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Chapter 11

Chapter 11. Chemical Bonding. Bond – a force that holds groups of atoms of two or more atoms together and makes them function as a unit Bond Energy – the amount of energy required to break the bond. Types of Chemical Bonds 11.1. Types of Bonds: 4 TYPES. Metallic.

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Chapter 11

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  1. Chapter 11 Chemical Bonding

  2. Bond – a force that holds groups of atoms of two or more atoms together and makes them function as a unit Bond Energy – the amount of energy required to break the bond Types of Chemical Bonds 11.1

  3. Types of Bonds: 4 TYPES Metallic • Cations packed in “a sea of electrons”Metals • Metals consist of closely packed cations floating in a “sea of electrons”. • All of the atoms are able to share the electrons. • The electrons are not bound to individual atoms.

  4. Type 1: Metallic • Properties of Meatals • Good conductors • Ductile • Malleable • Electrons act as a lubricant, allowing cations to move past each other

  5. Metals have a Crystalline Structure Example: Body Centered Cubic (Chromium) • Packed spheres of the same size and shape: • Body Centered Cubic • Face Centered Cubic • Hexagonal Close Packed

  6. More examples Face-Centered Cubic (gold) picture

  7. Last example Hexagonal Close-Packed (zinc) picture

  8. Type 2: IONIC IONIC picture • Bond between closely packed, oppositely charged ions • Bond between a metal and a nonmetal • hard solid @ 22oC • high mp temperatures • nonconductors of electricity in solid phase • good conductors in liquid phase or dissolved in water (aq)

  9. Covalent Bonding (2 types) Instead of gaining or losing electrons atoms can get stable by sharing electrons This is always between two non-metals. Two fluorine atoms, for example, can form a stable F2 molecule in which each atom has 8 valence electrons by sharing a pair of electrons. In covalent bonds they can share more than two electrons

  10. Type 3&4: COVALENT COVALENT picture • Electrons are shared • Have low melting, boiling points • Do not conduct electricity when melted or dissolved in water • relatively soft solidsas compared to ionic compounds at room temp

  11. Covalent bond –subtype #1 Non-polar Covalent picture • When two of the same elements bond they are called diatomic molecules, some examples of this are Hydrogen H2, Oxygen O2 and Nitrogen N2. • The atoms in these bonds would have the same electronegativities. This means that both atoms attract the shared electrons to that same extent.

  12. Covalent Bonds – subtype #2 POLAR COVALENT picture • Unequal sharing of electrons

  13. Dipole Moment 11.3 • A molecule that has a center of positive charge and a center of negative charge • Dipole often represented by an arrow • Points towards negative charge center and its tail indicates the positive charge center

  14. Review: 3 types of bonds thus far x

  15. Lewis Dot Structures • Show valence electrons • Use group number to figure it out

  16. Lewis Structures Section 11.6

  17. The Octet Rule The octet rule says that atoms tend to gain, lose or share electrons so they have eight electrons in their outer shell. There are some exceptions to the octet rule (imagine that) BF3 BCl3 PF5 SF6

  18. Follow the interactive website! Ionic Bonding: (this should be review) http://www.youtube.com/watch?v=T40sM8-SXso Covalent Bonding: http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH6404

  19. Drawing Lewis Structures Arrange the element symbols. Central atoms are generally those with the highest bonding capacity. Carbon atoms are always central atoms Hydrogen atoms are always peripheral atoms Add up the number of valence electrons from all atoms. For polyatomic ions, add one electron for each negative charge and subtract one for each positive charge. Draw a skeleton structure with atoms attached by singlebonds. Complete the octets of peripheral atoms. Place extra electrons on the central atom. If the central atom doesn’t have an octet, try forming multiple bonds by moving lone pairs.

