UNIT 3: CONCEPTS OF CHEMICAL BONDING. Chapter Chapter

Transcription

1 UNIT 3: CONCEPTS OF CHEMICAL BONDING Chapter Chapter

2 UNIT 3.1 Chapter 8.1, 8.2 2

3 CHEMICAL BONDS Atoms or ions that are strongly attached to one another Chemical bonds will form if potential energy decreases (becomes more stable) 3

4 8.1: TYPES OF BONDS 1. Ionic: electrostatic attraction between oppositely charged ions (typically between a metal and a nonmetal) 2. Covalent: sharing of e- between two atoms (typically between nonmetals) molecules created 3. Metallic: sea of e- ; bonding e- are relatively free to move throughout the 3D structure 4. Covalent Network: atoms bond with strong directional covalent bonding that lead to giant molecules and networks. Examples: carbon and silicon: diamond, graphite, silicon oxide (sand) 4

5 LEWIS SYMBOLS Valence e-: e - in highest energy level and involved in bonding; all elements within a group on periodic table have same # of valence e - Valence electrons are the outer s and p Lewis symbol (or electron-dot symbol): Shows a dot only for valence e - of an atom or ion. Place dots at top, bottom, right, and left sides and in pairs only when necessary (Hund s rule). Primarily used for main group elements only C : N Ex: Draw the Lewis symbols of C and N. 5 Gilbert N. Lewis ( )

6 THE OCTET RULE Atoms tend to gain, lose, or share e- until they are surrounded by 8 valence e- (have filled s and p subshells) and are thus energetically stable. Exceptions do occur (and will be discussed later.) 6

7 METALLIC BONDING Metallic elements have low I.E.; this means valence e - are held loosely. A metallic bond forms between metal atoms because of the movement of valence e - from atom to atom to atom in a sea of electrons. The metal thus consists of cations held together by negatively-charged e - "glue. This results in excellent thermal & electrical conductivity, ductility, and malleability. A combination of 2 metals is called an alloy. 7

8 Free e - move rapidly in response to electric fields, thus metals are excellent conductors of electricity. Free e - transmit kinetic energy rapidly, thus metals are excellent conductors of heat. Layers of metal atoms are difficult to pull apart because of the movement of valence e-, so metals are durable. However, individual atoms are held loosely to other atoms, so atoms slip easily past one another, so metals are ductile.

9 METAL ALLOYS An alloy is best defined as a substance that contains a mixture of elements and has metallic properties. There are two types of alloys: Substitutional alloy- some of the host metal atoms are replaced by other metal atoms of similar size. Interstitial alloy- is formed when some of the holes in the closest packed lattice are occupied by smaller 9

10 IONIC BONDING Ionic bonds do not form molecules An ionic formula is an empirical formula (smallest whole number ratio of atoms) and doesn t show what the structure looks like 10

11 IONIC BONDING Results as atoms lose or gain e - to achieve a noble gas e - configuration; is typically exothermic. The bonded state is lower in energy (and therefore more stable). Electrostatic attraction results from the opposite charges. Occurs when diff. of EN of atoms is > 1.7 (maximum is 3.3: CsF) Can lead to interesting crystal structures (Ch. 11) Ionic compounds are brittle solids with high 11

12 IONIC BONDING Use brackets when writing Lewis symbols of ions. Ex: Draw the Lewis symbol of sodium fluoride. [ Na ] 1+ [: F :] -1 12

13 LATTICE ENERGY Tells you the amount of energy it takes to break an ionic bond (If the lattice energy is negative its showing the amount of energy released when the ionic bond formed) Larger lattice energy means stronger ionic bond 13

14 LATTICE ENERGY H lattice = energy required to completely separate 1 mole of solid ionic compound into its gaseous ions H lattice Q Q r r Electrostatic attraction (and thus lattice energy) increases as ionic charges increase and as ionic radii decrease. Ex: Which has a greater lattice energy? 14 NaCl or KCl NaCl or MgS

