What the Numbers on the Periodic Table Mean

Are you confused by all the numbers on a periodic table? Here's a look at what they mean and where to find important elements.

Element Atomic Number

One number you will find on all periodic tables is the atomic number for each element. This is the number of protons in the element, which defines its identity.

How to Identify It: There isn't a standard layout for an element cell, so you need to identify the location of each important number for the specific table. The atomic number is easy because it is an integer that increases as you move from left to right across the table. The lowest atomic number is 1 (hydrogen), while the highest atomic number is 118.

Examples: The atomic number of the first element, hydrogen, is 1. The atomic number of copper is 29.

Element Atomic Mass or Atomic Weight

Most periodic tables include a value for atomic mass (also called atomic weight) on each element tile. For a single atom of an element, this would be a whole number, adding the number of protons, neutrons, and electrons together for the atom. However, the value given in the periodic table is an average of the mass of all isotopes of a given element. While the number of electrons does not contribute significant mass to an atom, isotopes have differing numbers of neutrons, which do affect mass.

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How to Identify It: The atomic mass is a decimal number. The number of significant figures varies from one table to another. It's common to list values to two or four decimal places. Also, the atomic mass is recalculated from time to time, so this value may change slightly for elements on a recent table compared with an older version.

Examples: The atomic mass of hydrogen is 1.01 or 1.0079. The atomic mass of nickel is 58.69 or 58.6934.

Element Group

Many periodic tables list numbers for element groups, which are columns of the periodic table. The elements in a group share the same number of valence electrons and thus have many common chemical and physical properties. However, there wasn't always a standard method of numbering groups, so this can be confusing when consulting older tables.

How to Identify It: The number for the element group is cited above the top element of each column. The element group values are integers running from 1 to 18.

Examples: Hydrogen belongs to element group 1. Beryllium is the first element in group 2. Helium is the first element in group 18.

Element Period

The rows of the periodic table are called periods. Most periodic tables do not number them because they are fairly obvious, but some tables do. The period indicates the highest energy level attained by electrons of an atom of the element in the ground state.

How to Identify It: Period numbers are located on the left-hand side of the table. These are simple integer numbers.

Examples: The row starting with hydrogen is 1. The row starting with lithium is 2.

Electron Configuration

Some periodic tables list the electron configuration of an atom of the element, usually written in shorthand notation to conserve space. Most tables omit this value because it takes up a lot of room.

How to Identify It: This isn't a simple number but includes the orbitals.

Examples: The electron configuration for hydrogen is 1s¹.

Other Information on the Periodic Table

The periodic table includes other information besides numbers. Now that you know what the numbers mean, you can learn how to predict periodicity of element properties and how to use the periodic table in calculations.