1. Chemistry Chemical Bonding

2. The Development of Atomic Models
Dalton – solid, indivisible mass

3. Thomson – “plum-pudding” model
Negatively charged e- (raisins) stuck in positively charged proton dough
No neutrons

4. Rutherford – electrons surrounding a dense nucleus

5. Bohr model – elctrons arranged in symmetrical orbits around the nuclues
“planetary model”
Electrons in a given path have a fixed energy level

6. 5. Quantum mechanical model – modern mathematical description of the atom
Sodium atom:

7. Energy level – region around the nucleus where the electron is likely to be moving. An electron can jump from one level to another by absorbing energy.

8. Quantum – the amount of energy required to move an electron from its present energy level to the next higher one “quantum leap”

9. Quantum mechanical model – uses mathematical equations to describe the location and energy of electrons in an atom
Developed by Erwin Schrodinger
Electrons are not in definite paths
Their location is described in terms of probability of being in a certain region
Electron cloud (ceiling fan)
Conventionally, the border is drawn at 90% probability

10. Atomic orbital – region in space that an electron is likely to be in Electrons can be described by a series of 4 quantum numbers.

11. 1. Principle quantum number (n)
Describes the energy level
Values of 1, 2, 3, 4, etc.

12. 2. Azimuthal quantum number ( l )
Describes the shape of atomic orbitals
Sublevels
Values of 0 to n-1
0 = s, 1 = p, 2 = d, 3 = f

13. s = spherical, p = peanut shape, d&f = more complex shapes
d = “daisy” f = “fancy”
So if n = 1, then l can be 0 (s) = 1 sublevel
n = 4, then l can be 0 (s), 1 (p), 2(d) , 3(f), = 4 sublevels

14. s orbitals – spherical

15. p orbitals – dumbbell-shaped

16. d orbitals – daisy-shaped

17. f orbitals – fancy shapes

18. 3. Magnetic quantum number (ml)
Orientation of the orbital in space
Values of –l to +l
So s has 1 orbital
p has 3
d has 5
f has 7

19. 4. Spin quantum number (ms)
Values of +½ and -½
Each orbital can hold 2 electrons with opposite spins
Since spinning charged objects create a magnetic field, the electrons must spin opposite directions to minimize repulsion

20. electron spins

21. Ex. How many orbitals are in the following?
A. 3p D. 4p B. 2s E. 3d C. 4f F. 3rd energy level How many electrons can be in each of the above?

22. Are these possible? n l ml ms 1
+1/2 5 2 -5
-1/2 -1

23. Electron Configuration – ways in which electrons are arranged around nuclei of atoms.

24. Rules that govern filling of atomic orbitals:
1. Aufbau principle – electrons enter orbitals of lowest energy first.

26. 2. Pauli exclusion principle – An atomic orbital can describe at most two electrons. They must have opposite spins.

27. 3. Hund’s rule – When electrons occupy orbital of equal energy, one electron enters each orbital until all orbitals contain one electron with parallel spins.

29. 1s 2s 3s 4s 5s 6s 7s p 3p 4p 5p 6p d 4d 5d f

30. Na = 11 1s22s22p63s1 Cd =48 1s22s22p63s23p64s23d104p65s24d10

31. Practice: Write the electron configuration for the following elements:
Li O Sc

32. More practice: Identify each of the following atoms on the basis of its electron configurations.
a) 1s22s22p6 neon b) 1s22s22p63s1 sodium c) [Kr] 5s24d2 zirconium d) [Xe] 6s24f6 samarium

33. Ground state – lowest energy level for an electron
Ground state – lowest energy level for an electron. (Normal, nonexcited state)

34. Exceptional Electron Configurations:
Cr: expected: 1s22s22p63s23p64s23d4 actual: 1s22s22p63s23p64s13d5 Cu: expected: 1s22s22p63s23p64s23d9 actual: 1s22s22p63s23p64s13d10 Half-filled energy levels are more stable than other partially filled energy levels. There are other exceptions.

35. Light and Atomic Spectra
Electromagnetic radiation – a series of energy waves that includes radio waves, microwaves, visible light, infrared, and ultraviolet light, X-rays, and gamma rays.

36. Electromagnetic Spectrum

37. Parts of a wave:
Wavelength, l
crest
trough

38. Amplitude – height of the wave from the origin to the crest
Amplitude – height of the wave from the origin to the crest. Wavelength - l - distance between the crests Frequency – f (or n) – the number of wave cycles to pass a given point per unit of time. The units of frequency are 1/s, s-1, or Hertz (Hz)

39. c= lf where c = speed of light = 3.00x108 m/s or 3.00x1010 cm/s
As l increases, f decreases.

