Chemical Bonding. Burlingame High School

Transcription

1 Chemical Bonding

2 Electronegativity Is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling

3 Electronegativity Trends

4 Forms of Chemical Bonds There are 2 extreme forms of connecting or bonding atoms: Ionic Ionic complete complete transfer of electrons from one atom to another Covalent Covalent electrons electrons shared between atoms Most bonds are somewhere in between.

5 Nonpolar bond Non-polar & Polar Covalent Bonds» Electrons are shared equally Polar bond» Electrons are shared unequally Difference in electronegativites is a rough measure of character of bond between two elements

6 EN and Bond Type EN and only nonmetals involved and a metal is involved >2.0 Bond Type Nonpolar covalent Polar covalent Polar covalent Ionic Ionic

7 EN and Bond Type Polar Covalent EN Between 1.6 and 2.0 and only non metals are involved Ionic EN Between 1.6 and 2.0 and a metal is involved

8 Electronegativity Differences

9 Electronegativity Differences

10 Non-Polar Covalent EN = 0

11 Polar Covalent EN= 0.9

12 Ionic Bond EN = 2.1 Na + Cl- Na+ Na + + Cl - Cl

13 NaCl Crystal Alternating Na+ and Cl- 3-D lattice structure

14 Now let s do Bonding Worksheet #3. Electronegativity and Bond Type

15 Valence Electrons Electrons in the highest occupied energy level of an elements atom Determines the chemical properties of an element

16 Valence Electrons core and Electrons are divided between core and valence electrons. Na 1s 2 2s 2 2p 6 3s 1 Core = [Ne[ Ne] ] and valence = 3s 1 Br [Ar[ Ar] ] 3d 10 4s 2 4p 5 Core = [Ar[ Ar] ] 3d 10 and valence = 4s 2 4p 5

17 Valence Electrons

18 Valence Electrons the electrons at the outer edge of the atom.

19 Octet Rule Atoms tend to gain, lose, or share enough electrons to become surrounded by eight valence electrons. Metallic elements lose electrons Non-metallic elements gain electrons or share them.

20 Covalent Bonds and the Octet Rule Shared electrons are counted by both elements Single bond - sharing one pair of electrons F F Double bond - sharing two pairs of electrons O C O Triple bond - sharing three pairs of electrons N N

21 Now let s do Bonding Worksheet #4. Lewis Dot Diagrams

22 Lewis Structures G. N. Lewis Electron distribution is depicted with Lewis electron dot structures Electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS.

23 Lewis Structures Write electron dot notation for each type of atom in molecule Determine total # of valence electrons Arrange atoms to form skeleton structure for molecule. If Carbon is present - in center. Otherwise least electronegative in center. Hydrogen never in center Connect atoms by electron-pair bonds Add unshared pairs of electrons so each nonmetal is surrounded by 8 electrons Count the electrons to see it matches with the number of valence electrons» If too many electrons create double or triple bonds

24 Lewis Structures Atoms will form bonds so that they have a total of 4 pairs of electrons. These can be bonded pairs or lone pairs. Halogens Bonded Pairs 1 Lone Pairs 3 Oxygen Family Nitrogen Family Carbon Family Hydrogen

25 Covalent Bonds and the Octet Rule Shared electrons are counted by both elements Single bond - sharing one pair of electrons F F Double bond - sharing two pairs of electrons O C O Triple bond - sharing three pairs of electrons N N

26 Write the Lewis structure of NH 3. The total number of valence electrons is 1(N) 1 x 5 = 5 3(H) 3 x 1 = 3 8 Lewis Structures The skeleton structure is H N H H

27 Lewis Structures Connect the atoms with electron pairs 8 electrons needed to obey the octet rule Finish the structure by using remaining electrons as lone pairs H N H H Check that the final Lewis structure has the correct number of valence electrons (8) and each atom (not H) has 8 electrons.

28 Lewis Structures Write the Lewis structure of H 2 CO. The skeleton structure is H O C H

29 Lewis Structures Write the Lewis structure of SiCl 4. The skeleton structure is...

30 Lewis Structures Write the Lewis structure of CO 2. The skeleton structure is...

31 Now let s do Bonding Worksheet #5. Lewis Structures

32 Formation of a Covalent Bond

33 Covalent Bonding Covalent bonds result from the sharing of electron pairs as shown here for H 2

34 Formation of a Covalent Bond The bond arises from the mutual attraction of 2 nuclei for the same electrons. H A + H B H A H B Bond is a balance of attractive and repulsive forces.

