What Is Electron Configuration? 1s2 2s2 2p6 Explained
Open any chemistry textbook and you'll run into strings like 1s² 2s² 2p⁶ 3s¹. They look like a secret code — and in a way they are: this one line tells you almost everything about how sodium behaves. Once you can read it, the periodic table stops being a wall of boxes and starts making sense.
The short answer: an electron configuration is the "address list" showing how an atom's electrons are arranged — which shells and subshells they occupy, and how many are in each. The notation 1s² 2s² 2p⁶ reads as: 2 electrons in the 1s subshell, 2 in the 2s, and 6 in the 2p, for 10 electrons total (that's neon).
How to read the notation
Every chunk of the code has three parts. Take 2p⁶:
- 2 — the shell (energy level), counted outward from the nucleus. Bigger number = farther out and higher energy.
- p — the subshell type. Subshells come in four flavors you'll meet in an intro course: s, p, d, f.
- ⁶ — the superscript is the number of electrons living in that subshell. It is not an exponent; nothing is being multiplied.
Each subshell type has a fixed capacity:
| Subshell | Holds up to |
|---|---|
| s | 2 electrons |
| p | 6 electrons |
| d | 10 electrons |
| f | 14 electrons |
A whole shell holds up to 2n² electrons: shell 1 holds 2, shell 2 holds 8, shell 3 holds 18.
The three filling rules
Electrons don't scatter randomly — they fill in a predictable order, governed by three rules:
- The Aufbau principle ("building-up"): electrons fill the lowest-energy subshells first. The standard order for the first few is 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p. Yes, 4s fills before 3d — the 4s subshell sits slightly lower in energy for a neutral atom being built up.
- The Pauli exclusion principle: each orbital holds a maximum of 2 electrons, and they must have opposite spins.
- Hund's rule: within a subshell, electrons spread into empty orbitals one at a time before any orbital gets a second electron — like strangers on a bus taking empty seats before doubling up.
Worked examples
Try writing each configuration before reading the answer. Count total electrons: for a neutral atom, that equals the atomic number.
Hydrogen (Z = 1): one electron → 1s¹
Carbon (Z = 6): 2 fill the 1s, 2 fill the 2s, 2 go into 2p → 1s² 2s² 2p²
Oxygen (Z = 8): → 1s² 2s² 2p⁴
Sodium (Z = 11): → 1s² 2s² 2p⁶ 3s¹. Everything before that final 3s¹ is just neon's configuration, so chemists abbreviate it as [Ne] 3s¹ — the noble-gas shorthand. That lone 3s electron is sodium's single valence electron, and it's why sodium so eagerly forms Na⁺.
Calcium (Z = 20): → 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², or simply [Ar] 4s²
Iron (Z = 26): after [Ar] 4s² the next 6 electrons enter 3d → commonly written [Ar] 3d⁶ 4s²
The last block of the configuration maps straight onto the periodic table: elements filling an s subshell sit in the tall left columns (the s-block — helium being the well-known stray, parked top-right with the noble gases), p-fillers sit on the right (p-block), and the d-block is the transition-metal bridge in the middle. Read across a period and you are literally watching subshells fill.
Why configurations matter
Chemistry is done by the outermost electrons. Atoms with the same outer configuration behave alike — that's why lithium ([He] 2s¹), sodium ([Ne] 3s¹), and potassium ([Ar] 4s¹) all react as one family. Configurations also explain why noble gases are unreactive (full outer shells), why chlorine grabs one electron (one short of a full 3p), and — as you'll see later this week — why it costs so much energy to rip an electron out of a full shell.
Common mistakes to avoid
- Filling 3d before 4s. The Aufbau order goes 4s first: potassium is [Ar] 4s¹, not [Ar] 3d¹. (A twist for later courses: when transition metals form ions, the 4s electrons are the first to leave.)
- Ignoring Hund's rule. Nitrogen's three 2p electrons occupy three separate orbitals with parallel spins — they don't pair up in one orbital.
- Treating the superscript as math. 2p⁶ means "six electrons in the 2p subshell," not 2p × 2p × … The total electron count is found by adding superscripts.
- Expecting zero exceptions. Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹ — half-filled and filled d subshells bring extra stability. Intro exams mostly want the standard pattern, but these two are famous.
FAQ
What does 1s² 2s² 2p⁶ mean?
Two electrons in the 1s subshell, two in the 2s, six in the 2p — ten electrons total, which is the configuration of neon (and of ions like Na⁺ and F⁻ that have ten electrons).
Why does 4s fill before 3d?
For a neutral atom being built up, the 4s subshell is slightly lower in energy than 3d, so Aufbau order puts electrons there first. Once 3d starts filling, the energies shuffle — which is why 4s electrons are removed first when ions form.
Which element is 1s² 2s² 2p⁶ 3s² 3p⁴?
Add the superscripts: 2 + 2 + 6 + 2 + 4 = 16 electrons → sulfur.
What is the noble-gas shorthand?
Replace the inner-electron portion of a configuration with the previous noble gas in brackets: calcium's 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² becomes [Ar] 4s². It highlights the valence electrons — the ones that do chemistry.
The takeaway
An electron configuration is the map of where an atom's electrons live: shell number, subshell letter, electron count. Fill from the bottom up (Aufbau), two per orbital (Pauli), spread before pairing (Hund), and the code writes itself — and once you can read it, families of the periodic table, ion charges, and reactivity patterns all snap into place.
Configurations are all about the outer shell, so brush up with What Is a Valence Electron? Shells and Bonding and What Is an Atom? The Building Block of Everything. Next in this series: [Electron Shells vs Orbitals] and [What Is Ionization Energy?].
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