Chemistry: The Central Science

Transcription

1 Chemistry: The Central Science Fourteenth Edition Chapter 8 Basic Concepts of Chemical Bonding

2 Chemical Bonds Three basic types of bonds Ionic Electrostatic attraction between ions Covalent Sharing of electrons Metallic Free electron hold metal atoms together

3 Lewis Symbols G. N. Lewis developed a method to denote potential bonding electrons by using one dot for every valence electron around the element symbol. When forming compounds, atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons (the octet rule).

4 Ionic Bonding (1 of 2) Metals and nonmetals (except group 8A) Electron transfer Very exothermic

5 Ionic Bonding (2 of 2) One element readily gives up an electron (has a LOW ionization energy). Another element readily gains an electron (has a HIGH electron affinity). Arrow(s) indicate the transfer of the electron(s).

6 Properties of Ionic Substances Evidence of well-defined 3-D structures: Brittle High melting points Crystalline Cleave along smooth lines

7 Energetics of Ionic Bonding Born Haber Cycle Many factors affect the energy of ionic bonding. Start with the metal and nonmetal elements: N a(s) and C l 2 (g). Make gaseous atoms: N a(g) and C l(g). + Make ions: Na ( g) and Cl ( g). Combine the ions: N a Cl(s).

8 Energetics of Ionic Bonding We already discussed making ions (ionization energy and electron affinity). It takes energy to convert the elements to atoms (endothermic). It takes energy to create a cation (endothermic). Energy is released by making the anion (exothermic). The formation of the solid releases a huge amount of energy (exothermic). This makes the formation of salts from the elements exothermic.

9 Lattice Energy (1 of 2) Energy required to completely separate one mole of a solid ionic compound into its gaseous ions. That amount of energy is Released to Make the ionic compound (in the Born Haber cycle). NaCl( s) Na + ( g) + Cl ( g) H = + 788kJ/ mol lattice

10 Lattice Energy (2 of 2) Table 8.1 Lattice Energies for Some Ionic Compounds Compound Lattice Energy (kj/mol) Compound Lattice Energy (kj/mol) L i F 1030 M g C l L icl 834 S r C l L ii 730 Blank blank N af 910 M g O 3795 N acl 788 C a O 3414 N a B r 732 S r O 3217 N al 682 Blank blank K F 808 S c N 7547 K C l 701 blank blank K B r 671 blank blank C s C l 657 blank blank C s I 600 blank blank

11 Trends in Lattice Energy Lattice energy increases with: E Increasing charge on the ions Decreasing size of ions el κqq = d 1 2

12 Electron Configuration of Ions Main group metals lose electrons, resulting in the electron configuration of the previous noble gas. Nonmetals gain electrons, resulting in the electron configuration of the nearest noble gas. Transition metals do Not follow the Octet rule. Transition metals lose the Valence electrons First, Then lose the d- electrons necessary for the given ion charge.

13 Covalent Bonding In covalent bonds, atoms share electrons. There are several electrostatic interactions in these bonds: Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei For a bond to form, the attractions must be greater than the repulsions.

14 Lewis Structures Sharing electrons to make covalent bonds can be demonstrated using Lewis structures. We start by trying to give each atom the same number of electrons as the nearest noble gas by sharing electrons. The simplest examples are for hydrogen, H 2, and chlorine, C l 2, shown below.

15 Number of Bonds for Nonmetals The group number is the number of valence electrons. To get an octet, like the nearest noble gas, in the simplest covalent molecules for nonmetals, the number of bonds needed will be the same as the electrons needed to complete the octet.

16 Electrons on Lewis Structures Lone pairs: electrons located on only one atom in a Lewis structure Bonding pairs: shared electrons in a Lewis structure; they can be represented by two dots or one line, NOT both!

17 Multiple Bonds Some atoms share only one pair of electrons. These bonds are called single bonds. Sometimes, two pairs need to be shared. These are called double bonds. There are even cases where three bonds are shared between two atoms. These are called triple bonds.

18 Polarity of Bonds The electrons in a covalent bond are not always shared equally. Bond polarity is a measure of how equally or unequally the electrons in a covalent bond are shared. In a nonpolar covalent bond, the electrons are shared equally. In a polar covalent bond, one of the atoms attracts electrons to itself with a greater force.

19 Polar or Nonpolar Covalent Bonds In elemental fluorine, the atoms pull electrons equally. The bond is a nonpolar covalent bond. Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end, making it a polar covalent bond.

20 Electronegativity Electronegativity is the ability of an atom in a molecule to attract electrons to itself. On the periodic table, electronegativity generally increases as you go: from left to right across a period. from the bottom to the top of a group.

21 Electronegativity and Polar Covalent Bonds When two atoms share electrons unequally, a polar covalent bond results. Electrons tend to spend more time around the more electronegative atom. The result is a partial negative charge (not a complete transfer of charge). It is represented by δ. The other atom is more positive, or δ +.

22 Polar Covalent Bonds The greater the difference in electronegativity, the more polar is the bond. Table 8.2 Bond Lengths, Electronegativity Differences, and Dipole Moments of the Hydrogen Halides Compound Bond Length (Å) Electronegativity Difference Dipole Moment (D) HF HCl HBr HI

23 Dipoles When two equal, but opposite, charges are separated by a distance, a dipole forms. A dipole moment, µ, produced by two equal but opposite charges separated by a distance, r, is calculated: µ = Qr It is measured in debyes (D).

