The Periodic Table of the Elements

The periodic table is a chart that organises all the known chemical elements according to their atomic number, the number of protons in the nucleus, which uniquely identifies each element. Devised in its classic form by Dmitri Mendeleev in the nineteenth century, the table arranges elements so that those with similar chemical behaviour are grouped together, revealing a recurring, or periodic, pattern in their properties. Each element is shown with its atomic symbol, atomic number and atomic mass.

The horizontal rows are called periods and the vertical columns are called groups. Elements in the same group tend to share chemical properties because they have the same number of electrons in their outermost shell. The table therefore predicts how an element is likely to behave from its position alone. Atomic mass reflects the combined mass of protons and neutrons, and many elements occur as several isotopes, atoms of the same element with differing numbers of neutrons.

Several properties vary in regular trends across the table. Electronegativity, a measure of how strongly an atom attracts shared electrons, generally increases toward the upper right, while atomic size tends to grow toward the lower left. Other listed properties include melting and boiling points, density and orbital arrangement of electrons. The table separates broad classes such as metals, non-metals and the noble gases, and it remains one of the most useful organising tools in all of science.

Frequently asked questions

What does the atomic number tell you?

The atomic number is the number of protons in an atom's nucleus. It identifies the element and determines its position in the periodic table.

What is the difference between a group and a period?

Groups are the vertical columns, whose elements share similar chemical properties, while periods are the horizontal rows arranged by increasing atomic number.

What is electronegativity?

Electronegativity measures how strongly an atom attracts shared electrons in a bond. It generally increases toward the top right of the periodic table.