1. History of the Periodic Table

2. During the nineteenth century, chemists began to categorize the elements according to similarities in their physical and chemical properties. The end result of these studies was our modern periodic table.

3. Döbereiners’ Triads
Johann Döbereiner (c. 1829) described the idea of the triad
Triads were groups of three elements that shared similar properties
He also noted that when the elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately half way between that of the other two elements of the triad

4. Döbereiners’ Triads He listed three triads: Ca, Sr, Ba S, Se, Te
Cl, Br, I

5. Newlands Octaves
John Newlands (c. 1864) arranged the existing elements by increasing atomic mass
He noticed that properties reemerge after 8 elements
He called this the law of octaves
The Law of Octaves states that the properties of the elements seem to repeat in a periodic fashion after every 8.

6. Newlands Octaves

7. Newlands Octaves
The pattern was perfect up to calcium then became less convincing as some metals appeared unlike the non-metals to their left.
Note that Newlands did not always stick to a strict increase in number. He exchanged the positions of Zn and Y, presumably because he realised that Y resembled Bo (modern symbol B).
On 1st March 1865, he described his ideas at a lecture at the Chemical Society. Many critised his work saying he that he might have equally well listed the elements alphabetically.
Chemistry journals refused to publish his paper, supposedly because it was of a purely theoretical nature. Humiliated, Newlands went back to his work as chief chemist at a sugar factory.

8. Dmitri Mendeleev
In 1869 he published a table of the elements organized by increasing atomic mass.

9. Mendeleev
Russian teacher Dimitri Mendeleev (c. 1869) proposed the first true version of the periodic table
Mendeleev’s table was not complete in that he left “holes” where he thought elements would later be discovered.

10. Mendeleev
For example, Mendeleev proposed that there was an element with similar properties as silicon that should exist, though it had not been discovered
This element was called ekasilicon
Later experiments isolated ekasilicon. It is now known as Germanium today.
Properties of germanium were almost the same as predicted

12. Moseley
English scientist Henry Moseley (c. 1913) proposed the modern periodic table
He arranged the periodic table in order of increasing atomic number as opposed to atomic mass
Came up with periodic law
(def) the physical and chemical properties of the elements are periodic functions of their atomic numbers

13. Moseley
The modern periodic law states: The chemical and physical properties of the elements are periodic functions of their atomic numbers.
Put in other words, when the elements are arranged in order of increasing atomic number, there is a periodic repetition of their properties.

14. Moseley
His research was halted when the British government sent him to serve as a foot soldier in WWI. He was killed in the fighting in Gallipoli by a sniper’s bullet, at the age of 28. Because of this loss, the British government later restricted its scientists to noncombatant duties during WWII.

15. Mendeleev’s Periodic Table
Differences between Mendeleev's Periodic Table and the Modern Periodic Table
Mendeleev’s Periodic Table
Modern Periodic Table
Fewer Elements (some not yet discovered)
More Elements
Gaps present
No Gaps
Arranged in order of Atomic Mass
Arranged in order of Atomic Number
Group 0 Elements missing
Group 0 Elements included

16. Mendeleev’s periodic table versus the modern periodic table

17. Our periodic table can be arranged into three general sections

18. The horizontal rows of the periodic table are called PERIODS.

19. The elements in any group of the periodic table have similar physical and chemical properties!
The vertical columns of the periodic table are called GROUPS, or FAMILIES.

20. Alkali Metals

21. Alkaline Earth Metals

22. Transition Metals

23. Inner Transition Metals
These elements are also called the rare-earth elements.
Inner Transition Metals

24. Halogens

25. Noble Gases