How to Read the Periodic Table

Click on an entry to see the details.

The information shown in each cell
Each cell of the periodic table typically shows the atomic number at the upper left (or top), the element symbol prominently in the center (for example, C for carbon, O for oxygen), and the element name and atomic weight (average mass) below that. Because atomic weight is an average that reflects the natural proportions of an element's various isotopes, it is usually shown as a decimal rather than a whole number.
What a period (horizontal row) means
The horizontal rows of the periodic table are called periods, and elements in the same period share the same number of electron shells. Period 1 has 1 electron shell, period 2 has 2, and so on, with the number of electron shells increasing by one with each row down; the current periodic table has 7 periods in total.
What a group (vertical column) means
The vertical columns are called groups, numbered from group 1 to group 18. Elements in the same group tend to have similar chemical properties because they have the same number of outermost (valence) electrons. For example, group 1 (lithium, sodium, potassium, etc.) consists of highly reactive alkali metals, while group 18 (helium, neon, argon, etc.) consists of noble gases that rarely react at all.
The metal, nonmetal, and metalloid regions
Based on a staircase-shaped dividing line, the periodic table is split into metals, which make up most of the left side, and nonmetals, mostly in the upper right; near that dividing line sit metalloids (semimetals) — such as boron, silicon, and germanium — that possess some properties of both metals and nonmetals. Metals are generally shiny and conduct electricity and heat well, while nonmetals often have the opposite properties.
The concept of the s/p/d/f blocks
The periodic table can also be divided into s, p, d, and f blocks based on the type of orbital being filled with electrons. Groups 1 and 2 belong to the s block; groups 13 through 18 belong to the p block; the transition metals in the middle belong to the d block; and the lanthanides and actinides, set apart at the very bottom, belong to the f block — this reflects which type of orbital each element's last electron enters.
Why elements in the same group are similar
An element's chemical properties are determined primarily by the number of outermost electrons it has. Even though elements in the same group may differ in their number of electron shells (period), they share the same number of outermost electrons, giving them similar ways of bonding with other atoms and similar reactivity. This was the core principle that first allowed Mendeleev to organize the elements into a table according to their periodic properties.
Trends in electronegativity and atomic radius
Across the periodic table, electronegativity — the strength with which an atom attracts electrons when bonding with another atom — generally increases from the bottom left toward the upper right, with fluorine (F) having the highest value. Atomic radius, by contrast, tends to increase from the upper right toward the bottom left; this is because, within the same period, the increasing nuclear charge pulls electrons in more strongly, while within the same group, the number of electron shells increases.

How was the periodic table created?

The periodic table originated in 1869, when the Russian chemist Mendeleev arranged the elements known at the time in order of atomic weight and discovered a pattern of periodic properties, which he then organized into a table. It was later rearranged in order of atomic number (number of protons) and refined into today's form, and all 118 elements are now filled in.

It's easier if you understand atomic structure first

If the meaning of the atomic number and outermost electrons shown in each cell doesn't quite click for you, we recommend first reviewing our guide to the basics of atomic structure.

Frequently Asked Questions

Where can I look up detailed information on all 118 elements myself?

If you're curious about detailed information for individual elements, try our interactive periodic table tool, which contains data for all 118 elements. This page is a guide focused specifically on how to read the periodic table itself.

Which block do the transition metals belong to?

The transition metals, from group 3 to group 12, mostly belong to the d block; these elements commonly share the trait of being able to take on multiple oxidation states and often forming colored compounds.