24 Is a Compound Ionic or Covalent? Simplest approach: Metal + nonmetal is ionic; nonmetal + nonmetal is covalent. There are many exceptions: It doesn t take into account oxidation number of a metal (higher oxidation numbers can give covalent bonding). Electronegativity difference can be used; the table still doesn t take into account oxidation number. Properties of compounds are often best: Lower melting points mean covalent bonding, for example.

25 Writing Lewis Structures (Covalent Molecules) 1. Sum the valence electrons from all atoms, taking into account overall charge. If it is an anion, add one electron for each negative charge. If it is a cation, subtract one electron for each positive charge. PCl 3 Keep track of the electrons: 5 + 3(7) = 26

26 Writing Lewis Structures (1 of 6) 2. Write the symbols for the atoms, show which atoms are attached to which, and connect them with a single bond (a line representing two electrons). Keep track of the electrons: 26 6 = 20

27 Writing Lewis Structures (2 of 6) 3. Complete the octets around all atoms bonded to the central atom. Keep track of the electrons: 26 6 = 20; = 2

28 Writing Lewis Structures (3 of 6) 4. Place any remaining electrons on the central atom. 5. If there are not enough electrons to give the central atom an octet, try multiple bonds. Keep track of the electrons: 26 6 = 20; = 2; 2 2 = 0 (multiple bonds unnecessary here)

29 Writing Lewis Structures (4 of 6) 6. If there are not enough electrons to give the central atom an octet, try multiple bonds.

30 Writing Lewis Structures (5 of 6) Then assign formal charges. Formal charge is the charge an atom would have if all of the electrons in a covalent bond were shared equally. 1 Formal charge = (valence electrons) (bonding electrons) 2 ( all nonbonding electrons). This can be a method to determine structure.

31 Writing Lewis Structures (6 of 6) The dominant Lewis structure: is the one in which atoms have formal charges closest to zero. puts a negative formal charge on the most electronegative atom. As such, it can be used to decide which structure is best.

32 The Best Lewis Structure? Following our rules, this is the Lewis structure we would draw for ozone, O 3. However, it doesn t agree with what is observed in nature: Both O-to-O connections are the same.

33 Resonance (1 of 2) One Lewis structure cannot accurately depict a molecule like ozone. We use multiple structures, resonance structures, to describe the molecule.

34 Resonance (2 of 2) The organic compound benzene, C 6 H 6, has two resonance structures. It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring. Localized electrons are specifically on one atom or shared between two atoms; delocalized electrons are shared by multiple atoms.

35 Exceptions to the Octet Rule There are three types of ions or molecules that do not follow the octet rule: Ions or molecules with an odd number of electrons Ions or molecules with less than an octet Ions or molecules with more than eight valence electrons (an expanded octet)

36 Odd Number of Electrons Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

37 Fewer Than Eight Electrons (1 of 5) Elements in the second period before carbon can make stable compounds with fewer than eight electrons. Consider BF 3 : Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine. This would not be an accurate picture of the distribution of electrons in BF 3.

38 Fewer Than Eight Electrons (2 of 5) If filling the octet of the central atom results in a negative formal charge on the central atom and a positive formal charge on the more electronegative outer atom, don t fill the octet of the central atom.

39 Fewer Than Eight Electrons (3 of 5) When an element is in periods 3 through 6, it can use d-orbitals to make more than four bonds and be hypervalent. Examples: PF 5 and phosphate below (Note: Phosphate will actually have four resonance structures with five bonds on the P atom!)

40 Average Bond Enthalpies (4 of 5) Bond enthalpies are positive, because bond breaking is an endothermic process. These are averages over many compounds; not every bond for a pair of atoms has the same bond energy. Table 8.3 Average Bond Enthalpies (kj/mol) Single Bonds Blank Blank Blank Blank Blank Blank Blank C H 413 N H 391 O H 463 F F 155 C C 348 N N 163 O O 146 Blank Blank C N 293 N O 201 O F 190 C l F 253 C O 358 N F 272 O C l 203 C l C l 242 C F 485 N C l 200 O I 234 Blank Blank C Cl 328 N B r 243 Blank Blank Br F 237 C Br 276 Blank Blank S H 339 B r C l 218 C I 240 H H 436 S F 327 B r B r 193

41 Average Bond Enthalpies (5 of 5) Single Bonds Blank Blank Blank Blank Blank Blank Blank C S 259 H F 567 S C l 253 Blank Blank Blank Blank H C l 431 S B r 218 I C l 208 S i H 323 H B r 366 S S 266 I B r 175 S i S i 226 H I 299 Blank Blank I I 151 S i C 301 Blank Blank Blank Blank Blank Blank S i O 368 Blank Blank Blank Blank Blank Blank S i Cl 464 Blank Blank Blank Blank Blank Blank Multiple Bonds Blank Blank Blank Blank Blank Blank Blank C C 614 N N 418 O O 495 Blank Blank C C 839 N N 941 Blank Blank C N 615 N O 607 S O 523 Blank Blank C N 891 Blank Blank S S 418 Blank Blank C O 799 Blank Blank Blank Blank Blank Blank C O 1072 Blank Blank Blank Blank Blank Blank

42 Bond Enthalpy and Bond Length Multiple bonds are stronger than single bonds. We can also measure an average bond length for different bond types. As the number of bonds between two atoms increases, the bond length decreases.

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