Notes on Shape & review of electronegativity and bonding.

Your Assignment-due 1/11/12• 1. Finish the worksheet from

Friday• 2. Read Page 332-339 in your

book-read it carefully and go through all the practice problems so you understand them.

• 3. Answer #1-3 from page 339• 4. Read pg. 341• 5. From pg. 342-343 do questions:

5, 9, 10, 13-20, 22

I was watching Ironman II over the weekend,

the compound that helped

cure his blood

toxicity was lithium dioxide. Name 2 things

wrong with the name of

this compound.

DMA #1 1/12/12

Calculate the percent composition of: CaCl2

CH4

C3H8 DMA #2 1/13/12

What are the 7diatomic

elements? Hint-look on your periodic

table!DMA #3 1/24/12

The Valence shell electron pair repulsion theory – VSEPR Theory

• The shape of a molecule or ion is governed by the arrangement of the electron pairs around the central atom.

• The pairs of electrons arrange themselves to produce the minimum amount of repulsion between them.

• In other words, they spread out as far as they can!• You have to consider both bonding pairs and lone

pairs.

Four electron pairs around the central atom

• The simplest is methane, CH4. Determine its Lewis dot diagram.

• Four electron pairs arrange themselves in space in what is called a tetrahedral arrangement. A tetrahedron is a regular triangularly-based pyramid. The carbon atom would be at the centre and the hydrogens at the four corners. All the bond angles are 109.5°.

• Build CH4 & draw it.

Other examples with four electron pairs around the central atom

• Ammonia, NH3 – Determine its dot diagram• Nitrogen is only forming 3 bonds, one of the pairs

must be a lone pair. • The electron pairs arrange themselves in a tetrahedral

fashion as in methane.• Draw NH3

An Additional Factor• Lone pairs are in orbitals that are shorter and rounder than

the orbitals that the bonding pairs occupy. • Because of this, there is more repulsion between a lone pair

and a bonding pair than there is between two bonding pairs.• That forces the bonding pairs together slightly - reducing the

bond angle from 109.5° to 107°.

Remember this:

• Greatest repulsion lone pair - lone pair • lone pair - bond pair• Least repulsion bond pair - bond pair• When you describe the shape, you only take notice

of the atoms. Lone pairs are ignored although they influence the shape.

• Ammonia is pyramidal - like a pyramid with the three hydrogens at the base and the nitrogen at the top.

Water, H2O • Draw the dot diagram for H2O

• Oxygen has four pairs of electrons, two of which are lone pairs.

• These make a tetrahedral arrangement. T

• The bond angle closes slightly to 104°, because of the repulsion of the two lone pairs.

• The shape isn't described as tetrahedral, because we only "see" the oxygen and the hydrogens - not the lone pairs.

• Water is described as bent or V-shaped.

Three electron pairs around the central atom

• The simple cases of this would be BF3 or BCl3.

• Boron is in group 3, so starts off with 3 electrons.

• It is forming 3 bonds, adding another 3 electrons. There is no charge, so the total is 6 electrons - in 3 pairs.

• Because it is forming 3 bonds there can be no lone pairs. The 3 pairs arrange themselves as far apart as possible. They all lie in one plane at 120° to each other. The arrangement is called trigonal planar.

Electronegativity• Electronegativity is the ability of an atom to

attract electrons to form a chemical bond. • An atom's electronegativity is related to its

electron affinity and ionization energy. • (Electron affinity is the amount of energy that

is liberated by a gaseous element when an electron is added to it.)

• Ionization energy is the smallest amount of energy required to remove the most weakly bond electron from an atom of a gas.

What happens if two atoms of equal electronegativity bond together?

• Consider a bond between two atoms, A and B. Each atom may be forming other bonds as well as the one shown - but these are irrelevant to the argument.

• If the atoms are equally electronegative, both have attract the bonding pair of electrons, and so it will be found on average half way between the two atoms.

• A and B would usually have to be the same atom. You will find this sort of bond in, for example, H2 or Cl2 molecules.

What happens if B is slightly more electronegative than A?

• B will attract the electron pair more than A does.

• That means that the B end of the bond has more than its fair share of electron density and so becomes slightly negative.

• The A end (rather short of electrons) becomes slightly positive.

• In the diagram, “d" (read as "delta") means "slightly" - so d+ means "slightly positive".

Polar Covalent Bonding Animation

• http://www.tutorvista.com/content/chemistry/chemistry-iii/chemical-bonding/polar-covalent-animation.php

Defining polar bonds • This is described as a polar bond. • A polar bond is a covalent bond in which

one end is slightly positive and the other slightly negative.

• Examples include most covalent bonds. The hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical.

What happens if B is a lot more electronegative than A?

• In this case, the electron pair is dragged right over to B's end of the bond.

• To all intents and purposes, A has lost control of its electron, and B has complete control over both electrons. Ions have been formed.

• So-it is called an ionic bond

Summary

• No electronegativity difference between two atoms leads to a pure non-polar covalent bond.

• A small electronegativity difference leads to a polar covalent bond.

• A large electronegativity difference leads to an ionic bond.

Polar bonds and polar molecules

• In a simple molecule like HCl, if the bond is polar, so also is the whole molecule. What about more complicated molecules?

• In CCl4, each bond is polar.• The molecule as a whole, however, isn't polar - in the sense that it

doesn't have an end (or a side) which is slightly negative and one which is slightly positive. The whole of the outside of the molecule is somewhat negative, but there is no overall separation of charge from top to bottom, or from left to right.

By contrast, CHCl3 is polar.

• The hydrogen at the top of the molecule is less electronegative than carbon and so is slightly positive. This means that the molecule now has a slightly positive "top" and a slightly negative "bottom", and so is overall a polar molecule.

• A polar molecule will need to be "lop-sided" in some way.

Patterns of electronegativity in the Periodic Table

• The most electronegative element is fluorine. If you remember that fact, everything becomes easy, because electronegativity must always increase towards fluorine in the Periodic Table.