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dull gray, silver

Spectral lines of calcium
General properties
Name, symbol, number calcium, Ca, 20
Pronunciation /ˈkælsiəm/ KAL-see-əm
Element category alkaline earth metal
Group, period, block 24, s
Standard atomic weight 40.078(4)g·mol−1
Electron configuration [Ar] 4s2
Electrons per shell 2, 8, 8, 2 (Image)
Physical properties
Phase solid
Density (near r.t.) 1.55 g·cm−3
Liquid density at m.p. 1.378 g·cm−3
Melting point 1115 K, 842 °C, 1548 °F
Boiling point 1757 K, 1484 °C, 2703 °F
Heat of fusion 8.54 kJ·mol−1
Heat of vaporization 154.7 kJ·mol−1
Specific heat capacity (25 °C) 25.929 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 864 956 1071 1227 1443 1755
Atomic properties
Oxidation states +2, +1[1]
(strongly basic oxide)
Electronegativity 1.00 (Pauling scale)
Ionization energies
1st: 589.8 kJ·mol−1
2nd: 1145.4 kJ·mol−1
3rd: 4912.4 kJ·mol−1
Atomic radius 197 pm
Covalent radius 176±10 pm
Van der Waals radius 231 pm
Crystal structure face-centered cubic
Magnetic ordering diamagnetic
Electrical resistivity (20 °C) 33.6 nΩ·m
Thermal conductivity (300 K) 201 W·m−1·K−1
Thermal expansion (25 °C) 22.3 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3810 m/s
Young's modulus 20 GPa
Shear modulus 7.4 GPa
Bulk modulus 17 GPa
Poisson ratio 0.31
Mohs hardness 1.75
Brinell hardness 167 MPa
CAS registry number 7440-70-2
Most stable isotopes
Main article: Isotopes of calcium
iso NA half-life DM DE (MeV) DP
40Ca 96.941% >5.9×1021 y β+β+ 0.194 40Ar
41Ca trace 1.03×105 y ε - 41K
42Ca 0.647% 42Ca is stable with 22 neutrons
43Ca 0.135% 43Ca is stable with 23 neutrons
44Ca 2.086% 44Ca is stable with 24 neutrons
45Ca syn 162.7 d β 0.258 45Sc
46Ca 0.004% >2.8×1015 y ββ 0.988 46Ti
47Ca syn 4.536 d β 0.694, 1.99 47Sc
γ 1.297 -
48Ca 0.187% 4.3×1019 y ββ 4.274 48Ti
(not observed)
0.0058 48Sc
Very pure calcium metal, not corroded
Calcium metal in air, corroded

Calcium is a chemical element. Its symbol on the periodic table (a list of all the elements) is Ca. Its atomic number is 20. (The atomic number says where Calcium sits in the periodic table.) It has 20 protons and 20 electrons (if is an atom, see ion). The most common isotopes are Ca-40 and Ca-44. Its mass number is about 40.08. Calcium is very important in the human body, It is required for minerals and bones.


Physical properties

Calcium is a soft white-gray metal. It is an alkaline earth metal. It melts at a quite high temperature for a reactive metal. It is a little harder than lead. It has two allotropes. It does not conduct electricity as well as copper, but is much lighter in weight.

Chemical properties

File:Calcium air 2.theora.ogv It reacts with water to produce hydrogen and calcium hydroxide. It reacts with water very fast when it is powdered. When it is in a chunk, it starts reacting slowly because calcium hydroxide makes a coating that does not dissolve on the calcium. If a little acid is added to calcium hydroxide, it dissolves it, making the calcium react very fast. It burns when powdered to make a reddish flame. This makes calcium oxide. It also makes calcium nitride when heated. It can react with halogens to make calcium halides like calcium chloride with chlorine.

Calcium compounds

Calcium forms chemical compounds in the +2 oxidation state. Calcium compounds are colorless. Most calcium compounds are not toxic. They are needed in the human body, actually. They are unreactive as far as calcium ions go. Calcium oxide was used to make limelights, which have a flame heating calcium oxide and makes it glow very bright.


In the ground


Calcium is not found as a metal in the ground; it is too reactive. Calcium carbonate, also known as calcite, is the most common calcium mineral.

Calcium in cells

It is important to know how cells work. Many cells have calcium channels on their surface. These are openings where calcium ions can enter the cell. The cell is told to act and it opens the channels. Once in the cell calcium ions activates many proteins to do specific things. For example, when it goes into muscle cells, it makes them contract (shorten so the muscle pulls.) When it goes into nerve cells, it triggers electrical impulses that send a messages. When it goes into white blood cells it makes them fight germs.

Calcium ions are important to cells, but too many calcium ions can be bad. If a cell gets more calcium ion than it needs it can die. This is why the amount of calcium ion in cells is highly regulated. Conversely, not enough calcium ion is bad. Cells must have the right amount to function properly.

Sometimes cells are unhealthy and need to die in for the body to replace them with new, healthy cells. This keeps the whole organism healthy. Cells know when they should die and can trigger reactions to end their life cycles in many ways. When this happens it is called apoptosis, also known as a 'programmed cell death' (planned cell death.) One way cells accomplish apoptosis by taking in toxic levels of calcium ions.

Calcium is very important for the human body.

Calcium storage

Bones contain most of the calcium ion in the human body. If we need more calcium for our blood, muscles, or other tissue, it comes from the bones. If we have extra calcium it goes into bones.

Calcium as an element is not found in the human body, just calcium ions in the form of chemical compounds.

Calcium regulation

Organisms need to keep calcium ion levels very well controlled. High calcium levels are bad, and low calcium levels are bad.

The body controls this by changing

  • how much calcium we get from the food we eat
  • how much calcium we lose in urine
  • how much calcium is put in bones

The control of calcium in the body is called calcium metabolism.

The body controls calcium levels with many hormones. Calcitonin, Parathyroid hormone (acronym PTH), and Vitamin D are the most important hormones in calcium metabolism. (Vitamin D is a hormone but it is called a vitamin.)


Calcium metal is made by electrolysis of melted calcium chloride. It has to be very hot to melt it. The calcium metal is liquid.


As an element

Calcium is used in the reduction of other metals. It can also be used to make alloys with other metals.

As chemical compounds

Concrete used to make a building

Calcium compounds are also important in chemistry. It is important for making things. It is a part of cement which is needed to make concrete (a hard substance that many buildings are made from.)

Calcium is part of calcium oxide. Calcium oxide is used to make paper, pottery, food, and to purify water (make it good to drink.) Calcium carbonate is used as a calcium supplement. Calcium permanganate can be used as a rocket propellant.


Calcium is toxic as an element. It reacts with water and makes a strong base, calcium hydroxide. Calcium compounds are not toxic unless the anion is toxic. Calcium permanganate is only toxic because of the permanganate, not the calcium.

Related pages


  1. Krieck, Sven; Görls, Helmar; Westerhausen, Matthias (2010). "Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes". Journal of the American Chemical Society 132 (35): 100818110534020. doi:10.1021/ja105534w . PMID 20718434 .