Periodic table (noun, “peer-ee-AHH-dik TAY-bul”)
This is a chart that shows all the known chemical elements. The table is made up of over a hundred squares. Each square represents one element. A square contains one or two letters that stand for the element’s name, and numbers that tell about that element’s properties.
The location of each square in the table tells many things about each element. First, the elements are organized by atomic number, or how many protons they have. Those on top of the chart have the fewest protons. An element’s place also shows how likely it is to react. It also shows how its electrons are arranged.
During the mid-1800s, many chemists looked for patterns that explained how elements interacted. Back then, scientists didn’t know about the protons, neutrons and electrons that make up atoms. But they did understand that elements had different atomic weights. An atomic weight is the average weight of one atom of an element.
In 1869, the Russian chemist Dimitri Mendeleev lined up the 63 known elements in order by their atomic weights. He saw trends in the elements’ properties that varied over specific intervals, or periods. Other scientists were working on their own periodic tables, but Mendeleev published his table first.
The periodic table continued to grow as scientists discovered more elements. These include the noble gases, identified in 1890. This is a group of elements such as helium that don’t like to react with other elements. Starting in the 1940s, scientists found many new elements by colliding atoms or pieces of atoms.
At the end of 2018, chemists confirmed four elements that had never been observed before. That brought the number of known elements to 118 and completed the 7th row of the table.
In a sentence
The year 2019 marks the 150th anniversary of the periodic table, first conceived in 1869.