Presentation on theme: "Models of Chemical Bonding"— Presentation transcript:
1 Models of Chemical Bonding Chapter 9Models of Chemical Bonding
2 Models of Chemical Bonding 9.1 Atomic Properties and Chemical Bonds9.2 The Ionic Bonding Model9.3 The Covalent Bonding Model9.4 Between the Extremes: Electronegativity and Bond Polarity9.5 An Introduction to Metallic Bonding
4 Types of Chemical Bonding 1. Metal with nonmetal:electron transfer and ionic bonding2. Nonmetal with nonmetal:electron sharing and covalent bonding3. Metal with metal:electron pooling and metallic bonding
6 Lewis Electron-Dot Symbols For main group elements -The A group number gives the number of valence electrons.Place one dot per valence electron on each of the four sides of the element symbol.Pair the dots (electrons) until all of the valence electrons are used.Example:Nitrogen, N, is in Group 5A and therefore has 5 valence electrons.N:.
7 Lewis electron-dot symbols for elements in Periods 2 and 3
8 Ionic BondingIonic bonding results when there is a transfer of electrons between two atoms. Each atom achieves a full outer level of electrons. For many atoms in the 2nd and 3rd period, this would be 8 electrons, known as the octet rule.
9 SAMPLE PROBLEM 9.1Depicting Ion FormationPROBLEM:Use partial orbital diagrams and Lewis symbols to depict the formation of Na+ and O2- ions from the atoms, and determine the formula of the compound.PLAN:Draw orbital diagrams for the atoms and then move electrons to make filled outer levels. It can be seen that 2 sodiums are needed for each oxygen.SOLUTION:2s2pO2-3s3pNa2s2pO2 Na+3s3pNa:Na+ O.2Na+ + O 2-:Na2O
10 Three ways to represent the formation of Li+ and F- through electron transfer. Electron configurationsLi 1s22s1+F 1s22s22p5Li+ 1s2+F- 1s22s22p6Orbital diagramsLi+1s2s2pLi1s2s2pF-1s2s2p+F1s2s2p+Lewis electron-dot symbols.+F:LiLi++F -:
11 Born-Haber CycleLattice Energy - Energy released when ions come together forming an ionic solid.Remember Hess’s law states that the enthalpy change between two states is the same as the sum of enthalpies in a multistep process that goes between the same two states.H0f = H0 elements to atoms + H0 ions from atoms + H0latticeSo from this one equation if all of the H0 ‘s except one are known, e.g. H0lattice , the value can be calculated.
14 Covalent BondingCovalent bonding results when two electrons are shared in an orbital between two atoms. Each atom achieves a full outer level of electrons resulting in a lower energy system.The pair of electrons used are called the shared or bonding pair. In terms of the octet rule, this pair of electrons counts for both atoms in completing the octet.The electron pairs that are not involved in bonding belong only to the atom with which they are associated. These are called lone pairs. BOND ORDER - When only one pair of electrons are shared between two atoms, it’s called a single bond.If two pairs of electrons are shared covalently between two atoms, it’s called a double bond; three pairs, triple bond.
16 The attractive and repulsive forces in covalent bonding.
17 Bond Energy - The amount of energy required to break a bond Bond Energy - The amount of energy required to break a bond. The greater the energy, the stronger the bond.Bond breaking is an endothermic process, so bond breaking enthalpies are positive.
18 Multiple bonds result in stronger, shorter bonds. Bond Length - In general, the closer the electrons are held by the atoms, the shorter the bond length and the higher the bond energy.Multiple bonds result in stronger, shorter bonds.
19 SAMPLE PROBLEM 9.2Comparing Bond Length and Bond StrengthPROBLEM:Using the periodic table, but not Tables 9.2 and 9.3, rank the bonds in each set in order of decreasing bond length and bond strength:(a) S - F, S - Br, S - Cl(b) C = O, C - O, C OPLAN:(a) The bond order is one for all and sulfur is bonded to halogens; bond length should increase and bond strength should decrease with increasing atomic radius. (b) The same two atoms are bonded but the bond order changes; bond length decreases as bond order increases while bond strength increases as bond order increases.SOLUTION:(a) Atomic size increases going down a group.(b) Using bond orders we getBond length: S - Br > S - Cl > S - FBond length: C - O > C = O > C OBond strength: S - F > S - Cl > S - BrBond strength: C O > C = O > C - O
20 Electronegativity and Bond Polarity Electronegativity, (EN), is the ability of an atom to attract electron density of shared electrons.To the extent that an atom attracts extra electron density away from the other atom, it has a partial negative charge. The other atom has a corresponding positive charge. This creates a polar bond. The greater the difference in EN between the two atoms, the more polar the bond.The extreme “polar bond” is where the electrons are completely with one atom from the other. What has already been identified as an ionic bond.On the Pauling scale of EN, fluorine is the most electronegative with a value of 4.0. All other elements are less electronegative with Cesium being the least with a 0.7 value.
23 SAMPLE PROBLEM 9.3Determining Bond Polarity from EN ValuesPROBLEM:(a) Use a polar arrow to indicate the polarity of each bond: N-H, F-N, I-Cl.(b) Rank the following bonds in order of increasing polarity: H-N, H-O, H-C.PLAN:(a) Use Figure 9.16(button at right) to find EN values; the arrow should point toward the negative end.(b) Polarity increases across a period.SOLUTION:(a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0N - HF - NI - Cl(b) The order of increasing EN is C < N < O; all have an EN larger than that of H.H-C < H-N < H-O
24 Boundary ranges for classifying ionic character of chemical bonds. 3.0DEN2.0Boundary ranges for classifying ionic character of chemical bonds.0.0
25 Percent ionic character of electronegativity difference (DEN).