Covalent Bonds in Molecular Compounds

There are two types of covalent bonding 1. Non-polar bonding with an equal sharing of electrons. 2. Polar bonding with an unequal sharing of electrons. The number of shared electrons depends on the number of electrons needed to complete the octet. Use the END to determine if a bond is polar or non-polar

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1. Non Polar Bonds What does polarised/polarity mean? Opposites Non polar bonding results when two identical atoms equally share electrons between them.

Iodine (I2) and oxygen (O2) forms a diatomic (di – two, atomic – atoms) non-polar covalent molecule.

Range from non-polar to polar to ionic bonds- increasing inequality

2. Polar Bonds Polarized bonds occur in covalent molecules. One atom has greater electronegativity and therefore has a partial (slight) negative charge because it attracts the bonding electrons more strongly (remember electrons have a –ive charge). The other atom has a slight positive charge. Overall, we can say that the molecule has a dipole

Partial positive is represent by this symbol Partial negative is represented By this symbol

Which is more electronegative H or Cl? Cl has greater electronegativity The bonding electrons therefore spend more time near the Cl atom So the Cl side of the molecule becomes slightly negative (partial negative charge) and the H side becomes slightly positive making a polarised bond What about HF?

Polar Molecules Polar molecules are those containing polar bonds that do not cancel each other out Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, a molecule of water is polar because of the unequal sharing of its electrons in a "bent" structure, whereas methane is considered non-polar because the carbon shares the hydrogen atoms uniformly.

Polar The covalent bond between HCl is said to be polarized and the molecule is called a polar molecule It is also said to have a dipole because is has two charged ends

Intermolecular force: dipole-dipole When a molecule has a dipole, other molecules of the chemical nearby also have a dipole….they are thus attracted to each other because of electrostatic forces. Between polar molecules, there is a dipole-dipole force. The force between the molecules affects its physical and chemical properties. An example is that it increases its m.p. and b.p.

Non polar molecules 1- Non polar molecules are those that contain only non-polar bonds OR 2- Polar molecules in which polar bonds cancel each other out Cancellation of charge is the result of the shape of the molecule

Which shapes do you recognize? Which shapes are asymmetrical? x

Youtube clip http://www.youtube.com/watch?v=GQwpb5jyMq8

Quiz What happens to the electrons in a polar bonds? How do you know if a bond is polar? What is a non polar molecule? What is a dipole? What is the symbol for slightly negative? Can non polar molecules have polar bonds?

How to determine polar molecules There are two important factors 1. The polarity of the individual bonds in the molecule; 2. The shape or geometry of the molecule. Steps to take 1. Determine if a given individual bond is polar, Look at the difference between electronegativity of the atoms in the periodic table. If the difference is: 0.4 ≤ non polar ›0.4 -1.7 = polar

b) Determine the shape of molecule. Then there are 3 options i) if all bonds are non-polar, then the whole molecule is non-polar regardless of its shape. ii) If there are polar bonds and there is symmetry in the molecule so that the polarity of the bonds cancels out, then the molecule is non-polar. (symmetry around the central atom) iii) If there are polar bonds but there is no symmetry, the overall molecule is polar.

Which molecules are polar?

Determine if BF3 is polar or non polar 1- bonds are polar 2- but they are symmetrically arranged (trigonal planar) around the central boron atom. No side of the molecule has more negative or positive charge than another side, and so the molecule is nonpolar.

Predict whether the following molecules are polar or nonpolar: (a) BrCl; (a-1) Chlorine is more electronegative than bromine END= 0.8 - bond is polar a-2 Br-Cl is linear….consequently, BrCl molecule will be polar with chlorine carrying the partial negative charge:

Predict whether the following molecules are polar or nonpolar: (b) SO2 (b-1) Because oxygen is more electronegative than sulfur, the molecule has polar bonds. END= 0.8 (b-2) VSEPR predicts a bent shape. The bond dipoles do not cancel and the molecule is polar

Which of the following molecules contains polar bonds but is nonpolar? NH3 F2 CCl4 CH2Cl2

In CCl4, there are 4 bonds arranged symmetrically around the central carbon atom. Each bond is polar, you can tell due to the electronegativity differences between carbon and chlorine. Because the 4 atoms of chlorine are arranged symmetrically, the polarity of the bonds cancel each other out, making a non-polar molecule.

Determine if OF2 is polar or non polar The electronegativities of oxygen and fluorine, 3.44 and 3.98, respectively, produce a 0.54 difference that leads us to predict that the O-F bonds are polar. The molecular geometry of OF2 is bent. Such an asymmetrical distribution of polar bonds would produce a polar molecule.

When there are no polar bonds in a molecule, there is no permanent charge difference between one part of the molecule and another… thus the molecule is nonpolar. (ALWAYS)

What is the difference between a polar bond and a polar molecule? A polar bond is a charged bond. There is a positive and negative end that do not cancel each other out relative to the bond only A polar molecule contains polar bonds that do not cancel each other out overall within the molecule

Why is it important to determine if a molecule is polar or non-polar? A polar molecule has partially charged ends…in some ways this is like ionic compounds…this means that a polar molecule will have Higher mp, higher bp, Higher solubility in water Higher conductivity Than non-polar molecules