Biology » Chemical Foundation of Life » Atoms, Isotopes, Ions and Molecules: The Building Blocks

Organizing the Periodic Table

Organizing the Periodic Table

The organization of the periodic table is based on the total number of protons (and electrons). Understanding this helps us know the distribution of electrons among the outer shell. The periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located.


The periodic table shows the atomic mass and atomic number of each element. The atomic number appears above the symbol for the element and the approximate atomic mass appears below it. Image Attribution: OpenStax Biology

Electron configurations and the periodic table

Take a closer look at the some of the elements in the table’s far right column in the periodic table above.

  • The group 18 atoms helium (He), neon (Ne), and argon (Ar) all have filled outer electron shells. Consequently, this makes it unnecessary for them to share electrons with other atoms to attain stability. As a matter of fact, they are highly stable as single atoms. As a result of their non-reactivity we have named them the inert gases (or noble gases).
  • Now, compare this to the group 1 elements in the left-hand column. These elements, including hydrogen (H), lithium (Li), and sodium (Na), all have one electron in their outermost shells. That means that they can achieve a stable configuration and a filled outer shell by donating or sharing one electron with another atom or a molecule such as water. Hydrogen will donate or share its electron to achieve this configuration. On the other hand, lithium and sodium will donate their electron to become stable. As a result of losing a negatively charged electron, they become positively charged ions (Liand Na+).
  • On the other hand, group 17 elements, including fluorine and chlorine, have seven electrons in their outermost shells. So they tend to fill this shell with an electron from other atoms or molecules. This makes them negatively charged ions (F− and Cl).
  • Similarly, group 14 elements have four electrons in their outer shell. Carbon which is most important to living systems belongs to this group. As a result, this allows them to make several covalent bonds (discussed later) with other atoms.

Thus, the columns of the periodic table represent the potential shared state of these elements’ outer electron shells that is responsible for their similar chemical characteristics.

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