Do-Now: How can you experimentally determine if a compound is ionic or molecular? Good Morning! Today is Friday, December 4, 2015 HW Due: None

Conductivity Ionic Compounds Conducts electricity Molecular Compounds Does not conduct electricity WHY?

Structure of Ionic Compounds Ionic compounds are solids that contain identical formula units all attracted to one another due to the oppositely charged ions present Forms a crystal lattice Single formula unit of NaCl

Conductivity of Ionic Compounds Ionic compounds are unable to conduct on their own Ions are stuck tightly together due to opposite charges When an ionic compound is placed in water it becomes a good conductor

Conductivity of Ionic Compounds Water is able to pull the positive and negative ions apart from each other The positive and negative ions in the water allow electrons to flow Electricity is the movement of electrons

Conductivity of Covalent Compounds If water is able to pull apart a covalent compound, ions are not released Without ions present electrons cannot flow No electricity can be passed through the compound

Melting Point Ionic Compounds High MP Molecular Compounds Low MP WHY?

Intermolecular Forces Forces of attractions that exist between molecules in a compound The stronger the force, the more energy that is needed to pull the molecules in a compound apart This means that compounds with strong intermolecular forces will have a high melting point

IMF in Ionic Compounds The IMF found between molecules in ionic compounds are called electrostatic forces Occurs between opposite charges Strongest of all the intermolecular forces This is why ionic compounds have a higher melting point than covalent compounds

Breakdown of the Term Inter = Between Molecular = Molecules Force = Attraction The attractions between molecules!

IMF in Covalent Compounds There are two types of intermolecular forces in covalent compounds!

London Dispersion Forces Weakest intermolecular force Found between molecules in ALL covalent compounds May also be called “dispersion forces”

What Causes London Dispersion Electrons are constantly moving within a molecule There is a probability (chance) that the electrons all move to the same side of the molecule at once This creates what we call an instantaneous dipole (for an “instant” partial charges appear on the molecule).

Partial Charges Charges that are not always present in a molecule Represented by symbols ▫ δ- is written on the side of the molecule where the electrons are ▫ δ+ is written on the side of the molecule where the electrons are not

What Causes London Dispersion When an instantaneous dipole is formed in one molecule, the neighboring molecules all respond by shifting their electrons in the same direction. This is called an induced dipole!

What Causes London Dispersion The dipoles (instantaneous and induced) in the molecules cause the molecules to attract to each other. This attraction is the intermolecular force known as London Dispersion. Since the electrons are constantly moving, the dipoles are not always present. Therefore, the London Dispersion force is formed and broken constantly, making this a weak force.

Dipole - Dipole Visually, this looks very similar to London Dispersion, but it is called by a different name.

What Causes Dipole - Dipole Sometimes the electrons in covalent compound molecules are permanently stuck on one side of a molecule. This means the dipoles are always present in the molecules Attractions between the dipoles are permanent (do not break and form constantly like London dispersion forces) Stronger intermolecular force than London Dispersion

Hydrogen Bond A special case of Dipole – Dipole and is the strongest intermolecular force

What Causes Hydrogen Bonds When hydrogen is bonded to O, N, or F in one molecule and is then attracted to O, N, or F in a neighboring molecule of the same kind.

Conclusion What are the main differences between London dispersion forces and dipole-dipole forces in a covalent compound?

Homework None! Have a safe and fun weekend!