Electron Configurations and Covalent Bonding

1. #13 p. 229 in the book: What electron configurations do atoms usually achieve by sharing electrons to form covalent bonds? In forming covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases. symbol electron configuration helium He 1s 2 neon Ne [He]2s 2 2p 6 argon Ar [Ne]3s 2 3p 6 krypton Kr [Ar]3d 10 4s 2 4p 6 xenon Xe [Kr]4d 10 5s 2 5p 6 radon Rn [Xe]4f 14 5d 10 6s 2 6p 6 The s and p orbitals are full (8 electrons)

2. #14 How is an electron dot structure used to represent a covalent bond? An electron dot structure such as H:H (seen below) represents the shared pair of electrons of the covalent bond by two dots Please see the on-line textbook Chapter 8 section 2 simulation 6 CHEM ASAP! simulate the covalent bonding between molecules

4. #15 When are two atoms likely to form a double bond between them? A triple bond? Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons. Oxygen is an example of two atoms joining and sharing two pairs of elect- rons to form a double bond .

5. Nitrogen is an example of two atoms joining and sharing three pairs of elect- rons to form a triple bond .

6. #16 How is a coordinate covalent bond different from other covalent bonds? In most cova- lent bonds each atom contributes one electron to the bond. In a coordinate coval- ent bond both electrons are contributed by the same atom Oxygen has to kick in two additional electrons to allow both to have a full octet (eight total) valence electrons

7. #17 How is the strength of a covalent bond related to its bond dissociation energy? A large bond dissociation energy corresponds to a strong covalent bond. As you can see in the table on the right dif- ferent covalent bonds have different amounts of energy needed to break apart. A large dissociation energy (see N triple N) is a strong bond. A low dissociation energy (see N-O) is a weak bond.

8. #18 Draw the electron dot resonance structures for ozone and explain how they describe its bonding. When you have two equally valid options for dot structure as seen above you have RESONANCE. Click here to see a movie about how ozone is formed in the upper atmosphere and its importance in shielding the earth from Ultraviolet radiation Click here to see a movie about how ozone is destroyed by CFCs

9. #19 List three ways in which the octet rule can sometimes fail to be obeyed. The octet rule cannot be satisfied in molecules: 1. whose total number of valence electrons is an odd number. 2. There are also molecules in which an atom has FEWER THAN 8 valence electrons, 3. Molecules with MORE than a complete octet of valence electrons. 1. Nitrogen as an ODD number (5) of valence electrons and thus sometimes does not obey the octet rule Nitrogen has only 7 valence electrons O N O

10. #19 continued 2. The boron in BF 3 has a deficiency of valence electrons (only 6 total) so it doesnt obey the octet rule B F F F 3. In the case of PCl 5 and SF 6 the P and the S have MORE THAN the octet (P has 10 and S has 12)

11. #20 What kinds of information does a structural formula reveal about the compound it represents? It reveals the way the atoms are bonded together: ie; single bonds, double bonds, triple bonds, and bond angles, unshared pairs, etc.

12. #21 Draw electron dot structures for the following molecules which have onlysingle covalent bonds. H 2 S S H H

13. PH 3 H H H P

14. ClF Cl F Cl F

15. # 22. Use the bond dissociation energies of H 2 (435 kJ/mole) and of a typical carbon- carbon bond (347kJ/mole) to decide which bond is stronger. Explain your reasoning. The hydrogen bond is stronger due to its greater bond dissociation energy (the amount of energy needed to break it.)