1HHydrogen1.0081s¹
diatomic nonmetal

Hydrogen

One proton defines hydrogen. A neutral atom has just one electron. The simplest element connects the chemistry of water with the nuclear reactions that power the Sun.

Properties

At a glance

Atomic number
1
Atomic mass
1.008 u
Phase
Gas
Density
0.08988 g/L
Melting point
13.99 K
Boiling point
20.271 K
First ionization energy
1,312 kJ/mol
Electronegativity
2.2
Electron affinity
72.769 kJ/mol
Molar heat capacity
28.836 J/(mol·K)
Period
1
Appearance
colorless gas

Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.

Electronic structure

Hydrogen’s 1s ground state uses the standard nonrelativistic hydrogen model. Shapes shows the surface holding half its probability; Cloud shows dots where its electron is likely to be. Display size is illustrative.

Electrons by shell
Electron configuration

Preparing the orbital view…

1 proton0 neutrons

Hydrogen-1: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.

States of matter

Turn up the heat

298 KK stands for kelvin, the temperature scale used in science. It starts at absolute zero (0 K), the coldest temperature possible, so it has no negative numbers and no degree sign.

A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas

At this temperature, hydrogen is a gas.

At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.

Emission spectrum

A fingerprint made of light

Hpinkish purple

Every visible line of hydrogen ends on level 2, a family called the Balmer series. The bright red line at 656 nm, from level 3 down to level 2, is why clouds of glowing hydrogen in space look red in photographs.

Heated or electrified, hydrogen gives off light only at the exact wavelengths below. Each one comes from an electron falling between two energy levels, so the pattern is unique to the element, like a fingerprint.

Wavelength in nanometresA nanometre (nm) is a billionth of a metre. Our eyes see light from about 380 nm (violet) to 750 nm (red); a human hair is about 80,000 nm wide.
Hydrogen spectral lines

Click a line to see where it comes from.

656.3 nmred

An electron falls from level 3 down to level 2. It loses 1.89 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of red light. Bigger drops give bluer light.

Falls to level 1 give ultraviolet light (the Lyman series) and falls to level 3 give infrared (the Paschen series), so neither shows up in this visible range.

Energy1234570 eV

The strongest visible lines of neutral hydrogen from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.

Compared with other elements

How does it compare?

HydrogenGroup 1Other elements

Density

0.0000899 g/cm³The least dense of 99 elements

H

Melting point

14 KThe lowest melting point of 96 elements

H

Boiling point

20.3 K2nd lowest of 97

H

Electronegativity

2.216th highest of 96

H

First ionization energy

1,312 kJ/mol8th highest of 99

H

Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.

A bright loop of solar plasma rises above the edge of the Sun.

The fuel of a star

A solar prominence observed by the Solar Dynamics Observatory. The Sun’s energy originates much deeper, in its core.

⁠Image: Solar Dynamics Observatory / NASA

From hydrogen to helium

At the Sun’s core, extreme temperatures and pressure allow hydrogen nuclei to fuse into helium. These nuclear reactions release the energy that eventually reaches us as sunlight.

What makes an element?

The proton count is the defining feature. Every hydrogen nucleus has one proton. Isotopes can have different neutron counts, and ions can gain or lose electrons, without becoming a different element.

Keep exploring

Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.