What is helium's electron configuration?

Helium's electron configuration is 1s². Both electrons occupy the 1s orbital in the first principal energy level (n = 1). This completely fills the first shell and gives helium a full-shell noble-gas configuration.

What are the quantum numbers for helium's two electrons?

Both electrons share n = 1, l = 0, and ml = 0. They differ only in spin: the first electron has ms = +½ (spin-up) and the second has ms = −½ (spin-down). This satisfies the Pauli Exclusion Principle, which requires at least one quantum number to differ.

Why is helium chemically inert?

Helium's 1s orbital is completely filled with two electrons. A full valence shell (n = 1 holds a maximum of 2 electrons) means helium has no tendency to gain, lose, or share electrons. It belongs to Group 18 (the noble gases) precisely because of this stable configuration.

Does helium follow Hund's rule?

Hund's rule applies when multiple degenerate (equal-energy) orbitals are available. The 1s subshell contains only one orbital, so there is nowhere else for the second electron to go. Helium's two electrons must pair in the same orbital with opposite spins — Hund's rule is trivially satisfied by default.

What is the abbreviated electron configuration of helium?

Because helium itself is a noble gas, its configuration IS the core reference for Period 2 elements. Elements like lithium are written [He] 2s¹, where [He] abbreviates the filled 1s² core. Helium's own configuration cannot be abbreviated further.

Why does helium have the highest first ionization energy of all elements?

Helium's two electrons are in the n = 1 shell, very close to the nucleus with no inner shielding electrons. The nuclear charge (Z = 2) acts almost fully on each electron, requiring 24.59 eV (2372 kJ/mol) to remove one electron — the highest first ionization energy on the periodic table.

Helium Electron Configuration

Explore helium's 1s² electron configuration, orbital box diagram, all four quantum numbers for each electron, and key atomic properties in one interactive reference.

Shell Visualization

e⁻ 1 (mₛ = +½)

e⁻ 2 (mₛ = −½)

Two electrons orbiting the nucleus in shell n = 1, 180° apart.

Electron Configuration

1s 2

Principal level (n)

Orbital type

Electrons

Abbreviated: [He] — helium itself defines the noble-gas core used by Period 2 elements.

Orbital Box Diagram

↑ ↓

1s orbital is fully paired: spin-up (m_s = +½) and spin-down (m_s = −½).
The Pauli Exclusion Principle requires the two electrons to have opposite spins.

Quantum Numbers — Both Electrons

Symbol	Name	e⁻ 1	e⁻ 2	Meaning
n	Principal	1	1	First energy shell — closest to the nucleus.
l	Azimuthal	0	0	l = 0 denotes an s orbital (spherical shape).
m_l	Magnetic	0	0	Only one orientation for l = 0 (no directionality).
m_s	Spin	+½	−½	Opposite spins required by the Pauli Exclusion Principle.

The two electrons share the same n, l, and m_l values. They must differ in m_s — one spin-up, one spin-down.

Key Atomic Properties

Atomic Number (Z) 2 2 protons, 2 electrons

Atomic Mass 4.003 atomic mass units (u)

1st Ionization Energy 24.587 eV (2372 kJ/mol) — highest of all elements

2nd Ionization Energy 54.418 eV (5251 kJ/mol)

Electron Affinity ≈ 0 Does not form stable negative ions

Van der Waals Radius 140 pm (smallest noble gas)

Principles Applied

1
Aufbau Principle
Electrons fill orbitals from lowest to highest energy. The 1s orbital has the lowest energy, so both of helium's electrons go there first. With Z = 2, there are exactly enough electrons to fill the 1s orbital completely.
2
Pauli Exclusion Principle
No two electrons in the same atom can have an identical set of four quantum numbers. Because both electrons occupy 1s (n=1, l=0, ml=0), they must differ in spin: one is mₛ = +½, the other mₛ = −½. This is the key constraint that limits each orbital to at most two electrons.
3
Hund's Rule
Electrons in degenerate orbitals occupy separate orbitals with parallel spins before pairing. The 1s subshell contains only one orbital — there are no degenerate orbitals available. Hund's rule does not restrict the filling of helium's 1s orbital; pairing is the only option.

Summary

Helium (He) is the second element on the periodic table and the simplest noble gas, with an atomic number of 2. Both of its electrons occupy the 1s orbital, giving a full electron configuration of 1s². The first shell (n = 1) holds a maximum of two electrons — helium fills it completely, making it exceptionally stable. This tool provides an animated Bohr model, orbital box diagram, quantum number table for each electron, and key atomic properties.

How it works

The page loads helium's fixed electron configuration (1s²) automatically — no input needed.
The animated Bohr model shows two electrons orbiting the nucleus in the n = 1 shell, 180° apart.
The orbital box diagram displays the 1s orbital filled with one spin-up (↑) and one spin-down (↓) electron.
The quantum number table lists all four quantum numbers for each of the two electrons.
The properties panel shows atomic number, mass, ionization energy, and other key data.
The principles section explains how the Aufbau, Pauli, and Hund rules apply to helium.

Use cases

Verify helium's electron configuration quickly for chemistry homework or exams.
Study how the Pauli Exclusion Principle enforces opposite spins in a paired orbital.
Use as a reference when learning the Aufbau principle for heavier elements.
Understand why helium is chemically inert — its 1s orbital is completely full.
Compare helium's 1s² configuration with hydrogen's 1s¹ side by side.
Teach the concept of paired electrons and noble-gas stability in introductory chemistry.
Review ionization energy concepts — helium has the highest first ionization energy of all elements.
Prepare for AP Chemistry, IB Chemistry, or university-level quantum mechanics coursework.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu