1. Chapter 5
The Periodic Law

2. History of the Periodic Table
Section 5.1
History of the Periodic Table

3. 1860’s 63 elements discovered so far
no standardized method to compare atomic masses
Different chemists used different methods…very difficult to understand each other’s results

4. 1860 – Conference at Karlsruhe
Stanislao Cannizzaro introduced a method for accurately measuring the relative masses of atoms
chemists now agree on standard values for atomic mass
Now they could focus on finding relationships between atomic mass and other properties of the elements

5. Dmitri Mendeleev (Russian)
used the new mass values in a textbook that he was writing
tried to organize the elements according to their chemical and physical properties.

6. Chemical Periodicity
Dmitri noticed that when the elements were arranged in order of increasing atomic mass, certain similarities in their chemical properties appeared at regular intervals

7. Periodicity
Repeating patterns are referred to as being periodic (like a sine wave).

8. Published first periodic table by arranging cards of elements by their properties.
Broke the trend of arranging by mass number to have properties aligned (I[127] after Te[128]; boldly stated that some masses had been miscalculated!!).
Left gaps for predicted unknown elements. (ex. ekasilicon=germanium)
Dmitri Mendeleev: (1869)

9. Dmitri Mendeleev: (1869) By 1886, 3 missing elements were discovered
Mendeleev’s predictions were accurate, most chemists accepted his periodic table
Questions remained –
why did some elements need to be placed out of order of their atomic mass?
What was the reason for chemical periodicity?
Dmitri Mendeleev: (1869)

10. Henry Moseley - 1911 Examined the spectra of 38 different metals.
Realized that the elements in the periodic table were arranged in order of increasing nuclear charge (i.e., atomic number)
Consistent with Mendeleev’s ordering of elements by properties (Te – 52; I - 53)

11. Henry Moseley: (1911)
Established Periodic Law by determining atomic numbers of elements.

12. Periodic Law
Periodic Law - the physical and chemical properties of the elements are periodic functions of their atomic numbers.
elements are arranged by increasing atomic number, not mass;
elements with similar properties fall in the same group.

13. The Modern Periodic Table
An arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column, or group.

14. Since Mendeleev...
Chemists have discovered new elements, and synthesized new elements.
Three sets of elements were added to the table after Mendeleev:

15. Noble Gases (Group 18)
1894 Rayleigh and Ramsay discovered argon, and other noble gases were later discovered.
Due to their unreactivity a new group was proposed.

16. The Lanthanides (rare earths)
14 elements with atomic numbers from 58 to 71.
These elements are so similar in their chemical and physical properties that separating and identifying them was difficult.

17. The Actinides 14 elements with atomic numbers from 90 to 103.
Extremely rare and those beyond element #92 (U) are synthesized.

18. For groups 1,2, 13-18, the differences between the atomic numbers of successive elements in a group is as follows:
Li atomic # = 3
Na atomic # = (8 protons)
K atomic # = (8 protons)
Rb atomic # = (18 protons)
Cs atomic # = (18 protons)
Fr atomic # = (32 protons) 1 18 2 13 14 15 16 17 8 3 4 5 6 7 8 9 8 10 11 12 18 18 18 32 32 32
Periodicity

19. The Reason for Periodicity
Is explained by the arrangement of the number of electrons around the nucleus (maximum outer level electrons are shown below):
Period 1 - 1s2
period 2 - 2s22p6
period 3 - 3s23p6
period 4 - 4s23d104p6
period 5 - 5s24d105p6
Period 6 – 6s24f145d106p6
The Reason for Periodicity

20. Assignment
5.1 Textbook Problems