Electronegativity Electronegativity Covalent compounds form - - PDF document

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Electronegativity Electronegativity

δ+ δ– δ0 δ0 H Cl H H

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The basic units: ionic vs. covalent The basic units: ionic vs. covalent

• Ionic compounds form repeating units.
• Covalent compounds form distinct molecules.
• Consider adding to NaCl(s) vs. H2O(s):

H O H Cl

Na Na Cl

Cl Cl

Na Na

H O H H O H

• NaCl: 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.

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Holding it together Holding it together

Q: Consider a glass of water. Why do molecules of water stay together? A: there must be attractive forces. Intramolecular forces

• ccur between atoms

Intermolecular forces

• ccur between molecules
• We 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. Intramolecular forces are much stronger

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I I’ ’m not stealing, I m not stealing, I’ ’m sharing unequally m sharing unequally

• We described ionic bonds as stealing electrons
• In fact, all bonds share – equally or unequally.
• Note how bonding electrons spend their time:
• Point: the bonding electrons are shared in each

compound, but are not always shared equally.

• The greek symbol δ indicates “partial charge”.

H2 HCl LiCl

δ+ δ– δ0 δ0 + –

covalent (non-polar) polar covalent ionic

H H H Cl [Li]+[ Cl ]–

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Electronegativity Electronegativity

• Recall 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
• n ∆EN (electronegativity difference).
• ∆EN = higher EN – lower EN

NBr3: ∆EN = 3.0 – 2.8 = 0.2 (for all 3 bonds).

• 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:

HCl, CrO, Br2, H2O, CH4, KCl

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Electronegativity Answers Electronegativity Answers

HCl: 3.0 – 2.1 = 0.9 polar covalent CrO: 3.5 – 1.6 = 1.9 ionic Br2: 2.8 – 2.8 = covalent H2O: 3.5 – 2.1 = 1.4 polar covalent CH4: 2.5 – 2.1 = 0.4 covalent KCl: 3.0 – 0.8 = 2.2 ionic

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Electronegativity & physical properties Electronegativity & physical properties

• Electronegativity can help to explain properties
• f compounds like those in the lab.

δ+ δ– δ+ δ– δ δ+ δ δ– δ+ δ–

• Lets look at HCl: partial charges

keep molecules together.

• The situation is similar in NaCl,

but the attraction is even greater (∆EN = 2.1 vs. 0.9 for HCl).

• Which 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 –

+ +

–

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CaCl2 would have a lower melting/boiling point: CaCl2 = 3.0 – 1.0 = 2.0 CaF2 = 4.0 – 1.0 = 3.0 LiBr would have a lower melting/boiling point: KCl = 3.0 – 0.8 = 2.2 LiBr = 2.8 – 1.0 = 1.8 H2S would have a lower melting/boiling point: H2O= 3.5 – 2.1 = 1.4 H2S = 2.5 – 2.1 = 0.4 Note: other factors such as atomic size within molecules also affects melting and boiling points. ∆EN is an important factor but not the

• nly factor. It is

most useful when comparing atoms and molecules of similar size.

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Why oil and water don Why oil and water don’ ’t mix 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+ δ+ δ–δ+ δ+ δ–δ+ δ+ δ–δ+ δ+ δ–δ+ δ+ δ–δ+ δ+ δ–δ+ δ+ δ–δ+ The partial charges on water attract, pushing the oil (with no partial charge) out of the way.

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Chemical Symbols of Common Elements

Prepare a chart in your notes …

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Complete for elements:1-20, 26, 28- 30, 35, 47, 50, 53, 79, 80, 82, 92 Use the chart on left to complete the two last columns (only listed elements will have anything for these columns). He Helium 2 H Hydrogen 1 *Mnemonic *Latin Symbol Name Z Stannum Tin Natrium Sodium Argentum Silver Kalium Potassium Hydrargyrum Mercury Plumbum Lead Ferrum Iron Aurum Gold Cuprum Copper Latin name Element

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Mnemonics

• Mnemonics are ways to help you remember
• Used by A students and experts
• They are often rhymes or visual connections
• E.g. “Thirty days has September, April, June

and November, all the rest have 31”

• Or using your knuckles to remember months
• Iron (Fe) …
• Iron → strong → opposite is feeble → Fe
• A bad mnemonic for Cu is a copper cup (any

metal can be made into a cup)

• A good mnemonic is a cup full of pennies
• It may seem like more to know, but it works

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Best Mnemonics

Sodium (Na) Salt? ↔ Bad for BP ↔ (Na, don’t want it) North Atlantic salt water Potassium (K) Potatoes covered in ketchup Koala (or kangaroo) eating bananas Putting Special K into rolling paper

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Best Mnemonics

Iron (Fe) Ironing with Feet French for iron is Fer Iron = strong = giant = Fe-Fi-Fo-Fum Copper (Cu) Cu (see you) at copperfields Cop drinking out of a cup A cup filled with pennies

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Best Mnemonics

Silver (Ag) Almost gold Aging = grey hair = silver Tin (Sn) “Tin is Sin” Gold (Au) Always united (wedding ring) Gold = shiny = aura leaves changing gold colour = Autumn

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Best Mnemonics

Mercury (Hg) thermometer = hug to stay warm Huge globe (Hg), Hot gas, High gravity Lead (Pb) Peanut butter coming out of a pencil Pellets and buckshot Plumber uses lead pipes

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