35 Formation of a Covalent Bond

36 Properties of Covalent Compounds Gases, liquids, or solids with low melting points Covalent bonds are not broken to change state. Do not conduct electricity or conduct electricity poorly

37 Ionic Bond Ionic bonding: results from the electrostatic attraction between cations and anions. Formation of an ionic bond can be viewed as a transfer of electrons. Na + Cl Na+ + Cl -

38 NaCl Crystal Structure

39 Properties of Ionic Compounds High melting points Solids at room temp Soluble in polar solvents like water Insoluble in nonpolar solvents like hexane Molten compounds conduct electricity Aqueous solutions conduct electricity

40 Now let s do Bonding Worksheet #6. More About Bonding

41 Metallic Bonding Attraction of the free-floating valence electrons for the positively charged metal ions

42 Properties of Metals 1. Good conductors of electricity and heat 2. Ductile - can be drawn into wires 3. Malleable - can be hammered into shapes

43 Alloys 1. Metallic substances composed of 2 or more elements 2. Properties are superior to those of their component elements Most of the metals we actually use are alloys

44 Examples of Common Alloys Brass - Cu and Zn Sterling Silver (92.5% Ag and 7.5% Cu)» Harder and more durable than pure silver Bronze (7 parts Cu : 1 part Sn) Steel- Fe, Cr and other metals» Corrosion resistant, more ductile, harder and tougher than iron Amalgams - Alloys of Mercury (Hg, Ag & Zn)» Used in older fillings

45 Striking it Rich Obtain 3 pennies. Set 1 aside for comparison at the end of lab.(1) Have 2 coated with Zinc (Zinc metal in NaOH solution) Set 1 of the zinc coated pennies aside for comparison (2) Using metal tongs, put 1 penny in open flame. About 10 seconds on each side. Place penny in cool water. Compare this penny (3) to penny (1) and (2) Write a short paragraph explaining why each penny looks the way it does.

46 Valence Shell Electron Pair Repulsion theory VSEPR The most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions.

47 Structure Determination by VSEPR Electron pairs repel and molecules adjust their shapes so that valence electron pairs and bonds are as far apart as possible.

48 VSEPR Shapes Valence Shell Electron Pair Repulsion Linear Trigonal Planar Bent Tetrahedal Trigonal Pyramida l

49 VSEPR Build a picture of the shape of the molecule or ion by counting regions of high electron density. Each bonded electron counts as one region. Each unshared electron pair counts as one region.

50 Ammonia, NH3 There are 4 electron pairs at the corners of a tetrahedron. H Structure Determination by VSEPR N H H H N H The ELECTRON PAIR GEOMETRY is tetrahedral. H lone pair of electron in tetrahedral positi

51 Structure Determination by VSEPR Water, H 2 O H O H O H The electron pair geometry is TETRAHEDRAL. H The molecular geometry is bent.

52 Structure Determination by VSEPR Formaldehyde, CH 2 O H O C H The electron pair geometry is PLANAR TRIGONAL The molecular geometry is also planar trigonal.

53 Molecular Polarity Molecules such such as HCl and H 2 O can be POLAR (or dipolar). They have a DIPOLE MOMENT.. The polar HCl molecule will turn to align with an electric field. POSITIVE H Cl +δ δ NEGATIVE

54 Polarity Of Water Dipole present = polar molecule

55 Carbon Dioxide No Dipole present = Nonpolar molecule Nonpolar molecule even though it contains polar bonds. This is due to the symmetry of the molecule

56 Molecular Polarity Molecules will be polar if bonds are polar the molecule is NOT symmetric Symmetric molecules

57 Molecular Polarity O C O CO 2 is NOT polar even though the CO bonds are polar.

58 Now let s do Bonding Worksheet #7. VSEPR shapes

59 Intermoleuclar Forces The forces of attraction between molecules Generally weaker than bonds that join atoms in molecules,ions in ionci compounds, or metals atoms in solid metals

60 Hydrogen Bonding An attractive force in which a hydrogen that is covalently bonded to an electronegative atom is also attracted to an unshared electron pair in a nearby atom.