Electron Configurations II

1. F. Schifano, Department of Science Bayonne High School, Bayonne NJ Electron Configurations II

2. Key Skills:

3. What are Valence Electrons? • The valence electrons of an element are the outermost electrons. • Only s and pelectrons in an atom’s highest energy level count as valence electrons.

4. Determining the # of Valence e-

5. Electron-Dot (Lewis) Diagrams Represent Valence Graphically • Each side represents a sub-orbital. • s-orbitals get both electrons first. • Each p sub-orbital gets one e- before any p-sub-orbital gets two. • This is called Hund’s Rule.

6. Writing Electron-Dot Diagrams

7. Practice • Determine the number of valence electrons in each of the following elements. Then draw their dot diagrams: • Ca • F • Br • Sr • N • O

8. Why are Valence Electrons Special? • Formation of Covalent Bonds • When molecules get close to each other, the valence electrons are attracted to the nucleus of the other atom.

9. Valence Electrons Form Bonds • If trading or sharing electrons would make both atoms more stable, this temporary attraction becomes a full-fledged chemical bond.

10. Valence Electrons Predict Chemical Behavior • If two elements have the same number of valence electrons, they will react in a very similar way. • They need similar changes in their electron configuration to become stable.

11. Practice • Which ones should have similar properties and react in similar ways? • Ca • F • Br • Sr • N • O

12. The Holy Grail: Valence = 8 • The most stable configuration of electrons most atoms can have is valence = 8, also known as the octet configuration. • In chemical reactions, most atoms just take the simplest path to valence = 8.

13. Noble Gases Are Already Stable • Neon, argon, krypton, radon, and xenon are all called noble gases. • They already have valence=8 . They don’t benefit from reacting, so they don’t react!

14. Same Column = Same Valence • The periodic table is designed in just such a way that elements with the same valence fall into the same vertical column.

15. Four Families

16. Special Cases • Hydrogen becomes stable at valence = 0 or at valence = 2. • It can give away an electron, becoming valence =0 (H+) or it can gain an electron, becoming valence =2 (H-). • Helium is already stable at valence =2. • Like the other noble gases, helium is already stable and doesn’t react. • Boron is stable at valence =6.

17. Writing Configurations Using the Periodic Table • An element’s position on the periodic table tells you the last electron that filled its orbitals: • Period (horizontal) = energy level • Block = orbital shape • Box number = #e- in the orbital. • Aufbau principle tells you all the rest!

18. Writing Configurations Using the Periodic Table • Sulfur is in the fourth box of the third row, in the p-block. • Its last electron is 3p4. • Aufbau principle says everything under 3p4 must be filled, so: • 1s2 2s2 2p6 3s2 3p4.

19. D-Block and F-Block Exceptions • If an element is in the d-block its last orbital will really be one energy level down from the row it’s in. • Last electron in chromium (Cr) = 3d4, NOT 4d4 • If an element is in the f-block its last orbital will really be two energy levels down from the row it’s in. • Last electron in plutonium (Pu) = 5f5, NOT 6f5

20. Noble Gas Notation • Longer configurations are a chore. • Noble gases can be used as starting points for longer configurations.

21. Using Noble Gas Notation

22. Noble Gas Notation Cr =1s2 2s2 2p6 3s2 3p6 4s2 3d4 Ar = 1s2 2s2 2p6 3s2 3p6 ____________________________ So we can write the configuration of Cr as [Ar] plus the difference: [Ar] 4s2 3d4

23. Questions • What are valence electrons?