An electron configuration table is a structured reference that (1) lists the maximum number of electrons in each subshell and (2) gives the Aufbau filling order used to write ground-state electron configurations. The table works because subshell energies generally increase in the order encoded by the diagonal rule, while electron placement must satisfy the Pauli exclusion principle and Hund’s rule.
1) Subshell types and capacities (core of an electron configuration table)
Subshell capacities follow directly from the number of orbitals in each subshell: \(s\) has 1 orbital, \(p\) has 3, \(d\) has 5, and \(f\) has 7. Each orbital holds at most 2 electrons.
| Subshell | Number of orbitals | Maximum electrons | Common periodic-table block |
|---|---|---|---|
| \(s\) | \(1\) | \(2\) | \(s\)-block |
| \(p\) | \(3\) | \(6\) | \(p\)-block |
| \(d\) | \(5\) | \(10\) | \(d\)-block (transition metals) |
| \(f\) | \(7\) | \(14\) | \(f\)-block (lanthanides/actinides) |
2) The Aufbau filling order used in an electron configuration table
The table’s filling order is commonly memorized as a sequence of subshells. A reliable list (ground-state, introductory general chemistry) is:
\[ 1s,\;2s,\;2p,\;3s,\;3p,\;4s,\;3d,\;4p,\;5s,\;4d,\;5p,\;6s,\;4f,\;5d,\;6p,\;7s,\;5f,\;6d,\;7p \]
Each subshell is filled up to its maximum (\(2,6,10,14\)) before moving to the next subshell in the list, until the total number of electrons equals the atomic number.
3) Visualization: diagonal rule (energy-order map)
4) Rules that an electron configuration table assumes
- Aufbau principle: electrons fill lower-energy subshells first (order given by the table).
- Pauli exclusion principle: an orbital holds at most 2 electrons with opposite spins (no two electrons in an atom share all four quantum numbers).
- Hund’s rule: within a set of degenerate orbitals (such as \(p\), \(d\), \(f\)), electrons occupy orbitals singly before pairing.
5) A practical electron configuration table (fill order with capacities)
The table below is used directly: add electrons subshell-by-subshell in the listed order until the atomic number \(Z\) is reached.
| Step | Subshell | Max electrons | Running total if filled |
|---|---|---|---|
| 1 | \(1s\) | \(2\) | \(2\) |
| 2 | \(2s\) | \(2\) | \(4\) |
| 3 | \(2p\) | \(6\) | \(10\) |
| 4 | \(3s\) | \(2\) | \(12\) |
| 5 | \(3p\) | \(6\) | \(18\) |
| 6 | \(4s\) | \(2\) | \(20\) |
| 7 | \(3d\) | \(10\) | \(30\) |
| 8 | \(4p\) | \(6\) | \(36\) |
| 9 | \(5s\) | \(2\) | \(38\) |
| 10 | \(4d\) | \(10\) | \(48\) |
| 11 | \(5p\) | \(6\) | \(54\) |
| 12 | \(6s\) | \(2\) | \(56\) |
| 13 | \(4f\) | \(14\) | \(70\) |
| 14 | \(5d\) | \(10\) | \(80\) |
| 15 | \(6p\) | \(6\) | \(86\) |
| 16 | \(7s\) | \(2\) | \(88\) |
6) Worked example using the electron configuration table
Example element: chlorine, \(Z=17\). Fill electrons using the table until 17 electrons are assigned.
- Fill \(1s\) (2 electrons): \(1s^2\) → total \(2\).
- Fill \(2s\) (2 electrons): \(2s^2\) → total \(4\).
- Fill \(2p\) (6 electrons): \(2p^6\) → total \(10\).
- Fill \(3s\) (2 electrons): \(3s^2\) → total \(12\).
- Remaining electrons: \(17-12=5\). Place 5 electrons into \(3p\): \(3p^5\).
Final ground-state electron configuration (chlorine).
\[ \mathrm{Cl:}\quad 1s^2\,2s^2\,2p^6\,3s^2\,3p^5 \]
Noble-gas shorthand: \[ \mathrm{Cl:}\quad [\mathrm{Ne}]\,3s^2\,3p^5 \]
7) Common cautions when using an electron configuration table
- Total-electron check: the exponents must sum to the atomic number for a neutral atom.
- Known exceptions: a few elements (notably in the \(3d\) series such as Cr and Cu) show slightly different ground-state configurations than a simple table prediction due to stability of half-filled or filled subshells.
- Ions require a rule adjustment: for transition-metal cations, electrons are removed from the highest principal quantum number first (commonly \(4s\) before \(3d\)), which is handled under electron configuration of ions.
An electron configuration table is most effective when used as a strict checklist: identify \(Z\), follow the filling order, respect subshell capacities, and verify that the total electrons placed equals \(Z\) for a neutral atom.