  20. Simple Rules • 1. Figure out number of electrons by counting the TOTAL valence electrons in whole compound • 2. Place the central element in the middle and surround it with the other elements • 3. Place single bonds between elements • 4. Place lone pairs around each element until there are a total of eight (Hydrogen only wants 2) • 5. Count total electrons surrounding the compound (don’t forget the bonds count as 2 electrons) • If electrons from #1 and #5 don’t match…. Erase electrons and put in double bond and recount

  21. Single, Double and Triple Bonds With Covalent bonds the elements can share two or more electrons A Single Bond is when 2 electrons are shared they are represented by a single line in bond diagrams A Double bond is when 4 electrons are shared they are represented by two lines in bond diagrams A Triple bond is when 6 electrons are shared they are represented by three lines in bond diagrams

  22. Lewis Dot Structures: H2CO

  23. Isomers – multiple correct structures for a single compound • (requires breaking bond to make new compound) • CH2Cl2 H Cl C Cl H Cl Cl C H H

  24. Electronegativity and Polarity Section 11.2

  25. Electronegativity Values The electronegativity values can be found in the periodic table The higher the value the higher the electronegativity The Pauling scale is used to measure electronegativity. It is a relative scale running from 0.7 to 4.0 (hydrogen = 2.2). The units for electronegativity are Pauling units.

  26. Electronegativity The ability of an atom to attract electrons when bonded • Nonmetals have high electronegativity • Metals have low electronegativity • Electronegativity increases across a period and decreases down a group. WHY???

  27. Electronegativity Chart Why would the metals have low electronegativity numbers? Why don’t the noble gases have electronegativity numbers?

  28. Nonpolar Covalent Bond • When electrons are shared between 2 atoms, a covalent bond is formed. • If the atoms are identical, e.g. Cl2, the electrons are shared equally (nonpolar) • Cl = 3.0 therefore the ∆EN = 3.0-3.0 = 0 • ∆EN = electronegativity Difference • 0 = nonpolar

  29. Polar Covalent Bond • If the electrons are shared between 2 different atoms, e.g. HBr, the sharing is unequal • The bonding electrons spend more time near the more electronegative atom • H = 2.1 and Br = 2.8 THEREFORE 2.8-2.1 = 0.7 • 0.7 = a polar covalent bond H Br

  30. Bond Type by Electronegativity Value • Remember the higher the atom’s electronegativity value, the closer the shared electrons tend to be to that atom when it forms a bond • Therefore, the polarity of a bond depends on the difference between the electronegativity values of the atoms forming the bond • The greater the difference, the more polar the bond.

  31. Electronegativity Differences • Why is there a gap between 1.7 and 2.0???? • If the two atoms are nonmetals =polar covalent bond • If nonmetal & metal = ionic bond 0 to 0.2 Nonpolar covalent 0.21 to 1.7 Polar covalent ≥2.0 Ionic Electronegativity Difference

  32. Sample Problems • Choose the bond that will be more polar • H-P or H-C • O – F or O – I • N – O or S – O • N – H or Si - H

  33. Sample Problems • Choose the bond that will be more polar • H-P or H-C • O – F or O – I • N – O or S – O • N – H or Si - H

  34. Polar Molecules (overall polarity of the molecule) • Note: Not all molecules with polar bonds are polar molecules • The dipoles in symmetrical molecules cancels out •  The bond is polar but the molecule is nonpolar

  35. How to determine polar molecules There are two important factors 1. The polarity of the individual bonds in the molecule; 2. The shape or geometry of the molecule. Steps to take • Determine if a given individual bond is polar, Look at the difference between electronegativity of the atoms in the perioidc table. If the difference is: 0.2 < non polar 0.2 - greater = polar

  36. b) Determine the shape of molecule. For now I will give them to you. Later you will figure out the shape yourself. i) if all bonds are non-polar, then the whole molecule is non-polar regardless of its shape. ii) If there is symmetry in the molecule so that the polarity of the bonds cancels out, then the molecule is non-polar. (symmetry arround the central atom) iii) If there are polar bonds but there is no symmetry the overall molecule is polar.

  37. Which molecules are polar?

  38. Which molecules are polar? For these two molecules, even though there are polar bonds the overall molecule is nonpolar because the molecule is symmetrical

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