Chemical Bonding Basic Concepts

Transcription

1 Chemical Bonding Basic Concepts

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3 Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons that particpate in chemical bonding. Group e - configuration # of valence e - 1 ns ns ns 2 np ns 2 np ns 2 np ns 2 np ns 2 np 5 7

4 Periodic table of elements (valence electrons)

5 Lewis Dot Symbols for the Representative Elements & Noble Gases 9.1

6 *Why do substances bond? *More stability *Atoms want to achieve a lower energy state

7 *Between a metal and a non-metal (I, II group and 16, 17 group of periodic table, metal hydride, between cation and acid residue) *Metals lose electrons becoming a cations, while non-metals gain electrons becoming anions. *An ionic bond is an electrostatic attraction between the oppositely charged ions.

8 The Ionic Bond Li + F Li + F - 1s 2 2s 1 1s 2 2s 2 2p 5 1s 2 1s 2 2s 2 2p 6 [He] Li Li + + e - e - + F F - Li + + F - Li + F - [Ne]

9 Electrostatic (Lattice) Energy Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. E = k Q +Q - r Q + is the charge on the cation Q - is the charge on the anion r is the distance between the ions Lattice energy (E) increases as Q increases and/or as r decreases. cmpd MgF 2 MgO LiF LiCl lattice energy 2957 Q= +2, Q= +2, r F - < r Cl -

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11 Ionic Structures *In an ionic compound (solid), the ions are packed together into a repeating array called a crystal lattice. *The simplest arrangement is one in which the spheres in the base are packed side by side. Opposite charges are attracted to each other. *Its called simple cubic packing (NaCl is an example)

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13 *Depend on the forces between the particles *The stronger the bonding between the particles, the higher the M.P and BP *MP tends to depend on the existence of a regular lattice structure

14 *An impurity disrupts the regular lattice that its particle adopts in the solid state, so it weakens the bonding. *They always LOWER melting points *Its often used to check purity of a known molecular covalent compound because its MP will be off, proving its contamination

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16 A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? F + F F F 7e - 7e - 8e - 8e - Lewis structure of F 2 single covalent bond lone pairs F F lone pairs lone pairs F F lone pairs single covalent bond

17 Lewis structure of water single covalent bonds H + O + H H O H or H O H 2e - 8e - 2e - Double bond two atoms share two pairs of electrons O C O or O C O 8e - 8e - 8e - double bonds Triple bond two atoms share three pairs of electrons N N 8e - 8e - triple bond or N N triple bond

18 Lengths of Covalent Bonds Bond Lengths Triple bond < Double Bond < Single Bond

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20 Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms (electrons are shared unequally) electron poor region electron rich region e - poor e - rich H F H F d + d -

21 Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond. Electronegativity - relative, F is highest electron poor region H electron rich region F

22 The Electronegativities of Common Elements

23 Classification of bonds by difference in electronegativity Difference Bond Type 0 Nonpolar Covalent 1.7 Ionic 0 < and <1.7 Polar Covalent Increasing difference in electronegativity Nonpolar Covalent share e - equally Polar Covalent partial transfer of e - Unequal sharing Ionic transfer e -

24 Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H 2 S; and the NN bond in H 2 NNH 2. Cs 0.7 Cl = 2.3 Ionic H 2.1 S = 0.4 Polar Covalent N 3.0 N = 0 NonPolar Covalent 9.5

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26 *A covalent bond that occurs between two atoms in which both electrons shared in the bond come from the same atom. *Both electrons from the nitrogen are shared with the upper hydrogen *Ammonium has 3 polar covalent bonds and 1 coordinate (dative) covalent bond. Properties do not differ from those of a normal covalent bond.

