Magnesium
Magnesium is a chemical element with symbol Mg and atomic number 12. It is a shiny gray solid which bears a close physical resemblance to the other five elements in the second column (Group 2, or alkaline earth metals) of the periodic table:they each have the same electron configuration in their outer electron shell producing a similar crystal structure. Magnesium is the ninth most abundant element in the universe.
At a glance
- Atomic number
- 12
- Atomic mass
- 24.305 u
- Phase
- Solid
- Density
- 1.738 g/cm³
- Melting point
- 923 K
- Boiling point
- 1,363 K
- First ionization energy
- 737.7 kJ/mol
- Electronegativity
- 1.31
- Electron affinity
- -40 kJ/mol
- Molar heat capacity
- 24.869 J/(mol·K)
- Period
- 3
- Appearance
- shiny grey solid
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Magnesium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
10 is the [Ne] core total across two shells.
Preparing the orbital view…
12 protons12 neutrons
Magnesium-24: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid
At this temperature, magnesium is a solid.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
MgNeutral atom · no charge
A neutral magnesium atom. Removing its first electron takes 738 kJ/mol.
Magnesium’s outer shell holds 2 electrons. Electron 3 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 5.3× as much energy as the one before. That’s why magnesium forms Mg²⁺ ions. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 2, then 8, then 2 bars. Read backwards, that’s 2, 8, 2: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 2 electrons before the first big jump are magnesium’s valence electrons, the ones it uses in chemical reactions. That’s why magnesium sits in group 2: every element in the group has the same number of outer electrons, so they react in similar ways.
- The numbers explain the charge. Removing electron 3 as well, to make Mg³⁺, would cost 7,733 kJ/mol. Forming a compound gives back far less energy than that, so Mg³⁺ doesn’t form in ordinary chemistry.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 3 | 738 | – | Measured |
| 2 | 3 | 1,451 | 2 | Measured |
| 3 | 2 | 7,733 | 5.3 | Measured |
| 4 | 2 | 10,543 | 1.4 | Estimated |
| 5 | 2 | 13,636 | 1.3 | Estimated |
| 6 | 2 | 18,020 | 1.3 | Estimated |
| 7 | 2 | 21,711 | 1.2 | Estimated |
| 8 | 2 | 25,658 | 1.2 | Estimated |
| 9 | 2 | 31,646 | 1.2 | Estimated |
| 10 | 2 | 35,457 | 1.1 | Calculated |
| 11 | 1 | 169,988 | 4.8 | Calculated |
| 12 | 1 | 189,368 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
Set a ribbon of magnesium alight
Magnesium is a light, silvery metal, but hold a strip of it in a flame and it bursts into one of the brightest lights in a school lab: a blinding white glare, hot enough to melt steel.
The ribbon weighs 0.24 g. Burn it and weigh what’s left.
Burning one gram of magnesium gives out about 25 kJ, roughly what it takes to bring a mug of water from cold to warm.
How would you put it out?
Pick one
Magnesium is burning in a heap. Which would you use to put it out?
Where you’ve seen it
- The first camera flash
Victorian photographers lit a puff of magnesium powder to get enough light indoors. Early flashbulbs used a twist of magnesium or aluminium foil inside glass.
- Flares and fireworks
Signal flares and the bright white sparkles in fireworks and sparklers owe their glare to burning magnesium.
- Why it doesn’t burn in your laptop
Solid magnesium alloys are used in laptops, cars and bikes: a thick piece carries heat away too fast to catch fire. Thin ribbon and powder are what burn so fiercely.
Masses from the element data’s atomic masses. Energy from magnesium oxide’s standard enthalpy of formation, 601.6 kJ/mol (NIST). The ribbon’s burn is sped up. Don’t try this without a teacher: the light can damage your eyes.
How does it compare?
Density
1.74 g/cm³17th lowest of 99
Melting point
923 K37th lowest of 96
Boiling point
1,363 K29th lowest of 97
Electronegativity
1.3136th lowest of 96
First ionization energy
738 kJ/mol40th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- MagnesiumRoyal Society of Chemistry
- MagnesiumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database