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

Preparing orbital…

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 the radius being calculated 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