Writing Electron Configurations and Orbital Notations

An electron configuration lists the subshells occupied by the electrons of an atom, with the number of electrons shown as a superscript, such as 1s² 2s² 2p⁴ for oxygen, while an orbital notation (orbital diagram) shows each orbital as a box and each electron as an arrow. In brief: to write a configuration, fill the subshells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p until all electrons are placed, and use the noble gas shorthand, for example [Ne] 3s² 3p⁵ for chlorine, to save space.

Electron configurations summarize an atom’s electronic structure, and from them chemists predict valence electrons, bonding, ion charges and magnetism. This article explains the step-by-step method, the shorthand notation, orbital diagrams, how to write configurations for ions and exceptions, shows the configurations of the first 36 elements in a table, works through examples and gives some history.

How do you write an electron configuration?

1. Find the number of electrons: for a neutral atom, it equals the atomic number.
2. Write the subshells in the filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
3. Fill each subshell to its capacity: s holds 2, p holds 6, d holds 10 and f holds 14 electrons.
4. Stop when all electrons are placed. The last subshell may be only partly filled.
5. Check that the superscripts add up to the total number of electrons.
6. Check for known exceptions (Cr, Cu and others).

What is the noble gas shorthand?

The inner electrons are the same as those of the previous noble gas, so you can replace them by the noble gas symbol in square brackets. Sodium, with 11 electrons, is 1s² 2s² 2p⁶ 3s¹, where the first ten electrons are the same as neon, so the shorthand is [Ne] 3s¹. The electrons that follow the noble gas core are the valence electrons (and, for transition metals, the d electrons).

How do you draw an orbital diagram?

Draw a box for each orbital: one for each s subshell, three for p, five for d and seven for f. Group the boxes by subshell and label them. Place arrows in the boxes following Hund’s rule and the Pauli principle: fill each box in a subshell with a single upward arrow first, and then add downward arrows to pair them. For nitrogen: 1s [↑↓], 2s [↑↓], 2p [↑] [↑] [↑]. In plain text, the diagram may be shown as 1s: ↑↓ | 2s: ↑↓ | 2p: ↑ ↑ ↑.

Electron configurations of the first 36 elements

By convention, the 3d subshell is written before 4s in the shorthand for atoms after calcium, because 3d is part of the third shell, although 4s is filled first.

How do you write the configuration of an ion?

For a cation, remove electrons from the subshell with the highest principal quantum number n first. This means that for transition metals the 4s electrons are removed before the 3d electrons, though 4s is filled first. For an anion, add electrons following the usual filling order.

• Na⁺: remove the 3s electron from [Ne] 3s¹ to get [Ne], or 1s² 2s² 2p⁶.
• Cl⁻: add one electron to [Ne] 3s² 3p⁵ to get [Ne] 3s² 3p⁶, which equals [Ar].
• Fe²⁺: remove the two 4s electrons from [Ar] 3d⁶ 4s² to get [Ar] 3d⁶.
• Fe³⁺: remove one more, from a 3d orbital: [Ar] 3d⁵. This half-filled subshell is stable.

Worked examples

1. Sulfur (16 electrons). 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁴ (16). Shorthand: [Ne] 3s² 3p⁴. Valence electrons: 2 + 4 = 6.
2. Orbital diagram of sulfur. 3s: ↑↓; 3p: ↑↓ ↑ ↑. There are 2 unpaired electrons.

3. Manganese (25 electrons). [Ar] 3d⁵ 4s². Check: 18 + 5 + 2 = 25 ✓. The 3d subshell is half-filled with 5 unpaired electrons.

4. Copper(II) ion. Copper is [Ar] 3d¹⁰ 4s¹. Cu²⁺ loses the 4s electron and one 3d electron: [Ar] 3d⁹. The ion has 1 unpaired electron and is paramagnetic, and its compounds are often blue.

5. Identify the element. The configuration [Kr] 5s² 4d¹⁰ 5p² has 36 + 2 + 10 + 2 = 50 electrons, so it is tin (Sn). It has 4 valence electrons (5s² 5p²), so it is in group 14.
6. Isoelectronic species. S²⁻, Cl⁻, K⁺ and Ca²⁺ all have 18 electrons and the configuration [Ne] 3s² 3p⁶, the same as argon.

How does the periodic table help?

The shape of the periodic table follows the filling order. Groups 1 and 2 form the s-block, groups 13 to 18 the p-block, the transition metals the d-block and the lanthanides and actinides the f-block. To write a configuration, count the position of the element in the table: the period number gives the shell, and counting across the blocks gives the number of electrons in the last subshell. For example, selenium is in period 4, group 16, so it ends in 4p⁴ (the fourth element of the p-block).

How did this notation develop?

Niels Bohr and Wolfgang Pauli worked on electron shell structure in the early 1920s. The spdf notation comes from spectroscopy, and the systematic electron configurations were developed for each element as the quantum theory was refined in the 1920s. The noble gas shorthand became popular because it highlights the valence electrons. The accepted configurations of many heavy elements are determined by experiment and calculation, and for some of them, such as lawrencium, the assignment is subject to continuing study.

Which mistakes are common?

Mistake 1: Making the superscripts the wrong total

Always check that the sum of the superscripts equals the number of electrons.

Mistake 2: Removing 3d electrons first when forming transition metal ions

The 4s electrons are removed first.

Mistake 3: Forgetting the exceptions

Chromium and copper have a half-filled or full 3d subshell, with only one 4s electron.

Mistake 4: Writing p⁷ or d¹¹

The capacities are 6 for p, 10 for d and 14 for f.