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 Melting point 842°C, 1548°F, 1115 K 
Period Boiling point 1484°C, 2703°F, 1757 K 
Block Density (g cm−3) 1.54 
Atomic number 20  Relative atomic mass 40.078  
State at 20°C Solid  Key isotopes 40Ca 
Electron configuration [Ar] 4s2  CAS number 7440-70-2 
ChemSpider ID 4573905 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 spiral shell and bones reflect the essential presence of calcium in all living things.
Calcium is a silvery-white, soft metal that tarnishes rapidly in air and reacts with water.
Calcium metal is used as a reducing agent in preparing other metals such as thorium and uranium. It is also used as an alloying agent for aluminium, beryllium, copper, lead and magnesium alloys.

Calcium compounds are widely used. There are vast deposits of limestone (calcium carbonate) used directly as a building stone and indirectly for cement. When limestone is heated in kilns it gives off carbon dioxide gas leaving behind quicklime (calcium oxide). This reacts vigorously with water to give slaked lime (calcium hydroxide). Slaked lime is used to make cement, as a soil conditioner and in water treatment to reduce acidity, and in the chemicals industry. It is also used in steel making to remove impurities from the molten iron ore. When mixed with sand, slaked lime takes up carbon dioxide from the air and hardens as lime plaster.

Gypsum (calcium sulfate) is used by builders as a plaster and by nurses for setting bones, as ‘plaster of Paris’.
Biological role
Calcium is essential to all living things, particularly for the growth of healthy teeth and bones. Calcium phosphate is the main component of bone. The average human contains about 1 kilogram of calcium.

Children and pregnant women are encouraged to eat foods rich in calcium, such as milk and dairy products, leafy green vegetables, fish and nuts and seeds.
Natural abundance
Calcium is the fifth most abundant metal in the Earth’s crust (4.1%). It is not found uncombined in nature, but occurs abundantly as limestone (calcium carbonate), gypsum (calcium sulfate), fluorite (calcium fluoride) and apatite (calcium chloro- or fluoro-phosphate).

Hard water contains dissolved calcium bicarbonate. When this filters through the ground and reaches a cave, it precipitates out to form stalactites and stalagmites.

Calcium metal is prepared commercially by heating lime with aluminium in a vacuum.
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Lime (calcium oxide, CaO) was the useful material obtained by heating limestone and used for centuries to make plaster and mortar. Antoine Lavoisier classified it as an ‘earth’ because it seemed impossible to reduce it further, but he suspected it was the oxide of an unknown element. In 1808, Humphry Davy tried to reduce moist lime by electrolysis, just as he had done with sodium and potassium, but he was not successful. So he tried a mixture of lime and mercury oxide and while this produced an amalgam of calcium and mercury, it was not enough to confirm that he’d obtained a new element. (Jöns Jacob Berzelius had conducted a similar experiment and also obtained the amalgam.) Davy tried using more lime in the mixture and produced more of the amalgam from which he distilled off the mercury leaving just calcium.

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) 2.31 Covalent radius (Å) 1.74
Electron affinity (kJ mol−1) 2.369 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 2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  40Ca 39.963 96.941 5.92 x 1021 EC-EC 
  42Ca 41.959 0.647
  43Ca 42.959 0.135
  44Ca 43.955 2.086
  46Ca 45.954 0.004 > 0.4 x 1016 β-β- 
  48Ca 47.953 0.187 4.4 x 1019 β-β- 
        > 7.1 x 1019 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 5.5
Crustal abundance (ppm) 41500
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) 65.2
Reserve distribution (%) Unknown
Top 3 producers
  • 1) China
  • 2) USA
  • 3) India
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)
647 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 17.2
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- 2.36
x 10-5
0.146 25.5 - - - - - - -
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Listen to Calcium Podcast
Transcript :

Chemistry in its element: calcium


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, welcome to this week's Chemistry in its Element, I'm Chris Smith. This week it's the turn of the element that gives us cement, plaster of Paris, our own bones, hard teeth and hard water.

Karen Faulds

Milk, cheese, yogurt, spinach, almonds. What element do they all have in common? It's calcium of course! But whilst most off us immediately think of food when someone mentions calcium (and I personally hold the old milk TV adverts accountable for this), it actually has a far bigger role in our lives than that. Calcium is all around us. The average human contains approximately 1kg of calcium, of which 99% is stored in our bones. It is the 5th most abundant element in the earth's crust, occurring widely as calcium carbonate which is more commonly known as limestone. It is also the fifth most abundant dissolved ion in seawater.

Calcium was named after the Latin term calx meaning lime, and is a reactive silvery metallic element found in Group 2 of the periodic table. It was first isolated in 1808 in England when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Today we obtain calcium through the electrolysis of a fused salt such as calcium chloride. Once exposed to air, elemental calcium rapidly forms a grey-white oxide and nitride coating. Unlike magnesium, calcium is quite difficult to ignite, but once lit, it burns with a brilliant high-intensity red flame.

