The Periodic Table

Development of the Periodic Table – There were only 23 at the time – Most were known since prehistoric times – Examples include gold, silver, carbon and oxygen Late 1790s- Antoine Lavoisier made a list of all the known elements

As technology evolved, more elements were found. By 1870, there were 70 known elements John Newlands noticed that when elements were arranged by increasing atomic mass, every eighth element has similar properties. Newlands had the right idea, but his arrangement didn’t work for every element.

In 1869, Dmitri Mendeleev proposed the first periodic table. His periodic table was arranged in order of increasing atomic mass and also had elements with similar properties in the same column. He left blanks for elements that had not been discovered, but he predicted would exist. – Some examples are scandium, gallium, and germanium

Mendeleev’s table had some of the elements in the wrong order. – Cobalt has a higher mass than nickel, but should come before nickel on the periodic table Since atomic mass isn’t the right way to arrange the elements, how should they be arranged?

In 1913, English chemist Henry Moseley determined what the problem was. The error was arranging the table by increasing atomic mass. It should be arranged by increasing atomic number. The statement that there is periodic repetition of properties when they are arranged by increasing atomic number is called the periodic law.