3.1: Atoms

An atom is a basic unit—it can’t be subdivided without losing its unique characteristics. Atom means uncut in Greek. Atoms have protons that have a positive charge, neutrons that are uncharged (neutral), and electrons that have a negative charge.

Protons and neutrons are in the center (nucleus) of atoms and have mass (weight). Electrons orbit the nucleus of the atom (the orbits are called electron shells). The weight (mass) of an electron is miniscule compared to that of a proton or neutron.

Atoms are uncharged—each atom has an equal number of protons and electrons. The positive charge of each proton is offset by the negative charge of each electron (Figure 3.1).

When speaking of specific atoms, the term element is often used. The number of protons in an atom determines which element the atom is. More than 100 protons can exist in an atom. Thus, there are more than 100 different elements, beginning with hydrogen with a single proton. Elements are listed by their number of protons (their atomic number) in the Periodic Table of Elements (Appendix A-2). Each element has a one- or two-letter symbol (Appendix A-3).

For example, the compound sodium chloride (table salt): NaCl: One atom of sodium: Na (from its Latin name natrium), and one atom of chlorine: Cl.

The number of protons is unique for each element, but the number of neutrons can vary. Atoms that have the same number of protons but differ in the number of neutrons are called isotopes (some, but not all, isotopes are radioactive). A carbon atom, for example, can have different numbers of neutrons, but always has 6 protons. (It would be boron with 5 protons, nitrogen with 7 protons.)

Most carbon atoms (about 99%) have 6 neutrons and 6 protons (carbon-12), but some have more neutrons. Carbon-13 (about 1% of all carbon atoms) is a nonradioactive isotope that has 7 neutrons and 6 protons.* Carbon-14 has 8 neutrons and 6 protons, and is radioactive. Its measurable radioactivity is used for such diverse purposes as determining the age of ancient relics and following the fate of molecules in lab animals and cells.

Because the body can’t change one element into another (i.e., the body can’t change the number of protons in an atom), each element the body needs must be taken in as food or in the air we breathe. For example, potassium (19 protons) and calcium (20 protons) are needed in the diet. The number of protons differs by only one, yet potassium and calcium are very different in their physical characteristics and functions in the body. One can’t substitute for the other, nor can one be made from the other. The same principle applies in chemistry as it does in biology. Recall how alchemists in the Middle Ages tried (unsuccessfully, of course) to turn lead (82 protons) into gold (79 protons).

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*Natural diamonds are 99% carbon-12 and 1% carbon-13. In 1990, scientists at General Electric made a diamond of only carbon-12. The occasional carbon-13 causes slight imperfections in the crystal. Diamonds are used as components of high-tech devices because they’re transparent, hard, and the best known heat conductor. Compared to ordinary diamonds, 100% carbon-12 diamonds conduct heat 50% better and withstand 10 times the intensity of laser radiation. Using these “super‑diamonds” in laser devices and miniaturized components (e.g., microchips) has immense practical value, e.g., enhanced heat conductivity in a microchip means that their components can be packed closer together, enabling computers to be smaller and work faster without overheating.