2The basic units: ionic vs. covalent Ionic compounds form repeating units.Covalent compounds form distinct molecules.Consider adding to NaCl(s) vs. H2O(s):ClNaClHONaClClNaNaHOHONaCl: atoms of Cl and Na can add individually forming a compound with million of atoms.H2O: O and H cannot add individually, instead molecules of H2O form the basic unit.
3Holding it togetherQ: Consider a glass of water. Why do molecules of water stay together?A: there must be attractive forces.Intramolecular forces are much strongerIntramolecular forces occur between atomsIntermolecular forces occur between moleculesWe do not consider intermolecular forces in ionic bonding because there are no molecules.We will see that the type of intramolecular bond determines the type of intermolecular force.
4I’m not stealing, I’m sharing unequally We described ionic bonds as stealing electronsIn fact, all bonds share – equally or unequally.Note how bonding electrons spend their time:H2HClLiClHCl[Li]+[ Cl ]–H+–0+–covalent (non-polar)polar covalentionicPoint: the bonding electrons are shared in each compound, but are not always shared equally.The greek symbol indicates “partial charge”.
5ElectronegativityRecall that electronegativity is “a number that describes the relative ability of an atom, when bonded, to attract electrons”.The periodic table has electronegativity values.We can determine the nature of a bond based on EN (electronegativity difference).EN = higher EN – lower ENNBr3: EN = 3.0 – 2.8 = 0.2 (for all 3 bonds).Basically: a EN below 0.5 = covalent, = polar covalent, above 1.7 = ionicDetermine the EN and bond type for these: HCl, CrO, Br2, H2O, CH4, KCl
7Electronegativity & physical properties CaCl2 would have a lower melting/boiling point:CaCl2 = 3.0 – 1.0 = 2.0CaF2 = 4.0 – 1.0 = 3.0Electronegativity can help to explain properties of compounds like those in the lab.Note: other factors such as atomic size within molecules also affects melting and boiling points. EN is an important factor but not the only factor. It is most useful when comparing atoms and molecules of similar size.+–Lets look at HCl: partial charges keep molecules together.+–LiBr would have a lower melting/boiling point:KCl = 3.0 – 0.8 = 2.2LiBr = 2.8 – 1.0 = 1.8+–+–The situation is similar in NaCl, but the attraction is even greater (EN = 2.1 vs. 0.9 for HCl).–+–+H2S would have a lower melting/boiling point:H2O= 3.5 – 2.1 = 1.4H2S = 2.5 – 2.1 = 0.4Which would have a higher melting/boiling point?NaCl because of its greater EN.For each, pick the one with the lower boiling point a) CaCl2, CaF2 b) KCl, LiBr c) H2O, H2S
8CaCl2 would have a lower melting/boiling point: CaF2 = 4.0 – 1.0 = 3.0Note: other factors such as atomic size within molecules also affects melting and boiling points. EN is an important factor but not the only factor. It is most useful when comparing atoms and molecules of similar size.LiBr would have a lower melting/boiling point:KCl = 3.0 – 0.8 = 2.2LiBr = 2.8 – 1.0 = 1.8H2S would have a lower melting/boiling point:H2O= 3.5 – 2.1 = 1.4H2S = 2.5 – 2.1 = 0.4
9Why oil and water don’t mix Lets take a look at why oil and water don’t mix (oil is non-polar, water is polar)+–+–The partial charges on water attract, pushing the oil (with no partial charge) out of the way.+–
10Chemical Symbols of Common Elements Prepare a chart in your notes …
11ZNameSymbol*Latin*Mnemonic1HydrogenH2HeliumHeElementLatin nameCopperCuprumGoldAurumIronFerrumLeadPlumbumMercuryHydrargyrumPotassiumKaliumSilverArgentumSodiumNatriumTinStannumComplete for elements:1-20, 26, 28-30, 35, 47, 50, 53, 79, 80, 82, 92Use the chart on left to complete the two last columns (only listed elements will have anything for these columns).
12Mnemonics Mnemonics are ways to help you remember Used by A students and expertsThey are often rhymes or visual connectionsE.g. “Thirty days has September, April, June and November, all the rest have 31”Or using your knuckles to remember monthsIron (Fe) …Iron strong opposite is feeble FeA bad mnemonic for Cu is a copper cup (any metal can be made into a cup)A good mnemonic is a cup full of penniesIt may seem like more to know, but it works
13Best Mnemonics Sodium (Na) Salt? Bad for BP (Na, don’t want it) North Atlantic salt waterPotassium (K)Potatoes covered in ketchupKoala (or kangaroo) eating bananasPutting Special K into rolling paper
14Best Mnemonics Iron (Fe) Ironing with Feet French for iron is Fer Iron = strong = giant = Fe-Fi-Fo-FumCopper (Cu)Cu (see you) at copperfieldsCop drinking out of a cupA cup filled with pennies
15Best Mnemonics Silver (Ag) Almost gold Aging = grey hair = silver Tin (Sn)“Tin is Sin”Gold (Au)Always united (wedding ring)Gold = shiny = auraleaves changing gold colour = Autumn
16For more lessons, visit www.chalkbored.com Best MnemonicsMercury (Hg)thermometer = hug to stay warmHuge globe (Hg), Hot gas, High gravityLead (Pb)Peanut butter coming out of a pencilPellets and buckshotPlumber uses lead pipesFor more lessons, visit