Molecular compounds

1. Linus Pauling 1932 came up with an easy way to determine the type of bond that will form when 2 atoms come together. He assigned a number to each element. This number represented the "holding and pulling powers" of the atom and expressed them as a single number. So all you have to do is subtract the 2 numbers and you will know the type of bond. i.e. If the difference is 1.7 or greater ionic Molecular compounds Sometimes when atoms come together it is not possible for ionization. The combination of the "pulling power" (anion) and the "holding power" (cation) is such that e- are not taken from one and given to another.

2. Types of bonds If the bond is between a metal and a non metal……………….. Ionic i.e.NaCl EN for Na is 0.9 and the EN for Cl is 3.2 3.2 – 0.9 = 2.3  very ionic Could a bond form between 2 non metals? Yes i.e. H2 Each atom is identical. The difference in EN would be 0. So how does this molecule exist? As the 2 hydrogen come together each will attract the others e-. But their electro negativitys are the same. Each e- ends up spending an equal amount of time orbiting each H atom. The electrons are being shared by each of the H atoms. (Like a tug of war.) So if you can imagine the e- moving back and forth really, really fast then each atom has the outer orbital full.  stable

3. Diatomic Molecules • If the two atoms bonded together are the same element then this is called a diatomic molecule. • i.e. H2 N2 F2 Cl2 O2 • See Table 3 page 209 for more examples

4. The Water Molecule Oxygen Group VI has a valence of -2 It has an electro negativity value of 3.5 Hydrogen needs 1 e- to fill its outer shell. Its EN value is 2.2. 3.5 – 2.2 = 1.5 This is less than 1.7 so the bond will be covalent. Just for interest. To be pure covalent the EN difference must be less than 0.5 So although the bonds in water are covalent they are not equal between the oxygen and the hydrogen. These are said to be Polar bonds which are bonds that result from an unequal sharing of the e-. One "end " of the molecule will be more –ve than the other.

5. The Water Molecule Notice how the 2 pairs of unshared e- in oxygen come together at the top and actually force the two unshared ones away. This is what gives water its shape and its polarity. Negative at the O end and less negative (positive) at the hydrogen end. This is why ionic substances will dissolve in water. The cations are attracted to the O end of H2O and the anions to the H end. Also note how the shared pair is represented by a line.

6. Salt Dissolving in Water

7. Covalent Bonds (page 206 – 209) • Covalent bonds are bonds between atoms where the e- are shared by the atoms. • Covalent bonds result in Molecular compounds. • Properties of Molecular Compounds Covalent bonds are not as strong as ionic bonds so molecular compounds : are more flexible. have lower melting points. And many like oil are not water soluble. (non polar)

8. Writing Formulae for Molecular Compounds • The ancients: These are compounds that we have known for centuries. So they have "common" names. • For example (see table 2 page 208) • H2O water • NH3 ammonia • H2S hydrogen sulphide • However there are rules for naming the vast number of molecular compounds that exist. • Prefixes are attached to an element to denote how many atoms of that element are present. • i.e. water is dihydrogen oxide • ammonia is nitrogen trihydride

9. Prefixes (table 3 page 209) Note: The rule for the ending is the same as for ionic compounds. They end in an -ide. i.e. Phosphorus + Chlorine , PCl3 is phosphorus chloride P2O3 =

10. Phosphorus trichloride • HW • Page 212 #1, 2, 3, 5, 6, 9 (good one!)