1. 5.1 History of the Periodic Table
Ch. 5 The Periodic Law
5.1 History of the Periodic Table

2. Introduction
By 1860, chemists had discovered more than 60 elements but did not have set ways to determine atomic masses.
First International Congress of Chemists met to discuss problem
Italian chemist, Stanislao Cannizzaro, proposed accurate way to measure atomic mass

3. Mendeleev Russian chemist, Dmitri Mendeleev
After hearing about meeting and new atomic masses discussed, he decided to use them in his new book
He wanted to create a way to organize the elements according to their properties
How would you approach this problem?

4. Mendeleev made notecards with each element described
he tried organizing them different ways and watched for patterns and trends

5. Mendeleev
when he arranged them by atomic mass, he found similar properties at certain intervals
Periodic- contains a repeating pattern
published the first periodic table in 1869

6. What is missing?

7. Mendeleev
left empty spaces where he predicted undiscovered elements should be
confirmed his predictions and persuaded other chemists
scandium (Sc)
gallium (Ga)
germanium (Ge)

8. Moseley 1911, Henry Moseley (English)
found that the pattern worked best if arranged by number of protons
Look at modern periodic table.
Do you see any out of order by atomic number? atomic mass?

9. Periodic Law
physical and chemical properties are periodic functions of their atomic numbers
when elements are arranged by atomic number, elements with the same properties appear at regular intervals.

10. Modern Periodic Table What is it?
an arrangement of elements in order of atomic number so that elements with similar properties fall in the same group
chemists made some very important discoveries after Mendeleev’s table

11. Lanthanides and Actinides
14 elements
58 (Ce) – 71 (Lu)
Actinides
14 elements
90 (Th) – 103 (Lr)

12. Noble Gases Argon: Helium:
1894 by Lord Rayleigh (English) and Sir William Ramsey (Scottish)
in atmosphere but since it is unreactive, no one had noticed it
Helium:
1868, discovered as component of sun
1895, Ramsey found on Earth

13. Noble Gases 1898, Ramsey found Kr and Xe
1900, Friedrich Dorn (German) found Rn
Ramsey proposed new group to hold them in between Group 1 and Group 17