Carbon
Carbon can form the rigid network of diamond or the sliding layers of graphite. The atoms are the same; the way they bond changes the material. Carbon’s ability to form varied chains also underpins the molecules of life.
At a glance
- Atomic number
- 6
- Atomic mass
- 12.011 u
- Phase
- Solid
- Density
- 3.513 (diamond); 2.2 (graphite) g/cm³
- Melting point
- Sublimes at 4,098 K
- Boiling point
- Sublimes at 4,098 K
- First ionization energy
- 1,086.5 kJ/mol
- Electronegativity
- 2.55
- Electron affinity
- 121.7763 kJ/mol
- Molar heat capacity
- 8.517 J/(mol·K)
- Period
- 2
- Appearance
- Unavailable
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Carbon’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.
Preparing the orbital view…
6 protons6 neutrons
Carbon-12: 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, carbon is a solid.
At normal pressure carbon never melts. It sublimes, turning straight from solid into gas. Liquid carbon needs roughly 100 times atmospheric pressure.
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
CNeutral atom · no charge
A neutral carbon atom. Removing its first electron takes 1,087 kJ/mol.
Carbon’s outer shell holds 4 electrons. Electron 5 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 6.1× as much energy as the one before.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 4, then 2 bars. Read backwards, that’s 2, 4: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 4 electrons before the first big jump are carbon’s valence electrons, the ones it uses in chemical reactions. That’s why carbon sits in group 14: every element in the group has the same number of outer electrons, so they react in similar ways.
- Removing electrons from carbon is expensive from the start (1,087 kJ/mol for the first). So in reactions carbon tends to gain or share electrons rather than lose them.
- 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 | 2 | 1,087 | – | Measured |
| 2 | 2 | 2,353 | 2.2 | Measured |
| 3 | 2 | 4,621 | 2 | Measured |
| 4 | 2 | 6,223 | 1.3 | Measured |
| 5 | 1 | 37,831 | 6.1 | Calculated |
| 6 | 1 | 47,277 | 1.2 | 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.
Watching atoms decay
Carbon-14 is made in the upper atmosphere by cosmic rays. Living things take it in until they die; after that it only decays, which lets archaeologists date bones, wood and cloth.
Half of any sample of C-14 decays every 5,700 years, turning into N-14.
Half-lives passed (1 half-life = 5,700 years)
No time passed · 0 half-lives · 100 of 100 atoms left
Each atom decays at a random moment, so no one can say which will go next, but about half of whatever is left goes in every half-life. The dashed curve is the prediction; the solid line is this sample. The new N-14 atoms decay too, on their own schedule, which the chain below follows.
Carbon dating
Drag the age of an old piece of wood or bone to see how much of its carbon-14 is left.
11,400 years old · 25% of its carbon-14 left · 2 half-lives
Scientists measure how much carbon-14 is left and work backwards to the age. Beyond about 50,000 years so little remains that it can’t be measured reliably.
From C-14 to stable N-14: 1 step
- C-14Carbon5,700 yearsβdecays by beta
- N-14Nitrogenstable
Arrows show how each atom decays: α gives off an alpha particle (2 protons and 2 neutrons), β turns a neutron into a proton, γ gives off a gamma ray.
Half-lives and decay modes from the IAEA Live Chart of Nuclides (evaluated ENSDF data). The chain follows each atom’s most common decay; a few steps have rare side branches.
How does it compare?
Density
1.82 g/cm³18th lowest of 99
Melting point
Carbon sublimes, turning straight from solid to gas at normal pressure, so it has no melting point.
Boiling point
Carbon sublimes, turning straight from solid to gas at normal pressure, so it has no boiling point.
Electronegativity
2.5510th highest of 96
First ionization energy
1,087 kJ/mol12th 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.
Change the connections
In graphite, each carbon bonds to three neighbours within a sheet. Weak interactions between sheets allow the layers to slide.
One six-membered ring in a graphite sheet; the network extends beyond these atoms.
A planet of carbon chemistry
The Blue Marble, assembled from satellite observations in 2002. Carbon circulates through living things, oceans, air, and rocks.
The ability to connect
Carbon forms strong bonds with other carbon atoms. Long chains and more complex arrangements make possible the diversity of organic molecules, from simple fuels to the molecules that living organisms build.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- CarbonRoyal Society of Chemistry
- Allotropes of carbonRSC Education
- Blue Marble 2002NASA
- CarbonWikipedia
- Carbon ground stateNIST
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database