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1: Shapes of the molecules and ions

Date. 1: Shapes of the molecules and ions. BIG picture. What skills will you be developing this lesson? ICT Numeracy Literacy Team work Self management Creative thinking Independent enquiry Participation Reflection How is this lesson relevant to every day life? (WRL/CIT).

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1: Shapes of the molecules and ions

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  1. Date 1: Shapes of the molecules and ions

  2. BIG picture • What skills will you be developing this lesson? • ICT • Numeracy • Literacy • Team work • Self management • Creative thinking • Independent enquiry • Participation • Reflection • How is this lesson relevant to every day life? (WRL/CIT)

  3. Shapes of Covalent Compounds • What is the importance of bonding in a compound? • Bonding determines the physical and chemical properties of the compound. • Covalent compounds form molecules and ions whose bonds are highly directional, giving them a fixed shape which determines the physical and chemical properties of the compound.

  4. Shapes of Covalent Compounds • Shape (structure) of enzymes control the rate of reaction in a biological system. • Example: organophosphates are commonly used insecticides. The molecules are exactly the right shape to interfere with an enzyme that controls nerve impulses in an insects body, so kiliing it.

  5. Shapes of Ionic Compounds

  6. Shapes of molecules and ions All you need to do is to work out how many electron pairs are involved in bonding and then arrange them to produce the minimum amount of repulsion between them. You have to include both bonding pairs and lone pairs. Finding out the shape of a molecule is easy!

  7. Al Al ELECTRON PAIR REPULSION THEORY “THE SHAPE ADOPTED BY A SIMPLE MOLECULE OR ION IS THAT WHICH KEEPS REPULSIVE FORCES TO A MINIMUM” Bonds are closer together so repulsive forces are greater Molecules contain covalent bonds. As covalent bonds consist of a pair of electrons, each bond will repel other bonds. Bonds are further apart so repulsive forces are less Bonds will therefore push each other as far apart as possible to reduce the repulsive forces. Because the repulsions are equal, the bonds will also be equally spaced All bonds are equally spaced out as far apart as possible

  8. C REGULAR SHAPES Molecules, or ions, possessing ONLY BOND PAIRS of electrons fit into a set of standard shapes. All the bond pair-bond pair repulsions are equal. All you need to do is to count up the number of bond pairs and chose one of the following examples... A covalent bond will repel another covalent bond BOND BOND PAIRS SHAPE ANGLE(S) EXAMPLE 2 LINEAR 180º BeCl2 3 TRIGONAL PLANAR 120º BCl3 4 TETRAHEDRAL 109.5º CH4 5 TRIGONAL BIPYRAMIDAL 90º & 120º PCl5 6 OCTAHEDRAL 90º SF6

  9. Be Cl Cl Be Cl 180° Cl Be Cl BERYLLIUM CHLORIDE Beryllium - has two electrons to pair up Chlorine - needs 1 electron for ‘octet’ Two covalent bonds are formed Beryllium still has an incomplete shell BOND PAIRS 2 LONE PAIRS 0 BOND ANGLE... SHAPE... 180° LINEAR

  10. ADDING ANOTHER ATOM - ANIMATION

  11. Cl Cl B Cl Cl BORON CHLORIDE B Boron - has three electrons to pair up Chlorine - needs 1 electron to complete ‘octet’ Three covalent bonds are formed; Boron still has an incomplete outer shell. BOND PAIRS 3 LONE PAIRS 0 Cl 120° Cl B BOND ANGLE... SHAPE... 120° TRIGONAL PLANAR Cl

  12. ADDING ANOTHER ATOM - ANIMATION

  13. H H H C H H C H 109.5° C H H H METHANE Carbon - has four electrons to pair up Hydrogen - 1 electron to complete shell Four covalent bonds are formed C and H now have complete shells BOND PAIRS 4 LONE PAIRS 0 BOND ANGLE... SHAPE... 109.5° TETRAHEDRAL

  14. F F F F P F F F 90° F F P 120° F F PHOSPHORUS(V) FLUORIDE P Phosphorus - has five electrons to pair up Fluorine - needs one electron to complete ‘octet’ Five covalent bonds are formed; phosphorus can make use of d orbitals to expand its ‘octet’ BOND PAIRS 5 LONE PAIRS 0 BOND ANGLE... SHAPE... 120° & 90° TRIGONAL BIPYRAMIDAL

  15. F F F F S S F F F F F F S F F F SULPHUR(VI) FLUORIDE Sulphur - has six electrons to pair up Fluorine - needs one electron to complete ‘octet’ Six covalent bonds are formed; sulphur can make use of d orbitals to expand its ‘octet’ BOND PAIRS 6 LONE PAIRS 0 90° BOND ANGLE... SHAPE... 90° OCTAHEDRAL

  16. Pentagonal bipyramidal

  17. O O O IRREGULAR SHAPES If a molecule, or ion, has lone pairs on the central atom, the shapes are slightly distorted away from the regular shapes. This is because of the extra repulsion caused by the lone pairs. BOND PAIR - BOND PAIR < LONE PAIR - BOND PAIR < LONE PAIR - LONE PAIR As a result of the extra repulsion, bond angles tend to be slightly less as the bonds are squeezed together.

