A carbon atom has six electrons around a nucleus of six protons and, usually, six neutrons. Measured the same way as the explorer's hydrogen atom, as twice the distance at which its outermost electrons are most likely found, it is about 0.13 nanometers across, or 130 picometers. A nanometer is a billionth of a meter, and a picometer is a thousandth of a nanometer.

Size

An atom has no rigid shape or size, so there are several ways to define it, and each gives a different number. The figure here comes from calculations of where the electrons are most likely to be. Two of carbon's six electrons stay close to the nucleus, and the other four sit farther out, as the explorer's picture shows. The outer four are most likely found about 65 picometers from the nucleus, which makes the atom about 130 picometers across.

For atoms in molecules, chemists often use a different measure. The covalent radius is half the distance between the centers of two bonded atoms. Two carbon atoms joined by a single bond sit 154 picometers apart on average, so by this measure a carbon atom is about 150 picometers across.

The van der Waals radius measures instead how closely two separate atoms can approach before they push each other apart. By that measure, a carbon atom is about 340 picometers across.

Size comparisons

• A carbon atom is about 1.2 times as wide as a hydrogen atom, which is 106 picometers across by the same measure.
• The DNA double helix, about 2 nanometers wide, is about 15 carbon atoms across.
• About 600,000–800,000 carbon atoms side by side would span the width of a human hair.
• If a carbon atom were enlarged to the size of a basketball, about 24 centimeters (9.4 inches) across, a human hair would be about 150–185 kilometers (90–115 miles) wide.

How carbon atoms join together

Carbon atoms tend to form four bonds, and they can link to each other in chains of many different lengths. This ability to form chains of many lengths is why carbon is essential to life. A double bond pulls two carbon atoms closer together than a single bond, to 134 picometers, and a triple bond to 120 picometers.

Pure carbon comes in several forms, including diamond, graphite and graphene. Diamond is hard enough to cut rock, while graphite is soft. Graphene is a sheet of carbon atoms only one atom thick, linked in a pattern of hexagons like chicken wire.

Seeing single carbon atoms

Individual carbon atoms are far too small for a light microscope, but they have been seen with electron microscopes. In 2008, researchers at Lawrence Berkeley National Laboratory used the TEAM 0.5 microscope to make images showing each carbon atom in a sheet of graphene. Graphene is fragile, so they used a gentler, lower-energy electron beam, which still picked out details 1 angstrom (100 picometers) apart.

Weighing atoms

Carbon also sets the scale for weighing atoms. Since 1960, carbon-12, the form of carbon with six neutrons, has been the reference for atomic masses. The unit used, the dalton, is defined as one-twelfth of the mass of a carbon-12 atom. About 99% of the carbon on Earth is carbon-12.

Sources

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• DNA Structure and Sequencing. In Biology 2e. OpenStax, 2018.
• Closest Look Ever at Graphene: Stunning Images of Individual Carbon Atoms From TEAM 0.5 Microscope. Lawrence Berkeley National Laboratory, 2008.
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