Electronegativity - Intramolecular Forces & Partial Charges · Title: Electronegativity -...
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Electronegativity δ + δ – δ 0 δ 0 H Cl H H The basic units: ionic vs. covalent • Ionic compounds form repeating units. • Covalent compounds form distinct molecules. • Consider adding to NaCl(s) vs. H 2 O(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. •H 2 O: O and H cannot add individually, instead molecules of H 2 O form the basic unit. Holding it together Q: 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 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”. H 2 HCl LiCl δ + δ – δ 0 δ 0 + – covalent (non-polar) polar covalent ionic H H H Cl [Li] + [ Cl ] – 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 on ΔEN (electronegativity difference). • ΔEN = higher EN – lower EN NBr 3 : ΔEN = 3.0 – 2.8 = 0.2 (for all 3 bonds). • Basically: a ΔEN below 0.5 = covalent , 0.5 - 1.7 = polar covalent , above 1.7 = ionic • Determine the ΔEN and bond type for these: HCl, CrO, Br 2 , H 2 O, CH 4 , KCl Electronegativity Answers Electronegativity & physical properties • Electronegativity can help to explain properties of compounds like those in the lab. 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 (ΔEN = 2.1 vs. 0.9 for HCl). • Which would have a higher melting/boiling point? • For each, pick the one with the lower boiling point a) CaCl 2 , CaF 2 b) KCl, LiBr c) H 2 O, H 2 S – + + – 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 + 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.
Transcript of Electronegativity - Intramolecular Forces & Partial Charges · Title: Electronegativity -...
Electronegativity
δ+ δ– δ0 δ0
H Cl H H
The basic units: ionic vs. covalent• Ionic compounds form repeating units.• Covalent compounds form distinct molecules.• Consider adding to NaCl(s) vs. H2O(s):
HOHCl Na
Na Cl
Cl
ClNa
NaHOHH
OH
• 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 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 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 ]–
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
on ΔEN (electronegativity difference).• ΔEN = higher EN – lower EN
NBr3: ΔEN = 3.0 – 2.8 = 0.2 (for all 3 bonds).• Basically: a ΔEN below 0.5 = covalent,
0.5 - 1.7 = polar covalent, above 1.7 = ionic• Determine the ΔEN and bond type for these:
HCl, CrO, Br2, H2O, CH4, KCl
Electronegativity Answers
Electronegativity & physical properties • Electronegativity can help to explain properties
of compounds like those in the lab.
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 (ΔEN = 2.1 vs. 0.9 for HCl).
• Which would have a higher melting/boiling point?
• For each, pick the one with the lower boiling 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+
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.
