The Periodic Table of Elements

The periodic table of elements is a tabular display of the 118 known chemical elements. Elements are arranged in the table by increasing atomic number, which is the number of protons in an atom's nucleus. The columns of the periodic table are called groups and the rows are called periods.

Dmitri Mendeleev and the Periodic System

The modern periodic table is based on the work of Dmitri Mendeleev, who published his version in 1869. Mendeleev noticed that certain properties recurred at regular intervals when elements were arranged by atomic weight (a measure of an atom's mass). He used this observation to develop a new way of arranging the elements, which he called the periodic system.

In Mendeleev's original version of the periodic table, there were gaps in some places where no element had yet been discovered that fit into that particular slot. As new elements were discovered over time, they were added to these gaps.

Atomic Structure and the Periodic Table

Today, we know that atoms are not indivisible particles but rather they are made up of smaller subatomic particles: protons, neutrons, and electrons. The number of protons in an atom's nucleus determines its atomic number and thus its place on the periodic table.

For example, hydrogen has one proton and therefore has an atomic number of 1; helium has two protons and an atomic number of 2; lithium has three protons and so on.

Groups and Chemical Properties

The groups on the periodic table represent different types of elements with similar chemical properties. There are four main groups:

• Alkali metals (group 1)
• Alkaline earth metals (group 2)
• Transition metals (groups 3-11)
• Post-transition metals (groups 12-18)

Each group contains elements with progressively higher atomic numbers as you move down a column from top to bottom.