UNIT 1: SECTION 3 - BONDING
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What are ionic bonds?
electrostatic attraction between atoms - v.strong
what are covalent bonds?
large difference in electronegativity between non-metals. electron pair found in the middle of cov. bond.
Two types of covalent structures.
simple covalent molecules & giant macromolecular structures.
Two macromolecular structures.
Graphite: Forms bond with 3 other Carbon atoms. Carries delocalised electrons; carries charge Sheets of hexagons; sheets held by weak IMF’s; layers can break off and slide. Strong cov. bonds; insoluble and high MP & BP. Layers far apart - low density; strong light equipment. Diamond: Forms bond with 4 other Carbons (tetra). No delocalised electrons; doesn’t carry charge. High MP & BP (strong cov. bonds); vibrations travel so it is good thermal conductor. v.tough - diamond drills.
What is a dative cov. bond?
When one atom donates its lone pair to an acceptor. shown by an arrow coming from donor.
What is a simple cov. molecule? What are properties of it?
Few atoms held by covalent bonds, e.g. H2O. Weak IMF’s hold simple cov. molecules together; low mp & bp. Insoluble in water (cov. bonds v.strong).
What is a giant ionic lattice? What are the properties of it?
Same units repeated e.g. NaCl Water-soluble (water polar) High MP. When molten, they conduct electricity.
What is the valence shell electron pair repulsion theory?
Charge clouds repel each other as much as they can; Lone Pair - Lone Pair Lone Pair - Bonding Pair Bonding Pair - Bonding Pair
What is the method to predict the shape?
1) Group of central atom + Number of bonded atoms. 2) Divide by two = number of electron pairs. 3) Number of bonding pairs = number of bonded atoms. 4) Any remnant pairs, Lone Pairs. 5) Work out shape from here.
Bond angles for 2 electron regions?
180 degrees
Bond angles for 3 electron regions?
120 degrees
Bond angles for 4 electron regions?
109.5 degrees
Bond angles for 5 electron regions?
90 & 120 degrees
Bond angles for 6 electron regions?
90 degrees
What are the most electronegative elements?
NOF
What does electronegativity mean?
The ability to attract electrons in a covalent bond.
What does a high electronegativity lead to?
Polar bond with a permanent dipole.
What is a dipole?
difference in charge caused by shift in electron charge density.
How can molecules be polar?
If molecules are unsymmetrical, the charge distribution is uneven and it is polar.
True or false: Polar molecules have permanent dipole-dipole IMF’s.
True because there is a permanent difference due to shift of electron charge density (due to high EN atom pulling electron pair).
What is the order of IMF strength?
Strongest: Hydrogen bonds, Permanent dipole-dipoles, London forces.
What are London Forces?
Forces which occur due to a temporary shift of electrons towards one side of charge cloud. LF’s can cause chain effect; neighbouring molecules repelled by slightly negative/positive end, and turn into temporary dipole and so on… Constantly created/destroyed making them have weak attraction.
One lattice which is held by LF’s?
Iodine lattice.
What effect does London Forces have on the physical properties of molecules?
Increases the Boiling point - • a larger molecule has a larger electron cloud; further distance for LF’s to cover. • longer, straighter molecules can also lie closer together; forces stronger. • IMF’s need to be overcome to boil.