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.