27 *Hydronium (H 3 O + ) *Carbon monoxide (CO)

28 Write the Lewis structure of nitrogen trifluoride (NF 3 ). Step 1 N is less electronegative than F, put N in center Step 2 Count valence electrons N - 5 (2s 2 2p 3 ) and F - 7 (2s 2 2p 5 ) 5 + (3 x 7) = 26 valence electrons Step 3 Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are number of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons F N F F

29 Write the Lewis structure of the carbonate ion (CO 3 2- ). Step 1 C is less electronegative than O, put C in center Step 2 Count valence electrons C - 4 (2s 2 2p 2 ) and O - 6 (2s 2 2p 4 ) -2 charge 2e (3 x 6) + 2 = 24 valence electrons Step 3 Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are number of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons Step 5 - Too many electrons, form double bond and re-check # of e - O C O O 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total = 24

30 The Incomplete Octet Exceptions to the Octet Rule BeH 2 Be 2e - 2H 2x1e - 4e - H Be H BF 3 B 3e - 3F 3x7e - 24e - F B F F 3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18 Total =

31 Exceptions to the Octet Rule Odd-Electron Molecules (radicals -very reactive) N 5e - NO O 6e - N O 11e -

32 *The shape of the molecule directly influences the overall polarity of the molecule. *If there is symmetry the charges cancel each other out, making the molecule non-polar *If there is no symmetry, then its polar Polarity

33 *Polar bonds do not guarantee a polar molecule *Ex: CCl 4 and CO 2 both have polar bonds, but both are NON-POLAR molecules. They have a dipole moment of zero *The greater the dipole moment, the more polar the molecule

34 *Polar molecules have higher melting and boiling points (for example the BP of HF is 19.5 C, and the BP of F 2 is 188 C). *Polar solvents dissolve ionic and polar molecules more efficiently than non-polar solvents

35 Dipole Moments and Polar Molecules electron poor region H electron rich region F d+ d-

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37 The bent shape creates an overall positive end and negative end of the molecule = POLAR The symetry of the molecule Cancels out the charges Making this NON-POLAR No overall DIPOLE

38 * Linear: * When two atoms attached to central atom are the same, the molecule will be Non-Polar (CO 2 ) * When the two atoms are different the dipoles will not cancel, and the molecule will be Polar (HCN) * Bent: * The dipoles created from this molecule will not cancel creating a net dipole moment and the molecule will be Polar (H 2 O)

39 * Pyramidal: * The dipoles created from this molecule will not cancel creating a net dipole and the molecule will be Polar (NH 3 ) * Trigonal Planar: * When the three atoms attached to central atom are the same, the molecule will be Non-Polar (BF 3 ) * When the three atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar (CH 2 O)

40 * When the four atoms attached to the central atom are the same the molecule will be Non- Polar * When three atoms are the same, and one is different, the dipoles will not cancel, and the molecule will be Polar

41 Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid

42 Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction

43 Hydrogen Bond Intermolecular Forces The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

44 Why is the hydrogen bond considered a special dipole-dipole interaction?

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46 Types of Crystals Metallic Bonds- electron sea model - metal cations in a sea of valence electrons the valence electrons do not belong to a single cation, but are delocalized and may move about Good conductors of heat and electricity nucleus & inner shell e - Cross Section of a Metallic Crystal mobile sea of e -

47 Metallic bond *Occurs between atoms with low electronegativities *Metal atoms pack close together in 3- D, like oranges in a box.

48 Conductivity *Delocalised electrons are free to move so when a potential difference is applied they can carry the current along *Mobile electrons also mean they can transfer heat well *Their interaction with light makes them shiny (lustre)

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50 *Depend on the forces between the particles *The stronger the bonding between the particles, the higher the M.P and BP

51 *It takes energy to break the bonds between the C 12 H 22 O 11 molecules in sucrose crystal structure. *It also takes energy to break the hydrogen bonds in water so that one of these sucrose molecules can fit into solution. *In order for sugar to dissolve, there must be a greater release of energy when the dissolution occurs than when the breaking of bonds occur.

52 *The energy needed to break the ionic bond must be less than the energy that is released when ions interact with water. *The intermolecular ion-dipole force is stronger than the electrostatic ionic bond *Breaks up the compound into its ions in solution.

53 *Soluble salt in water breaks up as NaCl (s) Na + (aq) + Cl - (aq)

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