Lesson Essential Question

Lesson Essential Question How do molecules interact with each other?

H H H H H H O O O Basic Units of Ionic vs. Covalent Ionic Compounds Form Repeating Units. Covalent Compounds Form Distinct Molecules. H2O: O and H cannot add individually, instead molecules of H2O form the basic unit. NaCl: Atoms of Cl and Na can add individually forming a compound with million of atoms.

Then how do water molecules stay together? There must be some kind of attractive force keeping them “stuck” together. Imagine a glass or water. O H H O H H O H H Intramolecular bonds determine the type of intermolecular force. This will be looked at later in the power point. Intermolecular forces are not considered in ionic bonding because there are no molecules. INTRAMOLECULAR forces occur between atoms. INTERMOLECULAR forces occur between molecules. Intramolecular forces are much stronger than Intermolecular forces.

0 + + 0 – – H2 HCl LiCl H H Cl H [Cl]– [Li]+ Electrons Are Not All Shared Equally in a 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. Covalent (non-polar) Ionic Polar Covalent 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. Recall that electronegativity is “a number that describes the relative ability of an atom, when bonded, to attract electrons” 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. You can calculate the nature of the bond by subtracting (ΔEN) the two electronegativities. To figure out which type of bond each compound has you need to look at electronegativity. Ionic Compounds do not have dipoles.

Overview Basically: a EN below 0.5 = covalent (non-polar) 0.5 - 1.7 = polar covalent above 1.7 = ionic

Try These • Determine the EN and bond type for these: • HCl 4. H2O • CrO 5. CH4 • Br2 6. KCl

Answer’s 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 and Physical Properties 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. NaCl because it has a higher electronegativity difference - ΔEN Which would have a higher melting and boiling point?

CaF2would have a higher melting/boiling point: CaCl2 = 3.0 – 1.0 = 2.0 CaF2 = 4.0 – 1.0 = 3.0 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. Which has the higher Boiling Point? CaCl2, CaF2 KCl, LiBr H2O, H2S KCl would have a higher melting/boiling point: KCl = 3.0 – 0.8 = 2.2 LiBr = 2.8 – 1.0 = 1.8 H20would have a higher melting/boiling point: H2O= 3.5 – 2.1 = 1.4 H2S = 2.5 – 2.1 = 0.4

Oil and Water Why do oil and water never mix?

– – – – – – – – – + + + + + + + + + + + + + + + + + + Oil is polar and water in non-polar (covalent) 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 oil oil oil oil oil oil oil oil

Cleansing Action of Soap

Lesson Essential Question