CHEM 121 Introduction to Fundamental Chemistry. Summer Quarter 2008 SCCC. Lecture 7.

Transcription

1 CHEM 121 Introduction to Fundamental Chemistry Summer Quarter 2008 SCCC Lecture 7

2 Forces Between Particles Noble Gas Configurations Ionic Bonding Ionic Compounds Naming Binary Ionic Compounds The Smallest Unit of Ionic Compounds Covalent Bonding Polyatomic Ions Shapes of Molecules and Polyatomic Ions The Polarity of Covalent Molecules

3 Electronegativity can be defined as: The tendency of an atom to pull electrons towards itself in a covalent bond. Electronegativity increases from left to right across a period and decreases going down a group. What element do you expect to be the most electronegative?

4 Note that electronegativity is not defined for the noble gases.

5 Atoms of the same type have identical electronegativities. When atoms of the same type form covalent bond electrons will be shared equally. This is called a nonpolar covalent bond. Atoms of different types have different electronegativities. When atoms of form covalent bonds the electrons will be shared unequally. This is called a polar covalent bond.

6 Table 4.4 in our text book gives numerical values for the electronegativities of selected elements. An important rule is if the numerical value for the difference in electronegativites of two atoms is > 2.1 then electrons are completely transferred and the bond is ionic.

7 When atoms with different electronegativities share electrons in a covalent bond the shared electrons will be closer to the more electronegative element resulting in a polar covalent bond. C O δ+ δ- What happens in a molecule with multiple covalent bonds?

8 We can define two types of covalent molecules containing polar covalent bonds: 1. If there is a symmetrical charge distribution about the central atom then the molecule is classified as nonpolar. δ F F δ B δ3+ F δ δ O δ2+ C δ O

9 2. If there is a nonsymmetric charge distribution about the central atom then the molecule is classified as polar. δ2- H O H δ+ δ+

10 When we name binary ionic compounds formed by simple ions we give the name of the cation followed by the name of the anion. e.g. NaCl is called sodium chloride, MgCl 2 is called magnesium chloride The name does not specify the number of ions of each type, this is meant to be understood.

11 How do we name covalent compounds? For covalent compounds there are three naming rules: 1. Give the name of the less electronegative element first. 2. Give the stem name of the more electronegative element and add the suffix ide. 3. Indicate the number of each atom using Greek prefixes.

12 You will need to learn the Greek numerical prefixes (Table 4.6): Number Prefix Mono- Di- Tri- Tetra- Penta- Hexa- Hepta- Octa- Nona- Deca-

13 Lets do some examples: Name the following: a) CO 2 b) CO c) N 2 O 5 d) NO 2 e) CS 2 a) Carbon dioxide b) Carbon monoxide c) Dinitrogen pentoxide d) Nitrogen dioxide e) Carbon disulfide As you can see there are some instances where the rules are broken.

14 To name compounds containing polyatomic ions we write the name of the cation followed by the name of the anion. The names of some polyatomic ions are given in Table 4.7. Memorize the following: Formula NH + 4 CO 2-3 CN - OH - NO - 3 PO 3-4 SO 2-4 Name Ammonium Carbonate Cyanide Hydroxide Nitrate Phosphate Sulfate

15 Some rules to keep in mind when writing the formulas for compounds containing polyatomic ions: 1. The overall charge of the compound must be zero. e.g. NH 4 Cl NH 4 Cl 2

16 2. If there is more than one of the ion parenthesis are put around the ion and the number indicated with a subscript. e.g. (NH 4 ) 2 Cr 2 O 7 Mg(CN) 2 Na 2 SO 4 Ba(OH) 2

17 Forces Between Particles Ionic Bonding (polyatomic ions or metal/nonmetal) Covalent Bonding (nonmetals) Molecules Network Solids Metallic Bonding attraction between sea of valence electrons and +ve core of atoms Interactions of polar molecules dipole-dipole interactions (H-bonding) dipole-ion interactions

18 We have previously talked about ionic crystalline compounds where we had lattice sites occupied by ions which formed ionic bonds with adjacent ions to form an infinitely repeating crystal.

19 We have talked about covalent bonds in molecules. Crystalline compounds form when we have covalent bonds between atoms at lattice sites. We have talked about covalent bonds in molecules. Crystalline compounds form when we have covalent bonds between atoms at lattice sites.

20 Another compound of this type is silica (quartz). Quartz has the formula SiO 2. There are two types of lattice sites in silica. In the first is a silicon atom that is covalently bonded to four oxygen atoms in a tetrahedral arrangement. In the second is an oxygen atom covalently bonded to two silicon atoms in a bent arrangement.

21 Quartz and diamond are an example of a class of substances called network solids. Network solids have extremely high melting points and hardness as the atoms they are composed of are held by strong covalent bonds.

22 In metals lattice sites are occupied by metal atoms. The valence electrons of metal atoms are loosely held and they move freely throughout the lattice. The attraction between the positive core of the metal atoms and this mobile sea of electrons is called a metallic bond.

23 When a molecule has an asymmetric distribution of charge we say it is polar or that it has a dipole. A molecule that has a dipole will have regions that have a net positive charge and regions that have net negative charge.

24 Electrostatic attraction between the positively and negatively charged regions of polar molecules is called a dipole-dipole interaction. Dipole-dipole interactions are weaker than ionic or covalent bonds. However, they are significant and effect boiling point, melting point and other properties.

25 It is also possible to have an electrostatic interaction between a polar molecule and an ion. These kinds of interactions are called dipole-ion interactions. We can also have dipole-dipole interactions between molecules of different types.

26 A special class of dipole-dipole interactions occurs between molecules that have a hydrogen atom covalently bound to a very electronegative element (O, N, S). This type of interaction is called a hydrogen bond.

27 Hydrogen bonding is very important in living systems. For example between base pairs in DNA.

28 Link these together and we get the DNA double helix.

29 Hydrogen bonding also determines how proteins fold to form characteristic structures such as β-sheets and α-helixes.

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