15 Arrange the following in order of increasing lattice energy: NaF, CsI, and CaO Answer: CsI < NaF < CaO 15

16 Transition metals typically form +1, +2, and +3 ions. It is observed that transition metal atoms first lose both s e-, even though it is a higher energy subshell. Most lose e- to end up with a filled or a half-filled subshell. 16

17 ORGANIC COMPOUNDS (CARBON COMPOUNDS) There are three basic types of hydrocarbons: (contain only carbon and hydrogen) Alkanes- single bonds (ends in ane) Alkenes- double bonds (ends in -ene) Alkynes-triple bonds (ends in yne) There are also functional groups that can Attach to the carbon chain. We will learn about Alcohols: -OH (ends in ol) And carboxylic acids: -COOH (ends in oic acid) 17

18 ORGANIC NOMENCLATURE There are three parts to a compound name: Base: This tells how many carbons are in the longest continuous chain. meth-1, eth-2, prop-3, but-4, pent-5, hex- 6, hept-7, oct-8, non-9, dec , PRENTICE-HALL, INC.

19 ORGANIC NOMENCLATURE There are three parts to a compound name: Base: This tells how many carbons are in the longest continuous chain. Suffix: This tells what type of compound it is. Alkanes end in -ane, alkenes end in -ene, alkynes end in -yne, alcohols end in -ol, and carboxylic acids end in oic acid 2009, PRENTICE-HALL, INC.

20 ORGANIC NOMENCLATURE There are three parts to a compound name: Base: This tells how many carbons are in the longest continuous chain. Suffix: This tells what type of compound it is. Prefix: This tells what groups are attached to the chain. 2009, PRENTICE-HALL, INC.

21 FORMULAS Lewis structures of alkanes look like this. They are also called structural formulas. They are often not convenient, though 2009, PRENTICE-HALL, INC.

22 FORMULAS so more often condensed formulas are used. 2009, PRENTICE-HALL, INC.

23 JUST KNOW THE NAMES OF THESE COMPOUNDS C 6 H 6 H 2 O 2 Benzene hydrogen perioxide

24 ISOMERS Isomers have the same molecular formulas, but the atoms are bonded in a different order. 2009, PRENTICE-HALL, INC.

25 UNIT 3.2 Chapter

26 CLASS STARTER 1. Which of the following would have the highest melting point. Explain KBr, CaO, MgO 2. write the formula a. Methane b. butane c. ethanol 26

27 COVALENT BONDS Covalent bonds form molecules The formula is not always empirical but shows what the molecule looks like A molecular formula shows what the molecule actually looks like Molecular formula: C 6 H 6 empirical: CH 27

28 MOLECULES- 2 OR MORE ATOMS BOUND TOGETHER THAT ACT AS A SINGLE, DISTINCT OBJECT These molecules are shown in ball and stick form. These are also represented in structural formulas like this: H O H N H H H H H C H

29 CONDENSED STRUCTURAL FORMULA Ball and Stick Structural Condensed

30 DIATOMICS, H N F O I CL BR Elements can exist as molecules. These elements can exist as shown. Notice that P and S have been added

31 COVALENT BONDING Atoms share e- to achieve noble gas configuration that is lower in energy (and therefore more stable). Polar covalent: (different elements) e- pulled closer to more EN atom and are shared unequally -Nonpolar covalent: (same elements) e- shared equally 31

32 H 2 nonpolar; the hydrogens share the electrons equally HF polar: fluorine pulls the electrons closer so they share the electrons unequally In a polar molecule, one end is partially positive and one is partially negative (Dipole) s + s - H F or H F (vector points to neg. end) 32

33 COVALENT BONDS a line between atoms shows that 2 electrons are being shared H F (single bond) Multiple bonds A double line shows that 4 electrons are being shared O=O (double bond) A triple line shows that 6 electrons are being shared N=N (triple bond) Bond length : triple < double < single Bond energy : triple>double>single 33

