The Periodic Law

History of the Periodic Table

Mendeleev’s Periodic Table• Classification of elements depended upon accurate

measurements of atomic mass.• Mendeleev’s first table was published in 1869.• Elements were arranged in order of increasing atomic

mass and similar chemical properties occurred at regular intervals.

• Approximately 60 elements were known at the time.• Mendeleev left empty spaces in his table and

predicted the properties of elements that had yet to be discovered.

Mendeleev’s Periodic Table

Unanswered Questions:

1. Why could most elements be arranged by increasing atomic mass but others could not?

2. What was the reason for chemical periodicity?

Moseley and the Periodic Law• (1911) – Mosely discovered that elements fit into

patterns when they were arranged by increasing nuclear charge (# of protons)

• The concept of atomic number became recognized as a means for identifying and classifying elements.

• This organization matched Mendeleev’s grouping by chemical properties

Periodic Law – physical and chemical properties of the elements are periodic functions of their atomic numbers.

The Modern Periodic Table

Noble Gases• Discovered between

1894-1898• Placed on the periodic

table between Group 17 and Group 1

• Unreactive elements

He

Ne

Ar

Kr

Xe

Rn

The Inner Transition MetalsLanthanides• Discovered in the early

1900’s• Similar chemical and

physical properties• Form 3+ ions

Actinides• Radioactive elements• Some are naturally

occurring; others have been created synthetically