  18. H BOND PAIRS 3 LONE PAIRS 1 TOTAL PAIRS 4 N H H N H AMMONIA • Nitrogen has five electrons in its outer shell • It cannot pair up all five - it is restricted to eight electrons in its outer shell • It pairs up only three of its five electrons • 3 covalent bonds are formed and a pair of non-bonded electrons is left • As the total number of electron pairs is 4, the shape is BASED on a tetrahedron

  19. H BOND PAIRS 3 LONE PAIRS 1 TOTAL PAIRS 4 N H H N H N N H H 107° H H H H AMMONIA • The shape is based on a tetrahedron but not all the repulsions are the same • LP-BP REPULSIONS > BP-BP REPULSIONS • The N-H bonds are pushed closer together • Lone pairs are not included in the shape N H ANGLE... 107° SHAPE... PYRAMIDAL H H

  20. H BOND PAIRS 3 LONE PAIRS 1 TOTAL PAIRS 4 N H H N H AMMONIA

  21. H BOND PAIRS 2 LONE PAIRS 2 TOTAL PAIRS 4 H O H O WATER • Oxygen has six electrons in its outer shell • It cannot pair up all six - it is restricted to eight electrons in its outer shell • It pairs up only two of its six electrons • 2 covalent bonds are formed and 2 pairs of non-bonded electrons are left • As the total number of electron pairs is 4, the shape is BASED on a tetrahedron

  22. H BOND PAIRS 2 LONE PAIRS 2 TOTAL PAIRS 4 H O H O WATER • The shape is based on a tetrahedron but not all the repulsions are the same • LP-LP REPULSIONS > LP-BP REPULSIONS > BP-BP REPULSIONS • The O-H bonds are pushed even closer together • Lone pairs are not included in the shape O 104.5° H H O O ANGLE... 104.5° SHAPE...ANGULAR H H H H

  23. Bent

  24. BOND PAIRS 4 LONE PAIRS 2 TOTAL PAIRS 6 F F F Xe Xe F F XENON TETRAFLUORIDE • Xenon has eight electrons in its outer shell • It pairs up four of its eight electrons • 4 covalent bonds are formed and 2 pairs of non-bonded electrons are left • As the total number of electron pairs is 6, the shape is BASED on an octahedron

  25. BOND PAIRS 4 LONE PAIRS 2 TOTAL PAIRS 6 F F F Xe Xe F F F F Xe F F XENON TETRAFLUORIDE • As the total number of electron pairs is 6, the shape is BASED on an octahedron • There are two possible spatial arrangements for the lone pairs • The preferred shape has the two lone pairs opposite each other F F Xe F ANGLE... 90° SHAPE...SQUARE PLANAR F

  26. CALCULATING THE SHAPE OF IONS • The shape of a complex ion is calculated in the same way a molecule by... • calculating the number of electrons in the outer shell of the central species * • pairing up electrons, making sure the outer shell maximum is not exceeded • calculating the number of bond pairs and lone pairs • using ELECTRON PAIR REPULSION THEORY to calculate shape and bond angle(s) • the number of electrons in the outer shell depends on the charge on the ion • if the ion is positive you remove as many electrons as there are positive charges • if the ion is negative you add as many electrons as there are negative charges • e..g. for PF6-add one electron to the outer shell of P • for PCl4+remove one electron from the outer shell of P

  27. N SHAPES OF IONS EXAMPLE Draw outer shell electrons of central atom

  28. N+ N SHAPES OF IONS EXAMPLE Draw outer shell electrons of central atom For every positive charge on the ion, remove an electron from the outer shell... For every negative charge add an electron to the outer shell... for NH4+ remove 1 electron for NH2- add 1 electron N NH2- NH4+

  29. SHAPES OF IONS H H H H H H N+ N+ N N EXAMPLE Draw outer shell electrons of central atom For every positive charge on the ion, remove an electron from the outer shell For every negative charge add an electron to the outer shell.. for NH4+ remove 1 electron for NH2- add 1 electron Pair up electrons in the usual way N NH2- NH4+

  30. SHAPES OF IONS H H H H H H N+ N+ N N EXAMPLE Draw outer shell electrons of central atom For every positive charge on the ion, remove an electron from the outer shell For every negative charge add an electron to the outer shell.. for NH4+ remove 1 electron for NH2- add 1 electron Pair up electrons in the usual way Work out shape and bond angle(s) from number of bond pairs and lone pairs. N NH2- NH4+ BOND PAIRS 4 LONE PAIRS 0 TETRADHEDRAL H-N-H 109.5° BOND PAIRS 2 LONE PAIRS 2 ANGULAR H-N-H 104.5°