34 ND ORDER An indication of bond strength and bond length Single bond: 1 pair of e- shared Ex: F 2 :F-F: Longest, weakest Double bond: 2 pairs of e- shared Ex: O 2 O=O Triple bond: 3 pairs of e- shared Ex: N 2 :N N: Shortest, strongest 34

35 DRAWING LEWIS STRUCTURES 1. Add up valence e- from all atoms in formula. If there is a charge, add e- (if an anion) or subtract e- (if a cation). 2. Draw the molecular skeleton : Place the least EN atom(s) in the center. (never H) Array the remaining elements around the center and connect them with a single bond. (When in doubt, put the element written first in the formula in the center of the molecule.) 3. Complete the octets of the outer (more EN) atoms first. 4. Place leftover e- on the central atom, even if it violates the octet rule. 5. If the central atom does not have an octet, create multiple bonds by sharing e- with the outer atoms. 35

36 LEWIS STRUCTURES 1. add up valence e - from all the elements in the formula 2. Add up the amount of e- each atom in the formula wants (all atoms want 8 except H=2, Be=4, B=6) 3. Subtract #1 from #2 this tells you the number of e - shared 4. Divide by two to find the number of bonds in the Lewis structure 5. Draw the molecular skeleton with correct number of bonds. remember H can only single 1.CO 2 has 16 valence electrons 2. CO 2 wants 24 total electrons = /2=4 bonds 5. O=C=O 6. :O=C=O: 36

37 EX: DRAW THE LEWIS STRUCTURE, AND NAME THE MOLECULE. SO 4 2- HCN C 2 H 4 NO

38 RESONANCE Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule. H Benzene, C 6 H 6 H H H H H H H H H H H The actual structure is an average of the resonance structures. The bond lengths in the ring are identical, and between those of single and double bonds.

39 RESONANCE BOND LENGTH AND BOND ENERGY Resonance bonds are shorter and stronger than single bonds. H H H H H H H H H H H H Resonance bonds are longer and weaker than double bonds.

40 RESONANCE IN OZONE, O 3 O O O O O O Neither structure is correct. Oxygen bond lengths are identical, and intermediate to single and double bonds

41 RESONANCE IN POLYATOMIC IONS Resonance in a carbonate ion: Resonance in an acetate ion:

42 THE LOCALIZED ELECTRON MODEL Lewis structures are an application of the Localized Electron Model L.E.M. says: Electron pairs can be thought of as belonging to pairs of atoms when bonding Resonance points out a weakness in the Localized Electron Model.

43 MODELS Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Models can be physical as with this DNA model Models can be mathematical Models can be theoretical or philosophical

44 FUNDAMENTAL PROPERTIES OF MODELS A model does not equal reality. Models are oversimplifications, and are therefore often wrong. We must understand the underlying assumptions in a model so that we don t misuse it.

45 BOND ORDER & RESONANCE STRUCTURES Bond order: single bond = 1, double bond=2, triple bond = 3 To determine bond order with resonance structures: Pick one bond and add up the integer bond order in one resonance structure to the same bond position in all other resonance structures. Divide the sum by the number of resonance structures to find bond order. 45

46 Which has shorter bonds? What is the bond order in each? SO 3 or SO 3 2- Answer: SO 3 Bond order for SO 3 is 1 1/3 bond order of SO 3 2- is 1 46

47 EXCEPTIONS TO THE OCTET RULE Odd-electron molecules: Ex: NO or NO 2 (involved in breaking down ozone in the upper atmosphere) Incomplete octet: H 2 He BeF 2 BF 3 47

48 Expanded octet: occurs in molecules when the central atom is in or beyond the third period, because the empty 3d subshell is used in the bonding PCl 5 SF 6 If you find the number of bonds mathematically, the math won t make sense and you ll know it has an expanded octet. Only use single bonds and add extra electrons to the central atom. (outside atoms are usually halogens) 48