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Bond ing and VSPER Theory

Bond ing and VSPER Theory. Bond Types. Ionic bonds Strongest (melting point?) Charged ions Metallic bonds Strong, but weaker than ionic “Sea of electrons” – all shared among all of them Covalent bonds Weakest (melting point?) Probably the most important substances of all are molecules.

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Bond ing and VSPER Theory

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  1. Bonding and VSPER Theory

  2. Bond Types • Ionic bonds • Strongest (melting point?) • Charged ions • Metallic bonds • Strong, but weaker than ionic • “Sea of electrons” – all shared among all of them • Covalent bonds • Weakest (melting point?) • Probably the most important substances of all are molecules

  3. Lewis Structure for Ions • When you need to draw a Lewis structure, always list the number of electrons you have first. This is especially important for when you have polyatomic ions (charged particles). • SO42- is a polyatomic ion – what would its Lewis structure be?

  4. SO42- • First, how many electrons? • S  6 • O  6 x 4 = 24 • Charge  2 extra e- • TOTAL  6 + 24 + 2 = 32 • Place S in the center, surrounded by the 4 O’s. Make a single bond first on each one. Then place electrons until you have a model that obeys the octet rule.

  5. NH4+ • Let’s try one more for ammonium. • N  5 • H  1 x 4 = 4 • Charge  -1 e- (a positive charge means we’re short electrons) • Total  5 + 4 – 1 = 8

  6. Electronegativity • Remember we can use electronegativity to calculate the polarity of a bond? • We find the electronegativity values for each atom in the bond on the periodic table (p. 174) • Then we subtract the smaller number from the bigger one – the difference tells us how polar the bond is. • 0.0 ------- 0.5 ----------- 1.7 ------------ 3.3 • nonpolar polar mostly ionic

  7. Practice with Electronegativity • Consider the compound HCl. Is this bond nonpolar covalent, polar covalent, or mostly ionic? • Hydrogen’s electronegativity is 2.2 • Chlorine’s electronegativity is 3.19 • What is the difference? • What about CCl?

  8. VSEPR Theory • Has anyone seen the movie Casino Royale? Know the name of the main girl? • Vesper. • Now I have to ask: what are the odds that a scriptwriterwould just happen toput a guy namedBond with a girlnamed Vesper?Clearly that writerknew their chemistry.

  9. Valence Shell Electron Pair Repulsion • Say that 5 times fast! • What does that name tell you about the theory? • It has to do with valence shell electrons, it seems they come in pairs, and they repel each other. • Why would electrons repel?

  10. Molecular Shape (Geometry) • VSEPR theory only applies to molecules (substances with only covalent bonds). • Its difficult to see VSEPR shapes on paper, so I’ll be showing you some shapes with a modeling kit.

  11. Counting Electron Pairs • In a molecule, there are two types of electron pairs: bonding pairs (those used to form bonds), and lone pairs (those not used to form bonds) • For counting purposes, double bonds and triple bonds count as a single electron pair. • Each electron pair wants to be as far away from all the others as possible. As you’ll see, this results in some very common shapes.

  12. Geometry and Polarity • Electronegativity can tell us more than just the polarity of a bond. It can tell us if a molecule will be polar overall. • First, consider CH4. What is the geometry? What is the polarity of each bond in the molecule? Is the molecule polar? • Look at CH3Cl. Calculate the polarity of each bond in the molecule. Is the molecule polar? • Last one: CCl4. Are the bonds polar? Is the molecule polar?

  13. Does polar bonds meana polar molecule? • No! If all the polar bonds in a molecule cancel out, the molecule will be nonpolar.

  14. Practice VSEPR with these • Determine the geometry of each of the following molecules, then determine if the molecules are polar or nonpolar overall. (Remember, a compound can have polar bonds and still be nonpolar!) • H2O XeF2 • SO2 NH3 • BF3 • (the ones in bold don’t obey the octet rule for the central atom)

  15. Your Unit Assignment • P. 181 (4, 6, 8, 10, 14, 20) • P. 225 (1, 5, 7, 10, 12, 14b,c,e, 19) • This assignment will be due the day before the unit test – you should have at least a week to finish it.

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