  31. Explain, in terms of electrons, how ammonia can react with hydrogen ions to form ammonium ions, NH4+. (2) Ammonium ion 1. lone pair on N2. forms dative / co-ordinate bond with H+

  32. SHAPES OF IONS N H H H H H H H H H N N+ N+ N N REVIEW BOND PAIRS 3 PYRAMIDAL LONE PAIRS 1 H-N-H 107° NH3 NH4+ BOND PAIRS 4 TETRAHEDRAL LONE PAIRS 0 H-N-H 109.5° BOND PAIRS 2 ANGULAR LONE PAIRS 2 H-N-H 104.5° NH2- http://www.chemguide.co.uk/atoms/bonding/shapesdouble.html

  33. MOLECULES WITH DOUBLE BONDS O C The shape of a compound with a double bond is calculated in the same way. A double bond repels other bonds as if it was single e.g. carbon dioxide O O C Carbon - needs four electrons to complete its shell Oxygen - needs two electron to complete its shell The atoms share two electrons each to form two double bonds

  34. O C 180° O C O BOND ANGLE... SHAPE... 180° LINEAR MOLECULES WITH DOUBLE BONDS The shape of a compound with a double bond is calculated in the same way. A double bond repels other bonds as if it was single e.g. carbon dioxide O O C Carbon - needs four electrons to complete its shell Oxygen - needs two electron to complete its shell The atoms share two electrons each to form two double bonds DOUBLE BOND PAIRS 2 LONE PAIRS 0 Double bonds behave exactly as single bonds for repulsion purposes so the shape will be the same as a molecule with two single bonds and no lone pairs.

  35. ELECTRON PAIR REPULSION THEORY • This theory can be used to interpret and predict the shape of molecules. It is based on the following ideas: • The electron pairs arrange themselves as far apart from each other as possible in order to minimise the repulsions • The repulsions between lone pairs is greater than that between lone pair and a bond pair than that between two bond pairs. • The number of sigma bond pairs of electrons and lone pairs in the molecule should be counted. • Any pi bond pairs should be ignored when working out the shape of the molecule .

  36. RECAP

  37. RECAP

  38. TEST QUESTIONS For each of the following ions/molecules, state the number of bond pairs state the number of lone pairs state the bond angle(s) state, or draw, the shape BF3 SiCl4 PCl4+ PCl6- SiCl62- H2S ANSWERS ON NEXT PAGE

  39. TEST QUESTIONS ANSWER For each of the following ions/molecules, state the number of bond pairs state the number of lone pairs state the bond angle(s) state, or draw, the shape BF3 3 bp 0 lp 120º trigonal planar boron pairs up all 3 electrons in its outer shell 4 bp 0 lp 109.5º tetrahedral silicon pairs up all 4 electrons in its outer shell 4 bp 0 lp 109.5º tetrahedral as ion is +, remove an electron in the outer shell then pair up 6 bp 0 lp 90º octahedral as the ion is - , add one electron to the 5 in the outer shell then pair up 6 bp 0 lp 90º octahedral as the ion is 2-, add two electrons to the outer shell then pair up 2 bp 2 lp 92º angular sulphur pairs up 2 of its 6 electrons in its outer shell - 2 lone pairs are left SiCl4 PCl4+ PCl6- SiCl62- H2S

  40. Predicting Molecular Geometry Predict the following molecular geometries.... • CH4 and PO43- • XeF4 • PCl5 • BrF5 (Ans.Tetrahedral) (Ans.Square planar. Why not tetrahedral?) (Ans.Trigonal bipyramidal) (Ans. Square pyramidal)

  41. Questions

  42. The Octet Rule is Often Violated • H, Be, B, Al violate the octet rule(< 8 valence electrons) e.g. BeCl2, BH3, AlCl3 • Nonmetals with a valence shell greater than n = 2 (e.g. P, Cl, Br, I, etc.) • May violate the octet rule when they are the CENTRAL atom (e.g. ClF5 ) • How can they do this? • Why doesn’t Fluorine violate the octet rule?

  43. Lewis Structures for Organic Compounds • Alkanes: CnH2n+2 • Methane, Ethane, Propane, Butane, Pentane, Hexane • Alkenes: CnH2nhave double bond(s) • One double bond: Ethene (ethylene), Propene (propylene) • Alcohols: CnH2n+1OHhave hydroxyl group(s) • methanol, ethanol • Carboxylic Acids: CnH2n+1COOH have carboxyl group(s) • Methanoic acid (formic acid), HCOOH • Ethanoic acid (acetic acid, CH3COOH

  44. Alkane

  45. Butane Click on the hyperlink to see: wire model, ball and stick model, space filling model and other carbon compounds

  46. Alkene

  47. ETHANOL Predicting Molecular Shapes with More Than One Central Atom The tetrahedral centers of ethanol.

  48. Halogenoalkane

  49. Aldehyde

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