d$, and $4p$. When forming a positive ion, electrons are removed from the $4p$ subshell first, then $4s$, before any electrons would be removed from $3d$. That is because the outer shell is $n=4$, not $n=3$.

For example:

• $Al^{3+}$ has lost three electrons from the neutral aluminum atom

• $Mg^{2+}$ has lost both $3s$ electrons

• $K^+$ has lost its single $4s$ electron

Negative ions

A negative ion, or anion, forms when electrons are added to the neutral atom. Added electrons go into the next available space in the partially filled outer subshell.

Useful patterns:

• group 17 atoms often gain 1 electron

• group 16 atoms often gain 2 electrons

• group 15 atoms often gain 3 electrons

When adding electrons:

• keep the filling order unchanged

• place the added electrons into the existing outer subshell before moving elsewhere

• check that the final total number of electrons matches the atomic number plus the magnitude of the negative charge

Common examples include $F^-$, $O^{2-}$, $S^{2-}$, and $Br^-$. Many negative ions end up with a full outer shell, which is why their final configuration often matches that of a nearby noble gas.

Electrons-in-boxes notation

Electrons-in-boxes notation shows orbitals as boxes and electrons as arrows.

This notation is useful because it shows not just how many electrons are present, but also how they are distributed between orbitals. In an orbital, a maximum of two electrons can be shown, and they must have opposite spins.

For the subshells you need here:

• an s subshell has one box

• a p subshell has three boxes

• a d subshell has five boxes

A full $p$ subshell is shown as:

Orbital (electron-in-box) diagrams illustrating boxes as orbitals and arrows as electron spins, including $2p$ filling patterns that follow Hund’s rule. The diagrams make it clear why a $p$ subshell uses three adjacent boxes and how electrons occupy each box singly before pairing. This directly supports drawing ions in box notation by adjusting electron number while keeping subshell structure consistent. Source

• [↑↓] [↑↓] [↑↓]

A full $s$ subshell is shown as:

• [↑↓]

When drawing ions in box notation:

• start from the neutral atom if needed

• adjust the electron number for the charge

• fill or empty the boxes in the correct outer subshell

• make sure the final number of arrows equals the number of electrons in the ion

Common exam pitfalls

Several mistakes appear often in ion configuration questions.

• Removing electrons from the wrong subshell: for positive ions, students often remove from $3d$ before $4p$ or $4s$ in period 4 p-block species.

• Forgetting $3d^{10}$ in period 4 full configurations: this gives the wrong total number of electrons.

• Using the atomic number incorrectly: remember that the atomic number identifies the neutral atom first.

• Forgetting the charge: always check whether electrons should be added or removed.

• Giving an atom instead of an ion: the final electron total must match the ion, not the neutral element.

• Mixing notations: if the question asks for 1s notation, give the full subshell configuration; if it asks for electrons-in-boxes notation, show boxes and arrows clearly.

A strong final check is to count the electrons in your completed configuration. If the ion is $2+$, the total should be two fewer than the atomic number. If the ion is $1-$, the total should be one more than the atomic number.