Electonegativity, Polar Bonds, and Polar Molecules

Transcription

1 Electonegativity, Polar Bonds, and Polar Molecules

2 Some Definitions Electronegativity: the ability of an atom to attract bonding electrons to itself. Intramolecular forces: the attractive force between atoms and ions within a compound. Intermolecular forces: the attractive force between molecules.

3 Polar Covalent Bonds When a chemical bond is formed, it is not always exclusively ionic or covalent. In the case of shared electrons between two identical atoms, the electrons are shared equally. However, this is not the case for a compound like hydrogen chloride, where electrons are shared between two different elements.

4 In this situation, the sharing is unequal, as the bonding electrons spend more time near one atom than near the other. Due to this unequal sharing of electrons, one atom will have a slightly positive charge while the other will have a slightly negative charge.

5 We indicate these slight charges by + and -. (The Greek letter delta,, indicates a small difference. ) The bond is somewhere between an ionic bond and a covalent bond and is called a polar covalent bond. Does HCl have a polar covalent bond? NH 3?

6 To determine if the bond is ionic, covalent or polar covalent, measure the difference in electronegativity of the atoms. EN > ionic bond EN < covalent bond

7 What type of bond do the following compounds have: CsF, HF, and F 2

8 Polar Molecules A molecule that is slightly positively charged at one end and slightly negatively charged at the other because of electronegativity differences is classified as a polar molecule. Not all molecules containing polar covalent bonds are polar molecules.

9 Take a look at CH 4 and HCl.

10 polar molecule: a molecule in which the uneven distribution of electrons results in a positive charge at one end and a negative charge at the other end non-polar molecule: a molecule in which the electrons are equally distributed among the atoms, resulting in no localized charges

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12 Intermolecular Forces There are three kinds of intermolecular forces. Two of these are classified as van der Waals forces.

13 Dipole-Dipole Forces Dipole-dipole forces: an attractive force acting between polar molecules As a polar molecule, hydrogen fluoride has a negatively charged fluoride end and a positively charged hydrogen end.

14 When two hydrogen fluoride molecules are next to one another, the positive end of one molecule is attracted to the negative end of the next molecule. dipole-dipole force

15 London Dispersion Forces London dispersion forces: an attractive force acting between all molecules and unbonded atoms, including nonpolar molecules.

16 Hydrogen Bonds Water, a polar molecule, consists of one atom of oxygen bound by single covalent bonds to two hydrogen atoms. Its structure is simple, but water exhibits some rather unusual properties: higher than expected melting and boiling points, high vapour pressure, high surface tension, and the ability to dissolve a large number of substances.

17 To explain these properties, we must consider the intermolecular forces that exist between water molecules. As a result of the large difference in electronegativity between the hydrogen and oxygen, the O-H bonds in a molecule of water are highly polar covalent. As a result, the hydrogen atoms of one water molecule exert a strong force of attraction on the oxygen atom of neighbouring water molecules.

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19 This is known as a hydrogen bond. Hydrogen bonds occur among highly polar molecules containing F-H, O-H and N-H bonds. Although a hydrogen bond is similar to a dipole-dipole force, it is stronger than any of the van der Waals forces.

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21 Special Properties of Water Unusually high Melting and Boiling Points Low Density of Ice High Surface Tension (a phenomenon that leads to the formation of a skin-like film on the surface of a liquid) High Specific Heat Capacity (the quantity of the energy that a certain mass of a substance can absorb, and warm up by 1ºC)

22 Hydrogen Bonds in Biochemistry Hydrogen bonds play a significant role in determining the shape and function of large, biologically important molecules.

23 Proteins consist of hundreds or even thousands of atoms. The chain of atoms folds into very specific threedimensional structures because of attractions between different parts of the chain.

24 A DNA molecule is made up of two long chains of structures called nucleotides. Hydrogen bonds readily form between the two helixes, holding them together like the rungs of the ladder