1. History of the
Periodic Table

2. Mendeleev and Chemical Periodicity
By 1860, more than 60 elements had been discovered.
At this time there was no method for determining an element’s atomic mass.
Communication between chemists was difficult.
In 1860, chemists formed the First International Congress of Chemists in Germany.
During the Congress a method for calculating atomic mass was agreed upon.

3. Dmitri Mendeleev decided to include the atomic masses in a chemistry textbook he was writing.
He also wanted to organize the elements based on their properties.
Mendeleev’s first periodic table was published in 1869.

4. Mendeleev created a table in which elements with similar properties were grouped together – a periodic table of the elements.
Mendeleev left several empty spaces in his periodic table. In 1871, the Russian chemist boldly predicted the existence and properties of the elements that would fill three of the spaces.
By 1886, all three elements had been discovered.

5. Moseley and the Periodic Table
Henry Moseley found that elements in the periodic table fit into patterns better when arranged in increasing order according to nuclear charge (number of protons/atomic number).
Moseley’s work led to both the modern definition of atomic number and the recognition that atomic number, not atomic mass, is the basis for the organization of the periodic table.

6. Periodic Law
Mendeleev’s principle of chemical periodicity is correctly stated in what is known as the periodic law.
Periodic Law – the physical and chemical properties of the elements are periodic functions of their atomic numbers.
In other words, when the elements are arranged in order of increasing atomic number, elements with similar properties appear at regular intervals.

7. Periodicity of Atomic Numbers

8. The Modern Periodic Table
The periodic table is the arrangement of the elements in order of their atomic numbers so that elements with similar properties fall into the same column, or group.

9. Perhaps the most significant addition to the periodic table came with the discovery of the noble gases.
In 1894 the noble gas argon was discovered (Sir William Ramsay) which led to a new group being added to the table.

10. In the early 1900s the development of the periodic table continued with the lanthanides (cerium to lutetium).

11. The discovery of the actinides (thorium to lawrencium) also added to the development of the periodic table.