ElectronegativityElectronegativity

+ – 0 0

H Cl H H

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

HO

H Cl Na

Na Cl

Cl

Cl

Na

Na H

O H

HO 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.

Holding it togetherHolding it togetherQ: Consider a glass of water. Why do

molecules of water stay together?A: there must be attractive forces.

Intramolecular forces occur between atoms

Intermolecular forces occur 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

I’m not stealing, I’m sharing unequallyI’m not stealing, I’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 indicates “partial charge”.

H2 HCl LiCl

+ –0 0 + –

covalent (non-polar) polar covalent ionic

H H H Cl [Li]+[ Cl ]–

ElectronegativityElectronegativity• 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 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, 0.5 - 1.7 = polar covalent, above 1.7 = ionic

• Determine the EN and bond type for these: HCl, CrO, Br2, H2O, CH4, KCl

Electronegativity AnswersElectronegativity Answers

HCl: 3.0 – 2.1 = 0.9 polar covalent

CrO: 3.5 – 1.6 = 1.9 ionic

Br2: 2.8 – 2.8 = 0 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

Electronegativity & physical properties Electronegativity & physical properties • Electronegativity can help to explain properties

of compounds like those in the lab.

+ –

+ –

+ –

+ –• 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

– +

+ –

CaCl2 would have a lower melting/boiling point:

CaCl2 = 3.0 – 1.0 = 2.0CaF2 = 4.0 – 1.0 = 3.0

LiBr would have a lower melting/boiling point:KCl = 3.0 – 0.8 = 2.2LiBr = 2.8 – 1.0 = 1.8

H2S would have a lower melting/boiling point:H2O= 3.5 – 2.1 = 1.4H2S = 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 only factor. It is

most useful when comparing atoms and molecules of

similar size.

CaCl2 would have a lower melting/boiling point:

CaCl2 = 3.0 – 1.0 = 2.0CaF2 = 4.0 – 1.0 = 3.0

LiBr would have a lower melting/boiling point:KCl = 3.0 – 0.8 = 2.2LiBr = 2.8 – 1.0 = 1.8

H2S would have a lower melting/boiling point:H2O= 3.5 – 2.1 = 1.4H2S = 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 only factor. It is

most useful when comparing atoms and molecules of

similar size.

Why oil and water don’t mixWhy 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.

Chemical Symbols of Common Elements

Prepare a chart in your notes …

Complete 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).

Z Name Symbol *Latin *Mnemonic

1 Hydrogen H2 Helium He

Element Latin nameCopper CuprumGold AurumIron FerrumLead PlumbumMercury HydrargyrumPotassium KaliumSilver ArgentumSodium NatriumTin Stannum

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

Best Mnemonics

Sodium (Na)Salt? Bad for BP (Na, don’t want it)North Atlantic salt water

Potassium (K)Potatoes covered in ketchupKoala (or kangaroo) eating bananasPutting Special K into rolling paper

Best Mnemonics

Iron (Fe)Ironing with FeetFrench for iron is FerIron = strong = giant = Fe-Fi-Fo-Fum

Copper (Cu)Cu (see you) at copperfieldsCop drinking out of a cupA cup filled with pennies

Best Mnemonics

Silver (Ag)Almost goldAging = grey hair = silver

Tin (Sn)“Tin is Sin”

Gold (Au)Always united (wedding ring)Gold = shiny = auraleaves changing gold colour = Autumn

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

For more lessons, visit www.chalkbored.com