Hydrogen – facts and properties
- Symbol
- H
- Atomic number
- 1
- Atomic mass
- 1.008 u
- Period
- 1
- Group
- 1
- Category
- diatomic nonmetal
- Electron configuration
- 1s¹
- Electronegativity
- 2.2
- Atomic radius
- 53 pm
- Valence electrons
- 1
- State at standard conditions
- gas
- Melting point
- -259.34 °C
- Boiling point
- -252.87 °C
- Density
- 0.0899 g/L
- Discovered
- 1766
Hydrogen is the simplest atom there is: one proton and one electron. It has atomic number 1 and opens the periodic table. Although it makes up most of the visible matter in the universe, hardly any of it exists as a free gas on Earth; almost all of it is locked up in water and in the molecules of living things. Today hydrogen is discussed as a clean fuel, but it has been a workhorse of the chemical industry for more than a century.
Position in the periodic table
Where hydrogen is found and how it is produced
Stars are made mostly of hydrogen. In their cores, hydrogen nuclei fuse into helium and release the energy that makes them shine, which is why hydrogen is by far the most common element in the universe. On Earth the picture is different. Hydrogen gas is so light that the planet's gravity cannot hold it: free H₂ molecules drift to the top of the atmosphere and escape into space, so the air contains only traces. Bound hydrogen, however, is everywhere. Every water molecule contains two hydrogen atoms; the hydrocarbons in natural gas and crude oil are built from hydrogen and carbon, and coal contains hydrogen too. Every living cell contains hydrogen in sugars, fats, proteins and DNA.
Most industrial hydrogen is made from natural gas by steam reforming. Methane and steam react over a nickel catalyst at high temperature to give hydrogen and carbon monoxide; a second step converts the carbon monoxide to carbon dioxide and more hydrogen. Because carbon dioxide is released, this is called grey hydrogen. The alternative is electrolysis: an electric current splits water into hydrogen and oxygen. If the electricity comes from wind or solar power, the product is called green hydrogen and no carbon dioxide is emitted. In the school laboratory, hydrogen is usually prepared by dropping zinc into dilute hydrochloric acid. Hydrogen was first recognised as a distinct substance by Henry Cavendish in 1766; Antoine Lavoisier later named it from the Greek for water-former, because burning it produces water.
Properties and reactions
With the electron configuration 1s¹, a hydrogen atom has a single electron, which is also its only valence electron. Two atoms share their electrons to form the H₂ molecule. The gas is colourless and odourless, and with a density of 0.0899 g/L it is the lightest of all gases. It condenses to a liquid only at -252.87 °C and freezes at -259.34 °C. Hydrogen sits in period 1 and group 1, but it is not an alkali metal: its electronegativity of 2.2 lies roughly midway between typical metals and non-metals. Hydrogen can therefore share electrons in covalent bonds, as in water and methane, or accept an electron to form the hydride ion, as in sodium hydride.
The best-known reaction of hydrogen is combustion. A mixture of hydrogen and oxygen explodes when ignited and forms water, releasing a great deal of heat. In the laboratory the squeaky pop test uses this reaction to identify the gas. Hydrogen also combines with the halogens to give hydrogen halides, and with nitrogen under pressure and heat to give ammonia. As a reducing agent it removes oxygen from many metal oxides; passing hydrogen over hot copper oxide leaves behind shiny copper. Three isotopes occur in nature. Ordinary hydrogen-1 has no neutron, hydrogen-2 (deuterium) has one, and hydrogen-3 (tritium) has two and is radioactive. Water made with deuterium, known as heavy water, is used in some nuclear reactors.
Uses in everyday life and technology
The chemical industry is the biggest consumer of hydrogen. In the Haber-Bosch process it is combined with nitrogen to make ammonia, the starting point for the fertilisers that feed a large share of the world's population. Oil refineries use hydrogen to strip sulfur from fuels and to crack heavy fractions into petrol and diesel. Hydrogen is also a raw material for methanol, and it is used to harden vegetable oils into spreadable fats such as margarine.
As an energy carrier, hydrogen is gaining ground. A fuel cell combines hydrogen and oxygen in a controlled way, producing electricity, heat and nothing but water as exhaust. Fuel-cell buses, lorries and trains are already on the road and on the rails. Steelmakers are testing hydrogen as a replacement for coke in the reduction of iron ore, which would turn one of the largest industrial sources of carbon dioxide into a process that emits water vapour. Liquid hydrogen fuels the upper stages of large rockets. Historically, hydrogen filled airships until the Hindenburg disaster shifted the industry to helium. In daily life you rarely meet hydrogen as a gas, yet it is present in the fertiliser on fields, the fuel in cars and the plastics around you.
Molecules containing this element
- Hydrogen H2
- Ammonia NH3
- Hydrogen bromide HBr
Quick question
Which of these elements has the highest electronegativity?
FAQ
Is hydrogen dangerous?
Hydrogen is not toxic, but it is highly flammable. Mixed with air over a wide range of concentrations it forms an explosive mixture that a tiny spark can ignite, and its flame is almost invisible in daylight. Because it is so light it disperses quickly outdoors, but in a closed room it collects under the ceiling. Laboratories work with small quantities, good ventilation and no naked flames.
Why is hydrogen in group 1 if it is not a metal?
Like the alkali metals, hydrogen has one valence electron, which is why it is placed at the top of group 1. Chemically it behaves very differently: it is a gas, it forms molecules and it does not give up its electron as readily as sodium or potassium. Some periodic tables therefore set it apart or show it above the halogens as well, since it is also one electron short of a filled shell.
What is the difference between grey, blue and green hydrogen?
The colours describe how the hydrogen was made. Grey hydrogen comes from natural gas and releases carbon dioxide into the atmosphere. Blue hydrogen is made the same way, but the carbon dioxide is captured and stored. Green hydrogen is produced by electrolysis of water using electricity from renewable sources, so no carbon dioxide is emitted at all.