Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group 13  Melting point 29.7646°C, 85.5763°F, 302.9146 K 
Period Boiling point 2229°C, 4044°F, 2502 K 
Block Density (g cm−3) 5.91 
Atomic number 31  Relative atomic mass 69.723  
State at 20°C Solid  Key isotopes 69Ga 
Electron configuration [Ar] 3d104s24p1  CAS number 7440-55-3 
ChemSpider ID 4514603 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image reflects on puns relating to the origin of the element’s name. Lecoq de Boisbaudran named the element after France (‘Gaul’ in Latin) and also himself, since Lecoq, which means ‘the rooster’ translates to ‘Gallus’ in Latin. A silvery metallic rooster is shown on a background of an antique map of France.
Gallium is a soft, silvery-white metal, similar to aluminium.
Gallium arsenide has a similar structure to silicon and is a useful silicon substitute for the electronics industry. It is an important component of many semiconductors. It is also used in red LEDs (light emitting diodes) because of its ability to convert electricity to light. Solar panels on the Mars Exploration Rover contained gallium arsenide.

Gallium nitride is also a semiconductor. It has particular properties that make it very versatile. It has important uses in Blu-ray technology, mobile phones, blue and green LEDs and pressure sensors for touch switches.

Gallium readily alloys with most metals. It is particularly used in low-melting alloys.

It has a high boiling point, which makes it ideal for recording temperatures that would vaporise a thermometer.
Biological role
Gallium has no known biological role. It is non-toxic.
Natural abundance
It is present in trace amounts in the minerals diaspore, sphalerite, germanite, bauxite and coal. It is mainly produced as a by-product of zinc refining.

The metal can be obtained by electrolysis of a solution of gallium(III) hydroxide in potassium hydroxide.
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Gallium was discovered in Paris by Paul-Émile Lecoq de Boisbaudran in 1875. He observed a new violet line in the atomic spectrum of some zinc he had extracted from a sample of zinc blende ore (ZnS) from the Pyrenees. He knew it meant that an unknown element was present.

What Boisbaudran didn’t realise was that its existence, and properties, had been predicted by Mendeleev whose periodic table showed there was a gap below aluminium which was yet to be occupied. He forecast that the missing element’s atomic weight would be around 68 and its density would be 5.9 g/cm3.

By November of 1875, Boisbaudran had isolated and purified the new metal and shown that it was like aluminium. In December 1875 he announced it to the French Academy of Sciences.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 1.87 Covalent radius (Å) 1.23
Electron affinity (kJ mol−1) 41.49 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  69Ga 68.926 60.108
  71Ga 70.925 39.892 > 2.4 x 1026 β- 


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk 7.6
Crustal abundance (ppm) 16
Recycling rate (%) <10
Substitutability Medium
Production concentration (%) 54
Reserve distribution (%) Unknown
Top 3 producers
  • 1) China
  • 2) Germany
  • 3) Kazakhstan
Top 3 reserve holders
  • Unknown
Political stability of top producer 24.1
Political stability of top reserve holder Unknown


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
373 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - 1.94
x 10-7
0.000565 0.114 4.98 84.4 - - - -
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Listen to Gallium Podcast
Transcript :

Chemistry in its element: gallium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Chris Smith

Hello and this week to the story of the element that's named after a rooster although the man here to tell us about it actually chickened out when it came to eating some of this chemical, although he did confess to giving it a quick lick. And to tell us how it tasted and why Gallium could hold the key to the next generation of LEDs, here's Andrea Sella.

Andrea Sella

When I was a child growing up in New York, some of the sweets most sort after by my classmates and me, with yellow and brown packs of highly coloured sugar coated chocolate pills bearing the characters M & M. You could pop them into your mouth one by one and suck them gently until the smooth surface became crumbling to reveal the smooth milk chocolate beneath; alternatively you cold cram your mouth with as many as you could and crunch them greedily to cause an explosion of sound, texture and flavour in your head. A secret pleasure that was hard to beat. I was reminded of all this when a colleague of mine who was having a lab clear out, knocked on my door and asked me knowing full well what my answer would be, 'Hi Andrea, would you like a lump of Gallium?', 'of course I would love some Gallium', I gurgled. The M & M of the elements; the one which reputedly melts in your mouth but not in your hand, he handed me a small plastic bag badly stained with black smudges. I undid the knot eagerly and there it was, a gleaming silvery lump bearing all the hallmarks of a metal that had been repeatedly melted and then refrozen.

