1. Mrs. Paparella Bonding Unit 11-06-06
Ionic Compounds vs Molecular Substances and Intermolecular Forces of Attraction
Mrs. Paparella
Bonding Unit

2. Ionic Compounds Ionic compounds are never called molecules.
Characteristics of ionic solids include
High melting point
Do not conduct electricity in the solid state
Are crystalline in structure (alternately have + - ions to form a geometric crystal lattice)

3. When you add water…
Adding water to an ionic solid, like NaCl, will dissolve the solid into the cations and anions. The ions link to the water molecules and will conduct electricity.
NaCl (aq) will conduct electricity. It forms hydrated ions through the process of hydration.

4. Molecule-Ion Attraction
When the + cations are attracted to the
– dipole on the oxygen end of the water molecule and the – anions are attracted to the + dipole of the hydrogen end of the water molecule, hydrated ions are formed. These can then conduct electricity.

5. Molecular Substances Always covalent Can exist as s, l or g
Characteristics include:
Soft
Poor conductors of heat and electricity
Low melting points
Examples include: water, sugar, ammonia, diatomic elements, carbon tetrachloride (CCl4)

6. Network Solids
Covalent bonds link atoms in a giant network- a.k.a. macromolecules
Characteristics:
Hard
Poor conductors of heat and electricity
High melting points
Examples: asbestos, diamond (C), SiC,
SiO2 (sand), graphite (C)

7. Van der Waals Forces A type of Intermolecular Attraction
Weak polar attractions between nonpolar molecules [CO2, N2, H2, CH4,etc]
These forces make it possible for H2, O2 and He to exist in the liquid and solid phases (normally gases) under special conditions of low temp and high pressure.
What happens when gases get cold and experience more pressure (less volume)?

8. Brrrrr… It’s crowded here.
When gases get cold they condense to a liquid. When they experience high pressure, they get crowded together in a smaller space and can form liquids or gases.

9. Trend with Van der Waals Forces
They get stronger as you go down a group because of the greater number of e- placed farther away from the nucleus.
Mass increases/van der Waals increase too.
This results in higher boiling point (BP) as you go down a group [Halogens and Noble Gases] or within a series of compounds.
Why?

10. It’s harder to get the molecules apart!!!
Remember in order to boil something, heat energy needs to be added to separate the liquid molecules from each other. The stronger the attraction between the molecules, the higher the temp needed to make it boil. The stronger van der Waals forces need to be overcome.
In the halogen group, these forces account for the 3 different phases. F-gas Cl- gas Br- liquid I- solid.

11. Many elements form compounds with hydrogen - referred to as "hydrides"
Many elements form compounds with hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group.
The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.

13. Hydrogen Bonding Another Type of Intermolelcular Attraction
Takes place between molecules that have Hydrogen in them and some highly electronegative element (like F, O, N)
Examples: H2O, HF, NH3

14. Hydrogen Bonding (cont’d)
Water has a high BP due to the h-bonding between the molecules. It takes more energy to separate the liquid molecules from each other.

15. H-bonding explains why H2O, HF and NH3 have much higher BPs.