40. As wavelength increases, frequency decreases.
As wavelength decreases, frequency increases.

42. C = lf l = c/f l = (3.00x108 m/s )/(2.73x1016 s-1) l = 1.10x10-8 m
Ex. A certain wavelength of yellow light has a frequency of 2.73x1016 s-1. Calculate its wavelength. Convert to nm.
C = lf l = c/f l = (3.00x108 m/s )/(2.73x1016 s-1) l = 1.10x10-8 m

43. Spectrum – series of colors produced when sunlight is separated by being passed through a prism.
ROY G. BIV
Red: longest wavelength, lowest frequency
Violet: shortest wavelength, highest frequency

45. Atomic emission spectrum – series of lines of colored light produced by passing light emitted by an “excited atom through a prism.” This can be used to identify the element. The atomic emission spectrum of hydrogen shows three series of lines. The lines in the UV region (Lyman series) represent electrons falling to n=1, lines in the visible region (Balmer series) represent electrons falling to n=2 and lines in the IR region (Paschen series) represent electrons falling to n=3.

48. Max Planck found that the energy emitted or absorbed by a body changes only in small discrete units he called quanta. He determined that the amount of radiant energy, E, absorbed or emitted by a body is proportional to the frequency of the radiation.
E=hf
E = energy (J) f = frequency
h = Planck’s constant, 6.626x10-34 Js

49. Einstein studied the photoelectric effect whereby light of sufficient frequency shining on a metal causes current to flow. The amplitude of the radiation was not important, the frequency was. This told him that light must be in particles, each having a given energy. Einstein proposed that electromagnetic radiation can be viewed as a stream of particles called photons: E=hf

50. Photoelectric Effect
light
Electron (photons)
metal

51. Energy of a photon: E = hf

52. E=hf E = (6.626x10-34 Js)(2.73x1016 s-1) E = 1.81x10-17 J
Example: Calculate the energy of an individual photon of yellow light having a frequency of 2.73x1016 s-1.
E=hf E = (6.626x10-34 Js)(2.73x1016 s-1) E = 1.81x10-17 J

53. Einstein’s special theory of relativity: E=mc2
Matter and energy are different forms of the same entity.

54. Going Further:

55. Louis deBroglie suggested that very small particles like electrons might also display wave particles and he came up with: deBroglie’s equation: l = h mv m = mass in kg v = velocity in m/s h = Planck’s constant, 6.626x10-34 Js

56. DeBroglie’s equation is used to find the wavelength of a particle
DeBroglie’s equation is used to find the wavelength of a particle. It was determined that matter behaves as through it were moving in a wave. This is important in small object such as electrons but is negligible in larger objects such as baseballs. Heavy objects have very short wavelengths.

57. Example: Calculate the wavelength of an electron traveling at 1
Example: Calculate the wavelength of an electron traveling at 1.24x107 m/s. The mass of an electron is 9.11x10-28g.
l = h mv 9.11x10-28 g 1 kg = 9.11x10-31 kg 1000g l = (6.626x10-34 Js) = (9.11x10-31 kg)(1.24x107 m/s) l = 5.87x10-11 m

58. End Going Further.

59. In the photoelectric effect, electrons (called photoelectrons) are ejected by metals (esp. alkali) when light of sufficient frequency shines on them. Red light won’t work. Photoelectric cells convert light energy into electrical energy. They are used in automatically opening doors and security systems.

60. Heisenberg’s Uncertainty Principle – it is impossible to determine accurately both the momentum and the position of an electron simultaneously.
We detect motion by electromagnetic radiation. This interaction disturbs electrons.

61. Einstein and Heisenberg!

62. Mendeleev- arranged elements in order of increasing atomic mass Moseley- arranged elements in order of increasing atomic number

63. Periodic Law - When the elements are arranged in order of increasing atomic number, there is a periodic pattern in their physical and chemical properties.

64. Be able to locate noble gases, representative elements, transition metals, inner transition metals.

65. Noble Gases have completely filled shells of electrons
similar electronic structures
He 1s2
Ne 1s2 2s2 2p6
Ar 1s2 2s2 2p6 3s2 3p6
Kr 1s2 2s2 2p6 3s2 3p6 4s2 4p6
etc.

66. Representative Elements
elements in A groups on periodic chart
representative - because they best represent what we know about elemental structure & periodicity

67. d - Transition Elements
elements in B groups on periodic chart
metals
have d electrons
transition from metals to nonmetals

68. f - Transition Elements
inner transition metals

69. Electron configuration using the periodic table:

71. Columns are called groups or families
Columns are called groups or families. Rows are called periods or series.

72. Shorthand electron configuration:

75. Periodic Trends in atomic size: covalent atomic radius - half the distance between the nuclei of atoms in a homonuclear diatomic molecule (like Cl2) Trends: 

78. Atomic size increases going down a group because electrons are added to higher energy levels that are farther from the nucleus. It decreases going across a period because as each proton is added to the nucleus an electron is being added to the same energy level. This “shell” of electrons is pulled closer in towards the nucleus.

