Bonding and Molecular Structure: CHAPTER 6

Introduction

        I.  What makes up the world around us?  all matter is composed of atoms

        II.  How are the atoms found in nature?  sometimes, but rarely, free (such as Ar in atmosphere and He in natural gas reserves);  atoms are usually found in mixtures and compounds where atoms are bound together

        III.  Chemical and physical properties are affected by how the atoms are bound together;  bonding in and the structure of compounds determines the direction of chemical reactions  (many biological reactions are directly dependent on the geometric shape of molecules)

        IV.  What is a chemical bond?  "a mutual electrical attraction between the nuclei and valence electrons of different atoms that bind the atoms together";  it is a force that holds two or more atoms together and allows them to function as a single unit

        V.  Why do chemical bonds occur?  bonds result from the tendency of nature to move towards lowest energy, the bonded atoms are lower in energy (more stable) than the separate atoms that compose it;  the valence electrons in bonded atoms are redistributed to make the atoms in the compound more stable;  atoms in a stable compound have valence electrons that are isoelectronic with noble gases (completed s and p sublevels), the octet rule is satisfied (duet for hydrogen)

        VI.  Information about bonds is learned through experimentation

        VII.  Different ways that particles interact

                A.  Electrostatic force:  force holding closely packed, oppositely charged ions together;  electrical attraction;  can be strong or weak

                B.  Intramolecular forces:  hold atoms together in a molecule;  strong forces;  chemical bonds

                C.  Intermolecular forces:  hold molecules together;  weak, act over only a short distance;  called van der Waals forces

Types of Chemical Forces

        I.  Ionic bond:   between dissimilar atoms,  like a main group metal and a nonmetal

                A.  transfer of an electron from one atom to another 

                B.  causes ions to be formed

                        1.  one atom loses an electron forming a cation or positive ion

                        2.  one atom gains an electron forming an anion or negative ion

                C.  ions are held together by an electrostatic force

                D.  ion pair has lower energy than the separate atoms

                E.  compounds with ionic bonds are called ionic compounds

                F.  properties:

                        1.  relatively high melting points  (NaCl melting point is 800 ºC)

                        2.  usually soluble in water and these solution conduct electricity

                        3.  molten and gas phases conduct electricity

                        4.  form crystalline solids from their solutions

                                a.  crystal lattice:  ions formed in an orderly arrangement

                                b.  lattice energy

        II.  Covalent bond:   between two similar atoms, two nonmetals

                A.  a pair of electrons is shared between the two atoms 

                B.  electrons are simultaneously attracted to both atoms, causes energy of the bonded atoms to be less than the energy of the separated atoms

                C.  compounds with covalent bonds are called molecules 

                D.  properties:

                        1.  relativity low melting points

                        2.  may or may not be soluble in water and these solutions may or may not conduct electricity

                        3.  not conductors of electricity in any phase

4. bond length:  potential  energy is stored in chemical bonds the distance between atoms in a

compound is always changing the energy in the chemical bond is always changing;  bond length is the distance atoms are apart when they are at their lowest potential energy

5.  bond energy:  the strength of a bond is determined by how much energy is needed to break

the chemical bond and form neutral isolated atoms

                        6.  bond angle:  average angle between axis of adjacently bonded atoms;  can't get exact measurement due to movement of atoms

                E.  Types    

                        1.  Nonpolar covalent:  the bonding electrons are shared equally, or near equally, by the bonded atoms, resulting in a balanced electrical distribution of charge;  the bonded atoms have an equal, or near equal, attraction for the bonding electrons

a.  100% nonpolar bonding covalent occurs between identical atoms, such as H2, O2, etc.

b.  electrons shared near equally between two atoms, such as a bond between C and N

                        2.  Polar covalent :  the bonding electrons are not shared equally between the two atoms resulting in an uneven electrical distribution of charge;  the bonded atoms have an unequal attraction for the bonding electrons

                                a.  dipole:  a polar bond caused by partial positive (∂+) and partial negative (∂-) charge; properties of the compound relate to the strength of the dipole  (∂ is called delta)

                                b.  dipolar:  a dipole moment,  caused by having a center of positive charge and a center of negative charge, represented by   –|–––> pointing toward the negative center

                                c.  polar vs. nonpolar molecules:  the arrangement of peripheral atoms around the central atom will determine if a molecule with polar bonds will be a polar molecule  (H2O vs. CH4)

                                        1.  polar molecule:  polar bonds are arranged asymmetrically in a molecule

                                        3.  nonpolar molecule:  polar bonds are arranged symmetrically in a molecule or nonpolar bonds in a molecule

