The Periodic Table: Mendeleev's Legacy and Electron Configurations

1. Chapter 3The Periodic Table The how and why

2. History • Dmitri Mendeleev - Russian scientist • taught chemistry in terms of properties. • Mid 1800’s - molar masses of elements were known. • Wrote down the elements in order of increasing mass. • Found a pattern of repeating properties.

3. Mendeleev’s Table • Grouped elements in columns by similar properties in order of increasing atomic mass. • Found some inconsistencies - felt that the properties were more important than the mass, so switched order. • Found some gaps. • Must be undiscovered elements. • Predicted their properties before they were found.

4. The modern table • Elements are still grouped by properties. • Similar properties are in the same column. • Order is in increasing atomic number. • A column of elements Mendeleev didn’t know about was added later. • The noble gases weren’t found because they didn’t react with anything.

5. Why? • The part of the atom another atom sees is the electron cloud. • More importantly the outside orbitals. • The orbitals fill up in a regular pattern. • The outside orbital electron configuration repeats. • Outside electrons are called valence electrons • The properties of atoms repeat.

6. Electron Arrangements repeat • The shape of the periodic table is a representation of this repetition. • When we get to the end of the row the outermost energy level is full. • Full energy level is the most stable • Noble gases do not react because they are already stable

7. Determining Valence Electrons • Valence electrons determine how a chemical will react. • For group A elements, the group number is the number of valence electrons

8. Valence Electrons • All elements would like to have 8 electrons (except H and He) • 8 electrons means full outside shell • 8 electrons means more stable

9. Electron Dot diagrams • A way of keeping track of valence electrons. • How to write them: • Write the symbol. • Put one dot for each valence electron • Don’t pair up until they have to X

10. The Electron Dot diagram for Nitrogen • Nitrogen has 5 valence electrons. • First we write the symbol. N • Then add 1 electron at a time to each side. • Until they are forced to pair up.

11. Write the electron dot diagram for • Na • Mg • C • O • F • Ne • He

12. Periodic Table Setup

13. Horizontal rows are called periods • There are 7 periods

14. Vertical columns are called groups. • Elements are placed in columns by similar properties. • Also called families

15. 8A0 1A • The elements in the A groups are called the representative elements 2A 3A 4A 5A 6A 7A

16. These are called the inner transition elements and they belong here The group B are called the transition elements

17. Group 1A are the alkali metals • Group 2A are the alkaline earth metals

18. Group 7A is called the Halogens • Group 8A are the noble gases

19. Transition Metals • Chromium, gold, silver, nickel, zinc, iron, etc. • Transition metals do not behave predictably • Their atomic structure is more complicated

20. Inner Transition Metals • Two rows “under” main periodic table • First row is lanthanides – rare earth metals • Second row is actinides – radioactive

21. Nonmetals • Don’t conduct electricity • Brittle as solids • Low melting points

22. Metalloids • Between metals and nonmetals • Semiconductors – conduct some electricity

23. Semiconductors • Electricity is the flow of electrons • Metals conduct electricity because their electrons are free to move • Nonmetals do not conduct because their electrons are locked in place

24. Semiconductors • Semiconductors work best when a small amount of another element is added • Called doping • Si (4 valence e-) doped with P (5 valence e-) gives an extra electron • Called n-type for negative charge

25. Semiconductors • P- type have one less electron and are positive • Can combine types to form pnp- or npn- type • Allow electronic devices to be small: laptop, hearing aids, cell phones