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Electronegativity • Covalent Compounds Form Distinct Molecules

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Electronegativity • Covalent Compounds Form Distinct Molecules The basic units: ionic vs. covalent • Ionic compounds form repeating units. Electronegativity • Covalent compounds form distinct molecules. • Consider adding to NaCl(s) vs. H2O(s): Cl Cl O δ+ δ– δ0 δ0 Na Na H H Cl Na Cl Na H O O H H H • NaCl: atoms of Cl and Na can add individually forming a compound with million of atoms. HCl HH •H2O: O and H cannot add individually, instead 1 2 molecules of H2O form the basic unit. Holding it together I’m not stealing, I’m sharing unequally Q: Consider a glass of water. Why do • We described ionic bonds as stealing electrons molecules of water stay together? • In fact, all bonds share – equally or unequally. A: there must be attractive forces. • Note how bonding electrons spend their time: + – Intramolecular H2 H H HCl H Cl LiCl [Li] [Cl] forces are 0 0 + – +– much stronger δ δ δ δ Intramolecular forces Intermolecular forces occur between atoms occur between molecules covalent polar covalent ionic • We do not consider intermolecular forces in (non-polar) ionic bonding because there are no molecules. • Point: the bonding electrons are shared in each • We will see that the type of intramolecular bond compound, but are not always shared equally. determines the type of intermolecular force. 3 • The greek symbol δ indicates “partial charge”.4 Electronegativity Electronegativity Answers • Recall that electronegativity is “a number that HCl: 3.0 – 2.1 = 0.9 polar covalent describes the relative ability of an atom, when bonded, to attract electrons”. CrO: 3.5 – 1.6 = 1.9 ionic • The periodic table has electronegativity values. Br2: 2.8 – 2.8 = 0 covalent • We can determine the nature of a bond based H O: 3.5 – 2.1 = 1.4 polar covalent on ∆EN (electronegativity difference). 2 • ∆EN = higher EN – lower EN CH4: 2.5 – 2.1 = 0.4 covalent NBr3: ∆EN = 3.0 – 2.8 = 0.2 (for all 3 bonds). KCl: 3.0 – 0.8 = 2.2 ionic • Basically: a ∆EN below 0.5 = covalent, 0.5 - 1.7 = polar covalent, above 1.7 = ionic • Determine the ∆EN and bond type for these: 5 6 HCl, CrO, Br2, H2O, CH4, KCl 1 Electronegativity & physical properties • Electronegativity can help to explain properties CaCl2 would have a lower Note: other factors of compounds like those in the lab. melting/boiling point: such as atomic size δ+ δ– CaCl2 = 3.0 – 1.0 = 2.0 within molecules + – • Lets look at HCl: partial charges δ δ CaF2 = 4.0 – 1.0 = 3.0 also affects melting keep molecules together. – δ + LiBr would have a lower + δ δ – and boiling points. δ melting/boiling point: ∆EN is an important • The situation is similar in NaCl, – + KCl = 3.0 – 0.8 = 2.2 factor but not the but the attraction is even greater LiBr = 2.8 – 1.0 = 1.8 only factor. It is (∆EN = 2.1 vs. 0.9 for HCl). + – H2S would have a lower most useful when • Which would have a higher melting/boiling point? melting/boiling point: comparing atoms NaCl because of its greater ∆EN. H2O= 3.5 – 2.1 = 1.4 and molecules of • For each, pick the one with the lower boiling H2S = 2.5 – 2.1 = 0.4 similar size. 7 8 point a) CaCl2, CaF2 b) KCl, LiBr c) H2O, H2S Why 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) δ+ δ+ δ+ δ+ δ– δ– + + + + Chemical Symbols of δ δ δ –δ δ– δ δ+ δ+ δ+ δ+ δ– δ– Common Elements The partial charges on Prepare a chart in your notes … δ + water attract, pushing + + δ δ δ – – δ the oil (with no partial δ + + + charge) out of the way. δ – δ δ 9 10 Z Name Symbol *Latin *Mnemonic Mnemonics 1 Hydrogen H • Mnemonics are ways to help you remember 2 Helium He • Used by A students and experts • They are often rhymes or visual connections Complete for Element Latin name • E.g. “Thirty days has September, April, June elements:1-20, 26, 28- Copper Cuprum and November, all the rest have 31” 30, 35, 47, 50, 53, 79, Gold Aurum • Or using your knuckles to remember months Iron Ferrum 80, 82, 92 • Iron (Fe) … Lead Plumbum Use the chart on left to •Iron → strong → opposite is feeble → Fe complete the two last Mercury Hydrargyrum • A bad mnemonic for Cu is a copper cup (any columns (only listed Potassium Kalium metal can be made into a cup) elements will have Silver Argentum • A good mnemonic is a cup full of pennies anything for these Sodium Natrium 11 • It may seem like more to know, but it works 12 columns). Tin Stannum 2 Best Mnemonics Best Mnemonics Sodium (Na) Iron (Fe) Salt? ↔ Bad for BP ↔ (Na, don’t want it) Ironing with Feet North Atlantic salt water French for iron is Fer Potassium (K) Iron = strong = giant = Fe-Fi-Fo-Fum Potatoes covered in ketchup Copper (Cu) Koala (or kangaroo) eating bananas Cu (see you) at copperfields Putting Special K into rolling paper Cop drinking out of a cup A cup filled with pennies 13 14 Best Mnemonics Best Mnemonics Silver (Ag) Mercury (Hg) Almost gold Aging = grey hair = silver thermometer = hug to stay warm Tin (Sn) Huge globe (Hg), Hot gas, High gravity “Tin is Sin” Lead (Pb) Gold (Au) Peanut butter coming out of a pencil Always united (wedding ring) Pellets and buckshot Gold = shiny = aura Plumber uses lead pipes leaves changing gold colour = Autumn 15 For more lessons, visit16 www.chalkbored.com 3.
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