Gallium, you see, melts at 30oC, which means that on a hot day, you hold it in your pocket at your peril. Surprisingly however it's not very volatile. In fact Gallium has the largest liquid range of any material known to man. Its boiling point is just over 2400oC. So unlike other liquid metals, there is no toxic vapour to worry about. Bizarrely as well, the metal contracts as it melts, rather like water. So solid Gallium floats on its liquid, a property shared only by a couple of other elements, Bismuth and Antimony. The reason for this weird melting behaviour has been a matter of argument and speculation for about 50 years. It's now fairly well established that Gallium surrounds itself with more of its neighbours when in the liquid than in the solid, although the reasons for this still remains obscure. 

Yet for all its strangeness the discovery of this odd element was no accident. Dmitri Mendeleev, the bearded Russian chemist who constructed the periodic table as we know it today, spotted a number of gaps and discrepancies in his arrangement. One of these was the absence of an element which he expected to fit below Aluminum. So confident was he in the correctness of his framework that he named the as yet undiscovered element ekaaluminium. Six years later in 1875, an ambitious French element hunter François Lecoq de Boisbaudran one of the earliest proponents of the new-fangled technique of spectroscopy spotted a line in the violet part of the visible spectrum at 417nm in a sample of zinc sulphide, he realized that this must come from a new element. Working in his home laboratory in spite of starting from some 52 kilos of an ore from the Pyrenees, it took three weeks for him to accumulate a couple of milligrams of the mysterious material. He then scaled up his extraction and took the product of his labours to Paris where he studied it further in Adolphe Wurtz's lab. Just before Christmas in 1875, Lecoq presented his results to the French academy proudly displaying a sample of almost 600mg, less than a gram of material harvested from 450 kilos of ore. And the name Lecoq patriotically chose to base it on the Latin name for France, Gallia; Gaul in English. But it was immediately pointed out that there might be something more to the name than met the eye. The Latin word for a rooster is Gallus, Lecoq, rooster, Gallium, get it. It seems he may have been a rather cunning linguist as well as a chemist. Either way, Lecoq could look back with some satisfaction at having helped to cement Mendeleev's table, was the foundation stone of chemistry. He then moved on to the intriguing mystery of the 'rare earths', ultimately isolating two more elements and conforming the existence of several more.

Gallium soon moved into the main stream of chemistry. Nowadays the metal itself finds few uses, but its compound with arsenic, gallium arsenide has for several years been touted as a possible replacement for Silicon. Since not only is it a semiconductor but it is one with a direct band gap, in other words it can be made to emit light, a property which is particularly useful for infrared but also visible LEDs. Gallium arsenide solar cells are also much more efficient than those made of conventional Silicon and are being used in solar powered cars and in space probes.

But I'm sure you really want to know is, if this really is the M & M element, what does it taste like? I knew you would ask. So I had a quick lick a couple of days back and the answer is it doesn't actually taste very much to be honest. There's a faintly astringent, metallic taste which lingers on your tongue for few hours. And when it is molten, sorry I'll leave that experiment for someone more intrepid than I.

Chris Smith

UCL chemist Andrea Sella with the story of gallium, the element that Lecoq allegedly named after himself. Next week we are meeting the metal that powers nuclear rectors but can also be lethal for another reason.

Polly Arnold

Because it is so dense DU is also used in shielding in the keels of boats and more controversially in the noses of armour piercing weapons. The metal has the desirable ability to self sharpen as it pierces a target rather than mushrooming upon impact, the way conventional tungsten carbide tipped weapons do.

Chris Smith

DU being Depleted Uranium of course and Edinburgh chemist Polly Arnold will be here to tell us its story as well as revealing why it actually makes very beautiful glass on next week's Chemistry in its element. I hope you can join us. I'm Chris Smith, thank you for listening and good bye.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at
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Visual Elements images and videos
© Murray Robertson 2011.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J.J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions(version 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.
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