F.  Single versus Multiple bonds

                        1.  triple bond length < double bond length < single bond length

                        2.  triple bond strength > double bond strength > single bond strength

                        3.  single bonds are called saturated

                        4.  double and triple bonds are called unsaturated

III.  Polyatomic ions:  two or more atoms covalently bonded as a unit with a charge on the unit

A.  the bonded unit can attract extra electrons so the unit has a negative charge - CO3-2     

B.  the bonded unit can lose electrons so the unit has a positive charge - NH4+1  

C.  bonds as one ion

        IV.  Metallic bond:  strong, nondirectional bonding formed by the sharing of delocalized electrons

                A.  "sea of electrons":  overlapping outer orbitals in metals allow valence electrons to move freely through the metal;  the electrons are not attracted to any single metal atom;  the "sea of electrons" surrounds the positive core of the metal atom, which floats in the sea 

                B.  Properties

                        1.  lustrous, shiny

                        2.  generally solids at room temperature

                        3.  malleable and ductile because positive ions can move within metal structure  (it is difficult to separate the atoms in a metal but the positive ions are easily moved around as long as they are in contact with one another)

                        4.  high melting points 

                        5.  heat conductors

                        6.  electrical conductors in all phases, because of electrons moving in the metal

                                a.  electrons must be in the conduction band to move

                                b.  moving electrons are called free or delocalized electrons

                                c.  degree of properties determined by the number of delocalized electrons

        V.  Network bonding (macromolecules):  strong, directional covalent bonds

        VI.  van der Waals forces - intermolecular forces

A.  Dipole-dipole: 

                        1.  normal attraction between polar-polar molecules;  an electrostatic force;  opposite sides of the molecules attract each other (opposite charges attract)

                        2.  hydrogen bonding:  very high dipole-dipole force and seen only when hydrogen is bonded to nitrogen, oxygen or fluorine:    these atoms have high electronegativities, they have unshared pairs of electrons and they have small atomic radii à the exposed bare proton of hydrogen has a high partial positive charge and the small size allows atoms to get very close è hydrogen is attracted to the unshared electrons of a neighboring molecule

        B.  London dispersion forces - caused by the random motion of electrons in molecules;  force increases with an increase in the size of the molecules - the more electrons there are the more distorted the electron cloud can become causing the force to be greater 

                        1.  instantaneous, temporary or momentary dipole

                        2.  induced dipole

Stability of Compounds

        I.  compounds are stable because the components of the compounds each have a noble gas electron configuration

II.  ionic compounds:  gain or lose electrons forming ions so that each ion has a noble gas

configuration

        III.  covalent compounds (molecule):   share electrons to complete the noble gas electron configuration

Predicting Type of Bond

        I.  Ionization energy:  energy needed to take away an electron from an atom

        II.  Electron affinity:  attraction an atom has for it's electrons

        III.  Electronegativity

                A.  definition:  an atoms tendency to attract electrons to itself when bonded to another atom

                B.  how different from electron affinity:  a bonded atom versus an isolated, nonbonded atom

                C.  general trend:  same as ionization energy and electron affinity:  decrease down a group and increase across a period

                D.  see table below for values or see figure 5-20 on page 151 in text.  Where does hydrogen fit in for the trend?

        IV.  Bond character is determined by:  the electronegativity difference between two atoms

                A.  high electronegativity differences result in ionic compounds

                B.  actual bonds are a combination of ionic and covalent components, the bond character is a percent of each

                C.  1.70 difference is the number sometimes used to determine if a bond is mainly ionic or covalent

                        1.  if the difference is 0 ≤ x ≤ 0.3, the bond is primarily nonpolar covalent

                        1.  if the difference is 0.1 < x < 1.70,  the bond is primarily polar covalent

                        2.  if the difference is 1.70 ≤ x, the bond is primarily ionic

 

 

1A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8A

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

1

H

 

 

 

 

 

 

ELECTRONEGATIVITY

 

 

 

 

 

He

 

2.20

2A

 

 

 

 

 

 

 

 

 

 

3A

4A

5A

6A

7A

 

 

3

4

 

 

 

 

 

 

 

 

 

 

5

6

7

8

9

10

2

Li

Be

 

 

 

 

 

 

 

 

 

 

B

C

N

O

F

Ne

 

0.96

1.50

 

 

 

 

 

 

 

 

 

 

2.02

2.56

2.81

3.37

4.00

 

 

11

12

 

 

 

 

 

 

 

 

 

 

13

14

15

16

17

18

3

Na

Mg

 

 

 

 

 

 

 

 

 

 

Al

Si

P

S

Cl

Ar

 

0.96

1.29

3B

4B

5B

6B

7B

8B

8B

8B

1B

2B

1.63

1.94

2.04

2.46

3.00

 

 

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

4

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

 

0.84

1.02

1.28

1.44

1.54

1.61

1.57

1.74

1.79

1.83

1.67

1.60

1.86

1.93

2.12

2.45

2.82

 

 

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

5

Rb

Sr

Y