Lesson Essential Question How do molecules interact with each other?
Basic Units of Ionic vs. Covalent Ionic Compounds Form Repeating Units. NaCl: Atoms of Cl and Na can add individually forming a compound with million of atoms. Covalent Compounds Form Distinct Molecules. H 2 O: O and H cannot add individually, instead molecules of H 2 O form the basic unit. H O H H O H H O H
Then how do water molecules stay together? There must be some kind of attractive force keeping them “stuck” together. INTRAMOLECULAR forces occur between atoms. INTERMOLECULAR forces occur between molecules. Imagine a glass or water. H H H H H H O O O Intramolecular forces are much stronger than Intermolecular forces. Intermolecular forces are not considered in ionic bonding because there are no molecules. Intramolecular bonds determine the type of intermolecular force. This will be looked at later in the power point.
Electrons Are Not All Shared Equally in a Bond. H2H2 HH 00 00 Covalent (non-polar) HCl H Cl Polar Covalent ++ –– LiCl [Li] + [ Cl ] – +– Ionic To figure out which type of bond each compound has you need to look at electronegativity. Recall that electronegativity is “a number that describes the relative ability of an atom, when bonded, to attract electrons” You can calculate the nature of the bond by subtracting (ΔEN) the two electronegativities. H has an electronegativity of 2.1. Since it is connected to another H the ΔEN is 2.1 – 2.1 = 0. This makes it a non-polar covalent bond. H has an electronegativity of 2.1 and Cl has an electronegativity of 3.0. The ΔEN is 3.0 – 2.1 = 0.9. This makes it a polar covalent bond. Li has an electronegativity of 1.0 and Cl has an electronegativity of 3.0. The ΔEN is 3.0 – 1.0 = 2.0. This makes it an ionic bond. Last part to add – Dipoles. These occur with the uneven sharing of the electrons in covalent bonds. Dipoles are shown by the δ symbol. δ- where electrons are more concentrated and δ+ where they are moving away from. Ionic Compounds do not have dipoles.
Overview Basically: a EN below 0.5 = covalent (non-polar) = polar covalent above 1.7 = ionic
Try These Determine the EN and bond type for these: 1.HCl 4. H 2 O 2.CrO 5. CH 4 3.Br 2 6. KCl
Answer’s HCl:3.0 – 2.1 = 0.9 polar covalent CrO:3.5 – 1.6 =1.9ionic Br 2 :2.8 – 2.8 =0covalent H 2 O:3.5 – 2.1 =1.4polar covalent CH 4 :2.5 – 2.1 =0.4covalent KCl:3.0 – 0.8 =2.2ionic
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 and boiling point? NaCl because it has a higher electronegativity difference - ΔEN Electronegativity and Physical Properties
Which has the higher Boiling Point? 1.CaCl 2, CaF 2 2.KCl, LiBr 3.H 2 O, H 2 S H 2 0 would have a higher melting/boiling point: H 2 O= 3.5 – 2.1 = 1.4 H 2 S = 2.5 – 2.1 = 0.4 CaF 2 would have a higher melting/boiling point: CaCl 2 = 3.0 – 1.0 = 2.0 CaF 2 = 4.0 – 1.0 = 3.0 KCl would have a higher melting/boiling point: KCl = 3.0 – 0.8 = 2.2 LiBr = 2.8 – 1.0 = 1.8 ONE IMPORTANT NOTE: There are other factors such as atomic size within the molecules that 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.
Oil and Water Why do oil and water never mix?
Oil is polar and water in non-polar (covalent) ++ –– ++ ++ –– ++ ++ –– ++ oil Opposite dipoles of water are attracted to each other. They are trying to get closer and closer pushing the oil molecules out of the way. ++ –– ++ ++ –– ++ ++ –– ++ oil ++ –– ++ ++ –– ++ ++ –– ++
Cleansing Action of Soap