By Janet Rae-Dupree, Pat DuPree

All matter — be it solid, liquid, or gas — is composed of atoms. An atom is the smallest unit of matter capable of retaining the identity of an element during a chemical reaction. An element is a substance that can’t be broken down into simpler substances by normal chemical reactions.

There are 98 naturally occurring elements in nature and 20 (at last count) artificially created elements for a total of 118 known elements. However, additional spaces have yet to be filled in on the periodic chart of elements, which organizes all the elements by name, symbol, atomic weight, and atomic number. The key elements of interest to students of anatomy and physiology are

  • Hydrogen: Symbol H

  • Oxygen: Symbol O

  • Nitrogen: Symbol N

  • Carbon: Symbol C

HONC your horn for the four organic elements. These four elements make up 96 percent of all living material.

Atoms are made up of the subatomic particles protons and neutrons, which are in the atom’s nucleus, and clouds of electrons orbiting the nucleus. The atomic weight, or mass, of an atom is the total number of protons and neutrons in its nucleus. The atomic number of an atom is its number of protons; conveniently, atoms that are electrically neutral have the same number of positive charges as negative charges.

Opposite charges attract, so negatively charged electrons are attracted to positively charged protons. The attraction holds electrons in orbits outside the nucleus. The more protons there are in the nucleus, the stronger the atom’s positive charge is and the more electrons it can attract.

Simplified models assume that electron particles orbit the nucleus at different energy levels, known as shells. But quantum theory best describes information about electron-states using wave functions, or orbitals.


Each orbital has a four-quantum-number address (characteristic energy, 3-D shape, orientation, and spin). The principal quantum number is the shell. The other numbers allow room to add distinct electrons. Shells are divided into subshells, and these are filled to completion in order of increasing energy.

  • The first shell holds only two electrons, of opposite spin (magnetism).

  • The second and third shells each hold eight electrons in four subshells and two spins.

  • The fourth shell (which can be found in elements such as potassium, calcium, and iron) holds up to 18 electrons. Higher shells also exist.

Other key chemistry terms that you need to know are

  • Isotopes: Atoms of an element that have a different number of neutrons and a different atomic weight than usual. In other words, isotopes are alternate forms of the same chemical element, so they always have the same number of protons as that element but a different number of neutrons.

  • Ions: Because electrons are relatively far from the atomic nucleus, they are most susceptible to external fields. Atoms that have gained or lost electrons are transformed into ions. Getting an extra electron turns an atom into a negatively charged ion, or anion, whereas losing an electron creates a positively charged ion, or cation.

    To keep anions and cations straight, think like a compulsive dieter: Gaining is negative, and losing is positive.

  • Acid: A substance that becomes ionized when placed in solution, producing positively charged hydrogen ions, H+. An acid is considered a proton donor. (Remember, atoms always have the same number of electrons as protons. Ions are produced when an atom gains or loses electrons.) Stronger acids separate into larger numbers of H+ ions in solution.

  • Base: A substance that becomes ionized when placed in solution, producing negatively charged hydroxide ions, OH. Bases are referred to as being more alkaline than acids and are known as proton acceptors. Stronger bases separate into larger numbers of OH ions in solution.

  • pH (potential of hydrogen): A mathematical measure on a scale of 0 to 14 of the acidity or alkalinity of a substance. A solution is considered neutral, neither acid nor base, if its pH is exactly 7. (Pure water has a pH of 7.) A substance is basic if its pH is greater than 7 and acidic if its pH is less than 7.

    The strength of an acid or base is quantified by its absolute difference from that neutral number of 7. This number is large for a strong base and small for a weak base. Interestingly, skin is considered acidic because it has a pH around 5. Blood, on the other hand, is basic with a pH around 7.4.