How to Write Electron Configuration: A Step-by-Step Guide

Step 1: Find the Atomic Number

The atomic number tells you how many electrons a neutral atom has. Carbon has atomic number 6, so it has 6 electrons to place. Iron has atomic number 26, meaning 26 electrons. You can find the atomic number on any periodic table; it is always the integer displayed prominently in each element's square. For ions, adjust the electron count: cations (positive ions) have fewer electrons than the atomic number, and anions (negative ions) have more.

Step 2: Learn the Orbital Filling Order

Electrons fill orbitals starting from the lowest energy level and working upward. The filling order is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Notice that 4s fills before 3d, and 5s fills before 4d. This occurs because the energy of an orbital depends on both its principal quantum number (n) and its angular momentum quantum number (l). The Madelung rule states that orbitals fill in order of increasing (n + l) value, with lower n values filling first when (n + l) is equal.

A useful visual tool is the diagonal rule: write the subshells in a column (1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, etc.) and draw diagonal arrows from upper right to lower left. Following the arrows gives the correct filling order. Alternatively, you can read the filling order directly from the periodic table's block structure.

Step 3: Fill Orbitals With Electrons

Each orbital type has a maximum capacity: s orbitals hold 2 electrons, p subshells hold 6 (three orbitals of 2 each), d subshells hold 10 (five orbitals of 2 each), and f subshells hold 14 (seven orbitals of 2 each). Fill each subshell completely before moving to the next, unless you run out of electrons. For oxygen (8 electrons): 1s2 2s2 2p4. The 2p subshell is not full (capacity is 6) because oxygen only has 8 electrons total.

Step 4: Apply Hund's Rule for Partially Filled Subshells

When a subshell is partially filled, electrons occupy individual orbitals before pairing up in the same orbital. In the 2p subshell of nitrogen (which has three 2p electrons), each of the three p orbitals gets one electron, all with the same spin direction. This minimizes electron-electron repulsion and produces a lower energy state. Carbon, with two 2p electrons, places one electron in each of two separate p orbitals rather than pairing them in one orbital.

Step 5: Use Noble Gas Shorthand

For elements beyond helium, you can abbreviate the inner electron shells by writing the symbol of the nearest preceding noble gas in brackets, followed by the remaining electrons. Iron (26 electrons) has the full configuration 1s2 2s2 2p6 3s2 3p6 4s2 3d6, which can be shortened to [Ar] 4s2 3d6 (argon accounts for the first 18 electrons). This shorthand makes configurations easier to write and read, especially for heavier elements.

Step 6: Check for Known Exceptions

A few elements deviate from the expected filling order because half-filled or fully filled d and f subshells are unusually stable. Chromium (expected [Ar] 4s2 3d4) is actually [Ar] 4s1 3d5, promoting one s electron to achieve a half-filled d subshell. Copper (expected [Ar] 4s2 3d9) is actually [Ar] 4s1 3d10, promoting one s electron to complete the d subshell. Similar exceptions occur with molybdenum, silver, and several f-block elements. These exceptions are worth memorizing for the most common cases.