Hydrogen (H)

A hydrogen atom is one proton and one electron, which makes it the only element whose exact quantum mechanics can be solved on paper. It is about 90% of all atoms in the universe by count, but only about 0.14% of the Earth's crust by mass, because the young Earth was too warm and too small to hold on to it. Almost all the hydrogen you will ever meet is locked into water or hydrocarbons rather than floating free.

Properties

Atomic number

1

Symbol

H

Standard atomic weight

1.008 u

Category

Nonmetal

Group

1

Period

1

Block

s-block

Electron configuration

1s¹

Electrons per shell

1

Melting point

13.81 K — -259.3 °C — -434.8 °F

Boiling point

20.28 K — -252.9 °C — -423.2 °F

Density

0.00008988 g/cm³

Electronegativity

2.2 (Pauling)

Oxidation states

+1, -1

Discovered

1766 by Henry Cavendish

Electron configuration

Full

1s¹

Noble gas shorthand

1s¹

Shells

K: 1

What it is used for

• Ammonia synthesis for fertiliser — by far the largest single use
• Removing sulfur from crude oil in refinery hydrotreaters
• Turning liquid vegetable oils into solid fats
• Fuel-cell buses, trains and forklifts
• Coolant inside large power-station generators

Hazards

• Flammable in air across an unusually wide range, roughly 4% to 75% by volume
• Burns with a flame that is nearly invisible in daylight
• Colourless and odourless, so leaks are not detectable by smell

IUPAC publishes an interval, [1.00784, 1.00811], because the deuterium fraction varies between natural waters; 1.008 is the conventional value. The density given is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 1

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Hydrogen sits in group 1 with lithium, sodium, potassium, rubidium, cesium, francium.