Electron Configuration

See how electrons fill atomic orbitals.

Pick An Element

Element

OOxygen

Atomic number

Atomic Notation

Proton: 8
Electron: 8
Neutron: 8

Electron configuration

$1s^{2}$ $2s^{2}$ $2p^{4}$

K shell
L shell

Shell distribution

One possible last-electron state

Selected state

2p^{\downarrow}

(n,\ell,m_l,m_s)

(2, 1, -1, -\frac{1}{2})

Degenerate orbitals have no unique filling order; this tuple uses one conventional assignment.

Electron Orbital in 3D

Occupied subshell

Orbital orientation

Positive phase Negative phase

1a_0 \approx 52.9\,\mathrm{pm}

2p_z

·2 electrons

The shaded lobes illustrate angular probability. Turn off Shape to explore dots sampled from $|\psi|^2$ . Blue and orange show opposite wavefunction phases, not electric charge. The surface omits radial nodes and is not a probability boundary.

The scale uses Bohr radii ( $1a_0 \approx 52.9\,\mathrm{pm}$ ). For this hydrogen-like orbital, 90% of radial probability lies within $8.0a_0 \approx 423\,\mathrm{pm}$ of the nucleus. These model sizes are not measured sizes of Oxygen. Real orbital orientations can combine magnetic states, so axis names do not correspond one-to-one with Aufbau $m_l$ labels.

Why are $s$ orbitals round and $p$ orbitals two-lobed?

The wave separates into a distance factor $R$ and a direction factor $Y$ . For $s$ , $Y$ is constant in every direction. For the pictured $p_y$ , $Y \propto \cos\theta$ : it changes sign across the horizontal nodal plane and is zero there. The lobes plot $|Y|^2$ in each direction.

Compare the shape

Angle from

+y

: $45^{\circ}$

$Y_1^0 \propto \cos\theta$ · signed amplitude $0.71$

Follow the equation to all orbital shapes.

Aufbau Diagram

$1s$
$2s$
$2p$
$3s$
$3p$
$4s$
$3d$
$4p$
$5s$
$4d$
$5p$
$6s$
$4f$
$5d$
$6p$
$7s$
$5f$
$6d$
$7p$

n\setminus\ell

s

p

d

f

Shell 1

Shell 2

Shell 3

Shell 4

Shell 5

Shell 6

Shell 7

Vocabulary

Electron configuration and quantum-number vocabulary
Symbol	Term	Meaning	Example
$Z$	Atomic number	The number of protons in the nucleus. For a neutral atom, it also equals the number of electrons.	Oxygen: $Z = 8$
$n$	Principal (shell) quantum number	The electron shell or main energy level, numbered from 1 to 7.	$n = 1$ is the K shell
$\ell$	Azimuthal (subshell) quantum number	The subshell and general orbital shape within a shell.	$\mathrm{s}:\ell=0,\quad\mathrm{p}:\ell=1,\quad\mathrm{d}:\ell=2,\quad\mathrm{f}:\ell=3$
$m_l$	Magnetic quantum number	The orbital angular-momentum component along a chosen axis. Real orbital shapes can combine different $m_l$ states.	For $p$ : $-1, 0, +1$
$m_s$	Spin-projection quantum number	The electron's spin component along a chosen axis.	$+\frac{1}{2}\text{ or }-\frac{1}{2}$
K–Q	Shell names	Letter names for the seven electron shells.	$\mathrm{K}\,(n=1),\;\mathrm{L}\,(n=2),\;\ldots,\;\mathrm{Q}\,(n=7)$
$s, p, d, f$	Subshells	The four subshell types, each containing a different number of orbitals.	Maximum: 2, 6, 10, and 14 electrons
$\square$	Orbital	A region that can hold at most two electrons with opposite spins.	One box in the Aufbau diagram
$\uparrow\;/\;\downarrow$	Electron arrows	Electrons and their spin directions inside an orbital.	$\uparrow\downarrow$ is a paired orbital
$2p^4$	Electron configuration	A compact description of how an atom's electrons occupy its subshells.	$2p^4$ = four electrons in the $2p$ subshell
Aufbau	Filling principle	The principle that lower-energy orbitals fill before higher-energy orbitals.	$1s\to 2s\to 2p\to 3s$
Neutral	Neutral atom	An atom whose proton and electron counts are equal, giving no net charge.	8 protons and 8 electrons
Ground state	Ground-state configuration	The lowest-energy electron arrangement of an atom.	The configuration shown by this page