To understand biology, you must understand how atoms can join together to form compounds or molecules. Both molecules and compounds are held together by bonds.

Of all the elements in the Periodic Table, living things use only a handful. The four most common elements found in living things are hydrogen, carbon, nitrogen, and oxygen, all of which are found in air, plants, and water. Then, several other elements exist in smaller amounts in living organisms, including sodium, magnesium, phosphorus, sulfur, chlorine, potassium, and calcium. These elements are used in reactions inside the body.


Most often, the elements such as sodium, magnesium, chlorine, potassium, and calcium circulate in the body as electrolytes. Electrolytes are substances that release ions when they break apart in water. When in the “water” of the body, substances such as sodium chloride (NaCl) break apart into the ions Na(+) and Cl(-), which then are used in organs such as the heart or in cellular processes.


Ions are charged particles — that is, atoms with a positive or negative charge. Remember that inside atoms there are protons (which are positive) and neutrons (which are neutral), as well as electrons (which are negative) outside it. Ions are positive (+) when they have more protons than electrons; they are negative (-) when they have more electrons than protons.

Molecules and compounds

When atoms of the same element combine, they form molecules. Compounds are formed when molecules of two or more atoms are joined together. For example, because water is a combination of two different elements (hydrogen and oxygen), it is considered to be a compound. Another example of a compound is glucose, which combines several atoms of carbon, hydrogen, and oxygen:


Both molecules and compounds are held together by bonds, and bonds can be either ionic or covalent.

  • Ionic bonds: Ionic bonds hold atoms joined together in an ionic reaction. Ionic reactions occur when ions combine and the atoms involved lose or gain electrons. A simple example of an ionic reaction is the one between sodium Na(+) and chlorine Cl(-) to form table salt.

    Na is the symbol for the element sodium, and an ion of sodium has one more proton than electrons. Therefore, it is a positive ion with a “plus 1” charge. Remember that opposites attract; so, an ion with a positive charge is naturally attracted to an ion with a negative charge. Cl is the symbol for the element chlorine, and the chloride ion has one less proton than electrons. Therefore, it is a negative ion with a “minus 1” charge.

    When the sodium ion and the chloride ion come together, an ionic bond is formed, and the positive and negative charges on both ions are balanced when sodium “gives” an electron to chlorine. That is why the compound table salt is written as NaCl, with no plus or minus signs.

  • Covalent bonds: Covalent bonds are formed when atoms share electrons in a covalent reaction. The term covalence refers to the number of electron pairs that an atom shares with another atom. The more electron pairs that atoms share, the more stable they are. And, having stable atoms is a good thing. The bonds that form between the atoms that share electrons are called covalent bonds.