79. Size changes little in the transition metals because the electrons being added are core electrons.

80. Zeff = effective nuclear charge
Zeff = effective nuclear charge -actual pull of the nucleus on the valence electrons. Zeff = Zactual - effect of e- repulsions

81. Trends:  Increases from H to He Decreases from He to Li because 1s electrons shield 2s electrons Increases from Li to Be Decreases from Be to B because 2s shield 2p

82. Increases from B to C to N Decreases from N to O because of repulsion due to doubly occupied orbitals Increases from O to F to Ne Decreases from Ne to Na because 1s,2s & 2p shield 3s

83. Know exceptions to Zeff trends and reasons for these.

84. Ionization energy (IE) - energy required to remove the highest energy electron from a gaseous atom Li(g) + energy  Li+(g) + e- - depends on Zeff and size

86. Trends:  Ionization energy decreases down a a group because the valence electrons are farther from the nucleus and are thus held less tightly. Ionization energy increases across a period because atomic size decreases and valence electrons are held more tightly. Zeff increases.

88. 1st ionization energy = energy required to remove the first electron Al + energy  Al+ + e- 2nd ionization energy = energy required to remove the second electron Al+ + energy  Al2+ + e- 3rd ionization energy = energy required to remove the third electron Al2+ + energy  Al3+ + e-

89. For Na, the 1st ionization energy is fairly low but the 2nd would be high.

90. Know exceptions to ionization energy trends and reasons for them.

91. Ionic Size Anions are larger than the atoms from which they were formed. Know why!!! Cations are smaller than the atoms from which they were formed. Know why!!!!

92. Sizes of Ions Related to Positions of the Elements in the Periodic Table

93. Isoelectronic ions- a group of ions with the same number of electrons The one with the highest atomic number is the smallest in size (More protons pulling on the same # of electrons).

94. Na+, Mg2+, Ne, F-, O2-, N3- are isoelectronic
Na+, Mg2+, Ne, F-, O2-, N3- are isoelectronic. They all have 10 electrons. Mg2+ is the smallest because it has 12 protons pulling on 10 electrons. (The protons win the “tug of war”) N3- is the largest because it has 7 protons and 10 electrons. (The electrons win the “tug of war”)

95. Electron Affinity -energy change that occurs when an electron is added to a gaseous atom -usually exothermic Cl(g) + e-  Cl-(g)+ energy Trends:  (but many exceptions)

97. Electronegativity -relative tendency of an atom to attract shared electrons to itself Trends:  FONCl (Phone Call)

99. Elements with high electronegativity (nonmetals ) tend to gain electrons to form anions. Elements with low electronegativities (metals) often lose electrons to form cations.

100. Valence electrons- electrons in the highest occupied energy level of an atom. Valence electrons are the only electrons involved in the formation of chemical bonds.

101. Electron dot structures for atoms: -each dot represents a valence electron p p X s p

102. Examples: N

103. N

104. O

105. O

106. Xe

107. Xe

108. Al

109. Al

110. Na

111. Na

112. I

113. I

114. Si

115. Si

116. One of the major “driving forces” in nature is the tendency to go to lower energy. Atoms lose, gain or share electrons to become lower in energy and thus more stable.

117. Metals lose electrons easily to become positively charged cations
Metals lose electrons easily to become positively charged cations. They will usually lose their valence electrons to achieve a noble gas electron configuration. Na  Na+ + e- [Ne]3s1 [Ne] Al  Al3+ + 3e- [Ne]3s23p [Ne]

118. Some transition metals lose their highest energy level s and p electrons but still have d electrons remaining. Their electron configuration is not quite that of a noble gas but is still stable. It is called a pseudo-noble gas electron configuration. For example, zinc loses its two electrons in 4s but keeps the ten electrons in 3d.

119. Transition metals always lose their highest numerical energy level electrons first. Transition metals in the 4th period lose their 4s and 4p electrons before losing any from 3d. Metals in groups 3, 4, & 5 do this also.

120. Example: Fe [Ar]4s23d6 Fe2+ [Ar]3d6 Fe3+ [Ar]3d5

121. Nonmetals tend to gain electrons to become stable and form negatively charged anions. They achieve a noble gas electron configuration. Example: Cl e-  [ Cl ]- [Ne]3s23p5 [Ne]3s23p6 N + 3e-  [ N ]3- 1s22s22p s22s22p6

122. Ionic Bonding- the attraction of oppositely charged ions (cations and anions) When the electronegativity difference between two elements is large, the elements are likely to form a compound by ionic bonding (transfer of electrons). The farther apart across the periodic table two Group A elements are, the more ionic their bonding will be.

123. We can use Lewis dot formulas to represent the formation of ionic compounds. Na + Cl  Na+[ Cl ]- or NaCl Mg: N Mg2+ [ N ]3- Mg:  Mg or Mg3N2 Mg: N Mg2+ [ N ]3-

124. Properties of Ionic Compounds: They are usually crystalline solids with high melting points (>400oC) Their molten compounds and aqueous solutions conduct electricity well because they contain mobile charged particles.

125. Metals Metals form metallic solids that consist of positively charged metal cations in a “sea” of loosely held valence electrons. This arrangement allows metals to have their unique properties.

127. Metals are ductile (can be pulled into a wire) and malleable (can be hammered into a thin sheet) because the valence electrons act as “grease”, allowing the cations to slide past each other without colliding with each other and shattering. When ionic compounds such as NaCl are hammered, like-charged ions collide causing repulsion and the crystal shatters.

128. Metals can conduct electricity easily
Metals can conduct electricity easily. