How to Read Electron Configuration on the Periodic Table: A Step-by-Step Guide
Read an element’s electron configuration by using its position in the periodic table to identify the subshells being filled, then count electrons up to that element. For a neutral atom, the total number of electrons equals its atomic number.
The table’s blocks show which subshell is filling: the two left columns are the s-block, the ten middle columns are the d-block, and the six right columns are the p-block. The f-block is shown in two rows below the main table. The filling order follows subshell energy, as described by Encyclopaedia Britannica.
The basic reading method
Each subshell has a maximum capacity:
| Subshell | Maximum electrons |
|---|---|
| s | 2 |
| p | 6 |
| d | 10 |
| f | 14 |
For many neutral atoms, the filling sequence begins:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p
The period number helps identify the outer energy level, but it does not give the number before every subshell. In period 4, for example, the s-block fills 4s, the d-block fills 3d, and the p-block fills 4p. The d subshell has the principal number one lower than the period; the f subshell has the number two lower.
To read an element’s configuration:
- Start with the first element and follow the filling order.
- Fill each subshell to its capacity until you reach the element’s position.
- Use its position within the current block to find the number of electrons in the final subshell.
- Check that the superscripts add up to the element’s atomic number.
The block indicates the subshell type, while the element’s position within the block indicates how many electrons have been added to it. Group number alone is not a subshell count. For instance, an element in group 13 has one electron in the p subshell being filled, not 13. The block layout is shown on the IUPAC Periodic Table of the Elements.
Worked example: iron
Iron (Fe) has atomic number 26, so a neutral iron atom has 26 electrons. It is in period 4 and the d-block.
Follow the filling order up to iron:
- 1s² 2s² 2p⁶: 10 electrons
- 3s² 3p⁶: 8 more, bringing the total to 18
- 4s²: 2 more, bringing the total to 20
- 3d⁶: 6 more, bringing the total to 26
Iron’s configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. Its noble-gas shorthand is [Ar] 4s² 3d⁶. The [Ar] represents the first 18 electrons, which have the same configuration as argon.
Check the result by adding the superscripts: 2 + 2 + 6 + 2 + 6 + 2 + 6 = 26. The total matches iron’s atomic number. The NIST Atomic Spectra Database provides atomic data for checking configurations.
Noble-gas shorthand
Noble-gas shorthand replaces the inner electrons with the symbol of the preceding noble gas in brackets. The remaining subshells are written after it.
For iron, the preceding noble gas is argon, so the full configuration can be shortened to [Ar] 4s² 3d⁶. This notation is easier to read when an atom has many electrons. It also makes the outer subshells easier to spot.
When using shorthand, choose the noble gas before the element, then count forward through the relevant blocks. Make sure the electrons represented by the noble-gas symbol are included when checking the total.
What the filling order does and does not tell you
The aufbau principle describes a useful way to predict the ground-state configuration: electrons occupy lower-energy subshells before higher-energy ones. The order is not a literal path that electrons follow. It is a rule for writing the arrangement of electrons in an atom.
The familiar 4s-before-3d order applies when building configurations for many neutral atoms. For some elements, the measured ground-state configuration differs from the simplest prediction. Chromium and copper are common examples: their configurations are [Ar] 4s¹ 3d⁵ and [Ar] 4s¹ 3d¹⁰, respectively. When an exact configuration matters, consult a reliable atomic-data reference such as the NIST Atomic Spectra Database.
The simple table-reading method applies to ground-state neutral atoms. Ions have gained or lost electrons, so their configurations must be adjusted. For positive ions, electrons are generally removed from the outermost occupied energy level first. For example, Fe²⁺ has the configuration [Ar] 3d⁶: its two 4s electrons have been removed.
Key Takeaways
- Use the periodic table’s blocks to identify the subshell being filled: s, p, d, or f.
- Subshell capacities are s², p⁶, d¹⁰, and f¹⁴.
- In period 4, the common filling sequence includes 4s before 3d.
- For a neutral atom, the superscripts in the configuration must add up to its atomic number.
- Noble-gas shorthand replaces the inner electrons with the preceding noble gas.
- Treat the filling order as a prediction. Some atoms and ions have configurations that require a specific reference check.
