History and Development of the Periodic Table

1. Chapter 5 The Periodic Law

2. 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. 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 # = 11 (8 protons) • K atomic # = 19 (8 protons) • Rb atomic # = 37 (18 protons) • Cs atomic # = 55 (18 protons) • Fr atomic # = 87 (32 protons) 1 18 2 13 16 17 14 15 8 8 4 6 9 3 5 7 8 10 12 11 18 18 18 32 32 32 Periodicity

19. 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