The compounds of calcium are however much more useful than the element itself. Literature dating back to 975 AD shows that plaster of Paris (which is calcium sulphate) was used even then for setting broken bones. Calcium oxide (also known as lime or quicklime) is a major component of mortar and cement. The production of cement using calcium oxide has long been known; it was used by the Romans and also the Egyptians who built the Great Pyramid of Giza and Tutankhamen's tomb. Calcium fluoride is also well known for being insoluble and transparent over a wide range of wavelengths, making it useful for making cells and windows for infrared and ultraviolet spectrometers.

Our drinking water also contains calcium ions - more so in so called hard water areas. Hard water is the term used for water with a high proportion of calcium and magnesium (2 plus) ions. The calcium usually enters the water as it flows past either calcium carbonate, from limestone and chalk, or calcium sulfate, from other mineral deposits. Whilst some people do not like the taste, hard water is generally not harmful to your health. Although it does make your kettle furry! Interestingly, the taste of beer (something dear to my heart) seems related to the calcium concentration of the water used, and it is claimed that good beer should have a calcium concentration that is higher than that of hard tap water.

Calcium is what is known as an essential element, meaning that it is an element which is absolutely necessary for life processes. Which is what the old milk TV adverts were trying to tell us after all. Calcium is used to produce the minerals contained in bones, shells and teeth through a process called biomineralisation. Calcium phosphate (also known as hydroxyapatite) is the mineral component of bones and teeth and is a particularly good example of how organisms fabricate 'living' composite materials. Indeed, the different properties (such as stiffness) of bone are produced by varying the amount of organic component, mostly a fibrous protein called collagen, with which hydroxyapatite is associated. The bone in our body functions not only as a structural support, but also as the central Ca store. Thus, during pregnancy, bones tend to be raided for their Ca in a process called demineralisation. Bone does not last forever; a serious medical problem is osteoporosis which is the decalcification of bone. This loss of bone mass which occurs with increasing age makes bones more susceptible to breaking under stress and it occurs mainly in older people, especially women.

Calcium ions also play a crucial role in higher organisms as an intracellular messenger. Fluxes of Ca2+ trigger enzyme action in cells in response to receiving a hormonal or electrical signal from elsewhere in the organism. Calcium is also very important in helping blood to clot. When bleeding from a wound suddenly occurs, platelets gather at the wound and attempt to block the blood flow. Calcium, vitamin K, and a protein called fibrinogen help the platelets to form a clot. If your blood is lacking calcium or one of these other nutrients, it will take longer than normal for your blood to clot.

The ability to detect extremely small amounts of an element can be a very useful adaptation for an animal if that element is important to it. For example, hermit crabs, which inhabit second hand shells and change to newer, bigger shells as they grow, have the ability to recognise shells suitable for occupation not only by feeling for them, but apparently also by measuring the minute amount of calcium carbonate that is dissolved in the water around a shell. They can readily distinguish natural shells containing calcium carbonate from calcium-bearing replicas made from calcium sulphate. The concentration of calcium detected by the hermit crab is in the order of 4ppm or less, which is amazingly low.

So from strong teeth and bones, through to good tasting beer and ensuring hermit crabs find their perfect home -you can see that calcium really is an essential element.

Chris Smith

Well, I'm very at home with my hard water, and the local beer tastes quite good too, although I do get through quite a few kettles - indeed Russell Hobbs probably owe their buoyant share price just down to me. Well, maybe. That was Strathclyde University's Karen Faulds with the story of Calcium. Next week, if you were an element which one would you be?

Pat Bailey

If I had to choose a person to represent gold, then I guess it might be an ambitious young stockbroker, a bit flashy, and not great at forming relationships. For helium - an airy-fairy blonde with a bit of a squeaky voice, but with aspirations to join the nobility. And for boron? Well at first glance a boring, middle-aged accountant, maybe wearing brown corduroys and a tweed jacket . but with an unexpected side-to him in his spare time - skydiving, and a member of a highly dubious society that indulges in swapping partners.

Chris Smith

And you can get the inside story on Boron's swinging antics with Pat Bailey in next week's Chemistry in its Element. I'm Chris Smith, thank you for listening and goodbye.


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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  Help Text


Description :
In this experiment you will be observing and interpreting the changes when drops of solutions of various anions are added to drops of solutions of group 2 element cations.
Description :
Metals in group 2 of the periodic table are less reactive than those in group 1. This experiment indicates the relative reactivity of elements within the group.
Description :
Many elements react with oxygen on heating. These reactions and the properties of their products illustrate the periodic nature of the elements.
Description :
Many elements react with chlorine on heating. The reactions and the properties of the products illustrate the periodic nature of the elements. The reactions require less energy input to initiate than ...
Description :
Calcium carbonate is strongly heated until it undergoes thermal decomposition to form calcium oxide and carbon dioxide. The calcium oxide (unslaked lime) is dissolved in water to form calcium hydroxid...
Description :
Short videos of exciting demonstrations of the chemistry of everyday materials, taken from a lecture by Peter Wothers at the University of Cambridge.

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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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