Ions - When an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons, the atom becomes positively or negatively charged. This frequently happens for most atoms to have a full valence shell. This can happen either by gaining electrons to fill a shell that is more than half-full or by giving away electrons to empty a shell that is less than half-full, thereby leaving the next smaller electron shell as the new, full valence shell. An atom with an electrical charge—positive or negative—is an ion.

Potassium (K), for instance, is an important element in all body cells. Its atomic number is 19. It has just one electron in its valence shell. This characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron. The loss will cause the positive charge of potassium's protons to be more influential than the negative charge of potassium's electrons. In other words, the resulting potassium ion will be slightly positive. A positively charged ion is known as a cation.

Another element, fluorine (F), a component of bones and teeth, has an atomic number of nine. It has seven electrons in its valence shell. Fluorine is highly likely to bond with other atoms in such a way that it accepts one electron. When it does, its electrons will outnumber its protons by one, and it will have an overall negative charge. A negatively charged ion is known as an anion.

Ions are critical to normal body function, so their imbalances can lead to health problems. For instance, sodium ion imbalance could cause cardiovascular diseases. Low blood calcium levels can cause tetany — uncontrollable involuntary muscle contractions.

Molecules - Molecules are simply two or more atoms chemically bonded together. Molecules may consist of atoms of the same element, like a molecule of oxygen gas, formed by bonding between two oxygen atoms, respectively, or molecules may be formed by bonding between atoms of different elements, like a molecule of carbon dioxide gas, formed between oxygen and carbon atoms.

Compounds - A compound is a pure substance formed by bonding two or more atoms in a fixed proportion. For example, the compound glucose is an important body fuel. It is always composed of the same three elements: carbon, hydrogen, and oxygen, in the same relative amounts. In glucose, there are always six carbon and six oxygen units for every twelve hydrogen units.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 2.2 Chemical Bonds and Openstax, Anatomy and Physiology 2e, Section 2.1 Elements and Atoms: The Building Blocks of Matter.