Electronic configurations show how electrons are arranged in atoms. For this topic, you need to write accurate configurations for neutral atoms up to krypton using both full 1s notation and electrons-in-boxes diagrams.

What electronic configurations show

An electronic configuration tells you how the electrons in an atom are arranged. For a neutral atom, the number of electrons is the same as the atomic number, so the atomic number tells you how many electrons must be placed.

In 1s notation, each part of the configuration gives specific information:

• the number shows the main energy level

• the letter shows the subshell: s, p, or d

• the superscript shows the number of electrons in that subshell

For example, $3p^4$ means:

• energy level 3

• a p subshell

• 4 electrons in that subshell

A full electronic configuration must include all electrons in the atom, written in the correct order. For atoms with atomic numbers up to 36, the final subshell written is no higher than 4p.

Order in which subshells fill

Electrons fill the available subshells in order of increasing energy. For the first 36 elements, the order you need is:

• $1s,\ 2s,\ 2p,\ 3s,\ 3p,\ 4s,\ 3d,\ 4p$

This order must be learned accurately, because it is the basis of every configuration in this range.

Diagonal (Aufbau) filling diagram showing how subshells are filled in order of increasing energy. Following the arrows gives the sequence used in full configurations (e.g., …$3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$). This is especially useful for remembering why $4s$ is written before $3d$ for neutral atoms up to $Z=36$. Source

Each subshell has a maximum capacity:

• s holds 2 electrons

• p holds 6 electrons

• d holds 10 electrons

These capacities depend on how many orbitals are present in each subshell.

In full 1s notation, orbitals are not shown separately, but they are still important. An s subshell contains 1 orbital, a p subshell contains 3 orbitals, and a d subshell contains 5 orbitals.

A common difficulty is the position of 4s and 3d. For neutral atoms up to atomic number 36, 4s fills before 3d. This means potassium is $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^1$, and calcium is $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2$.

Useful checkpoint configurations

Some full configurations are especially useful as reference points:

• helium: $1s^2$

• neon: $1s^2\ 2s^2\ 2p^6$

• argon: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6$

• krypton: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^2\ 3d^{10}\ 4p^6$

If your configuration does not build correctly from these patterns, it should be checked.

Electrons-in-boxes notation

Electrons-in-boxes notation shows orbitals more clearly than 1s notation. Each box represents one orbital, and each arrow represents one electron.

This notation helps you see how electrons are arranged within a subshell, especially in p and d subshells.

The rules are:

• each box can hold a maximum of two electrons

• two electrons in the same box must have opposite spins

• when orbitals have the same energy, electrons go into separate boxes first before any pairing happens

So:

• a p subshell with 3 electrons is shown as [↑] [↑] [↑]

• a p subshell with 4 electrons is shown as [↑↓] [↑] [↑]

For box diagrams:

Orbital box template for $1s$, $2s$, and $2p$, showing how each orbital is represented as a box and each electron as an arrow. The diagram makes it explicit that an $s$ subshell has one orbital (one box), while a $p$ subshell has three orbitals (three boxes). This provides a visual bridge from subshell notation to electrons-in-boxes diagrams with correct spin pairing. Source

• an s subshell has 1 box

• a p subshell has 3 boxes

• a d subshell has 5 boxes

This makes it easier to represent atoms such as nitrogen, phosphorus, or the first-row d-block atoms, where several electrons occupy equal-energy orbitals.

Writing configurations for atoms up to atomic number 36

To write a configuration correctly:

• start with the atomic number

• place that number of electrons into subshells using the correct filling order

• do not exceed the maximum capacity of any subshell

• check that the total number of electrons written equals the atomic number

For atoms up to 20, configurations build from $1s$ to $4s$. After calcium, the next electrons go into 3d, so scandium begins the 3d series. After $3d^{10}$ is reached, the next electrons enter 4p, finishing at krypton.

Important exceptions

Most atoms up to 36 follow the standard filling order exactly, but two important neutral atoms must be learned as exceptions:

• chromium: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 3d^5\ 4s^1$

• copper: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 3d^{10}\ 4s^1$

These configurations are more stable than the expected alternatives because a half-filled or fully filled d subshell is especially favorable.

Common mistakes to avoid

• using shell patterns such as 2,8,8,2 when the question asks for full 1s notation

• writing 3d before 4s for neutral atoms in this range

• forgetting that p subshells have three orbitals and d subshells have five

• pairing electrons too early in box diagrams

• giving chromium as $4s^2\ 3d^4$ or copper as $4s^2\ 3d^9$

• forgetting to add the superscripts to the correct total number of electrons

Accuracy matters because one misplaced electron gives the wrong atom.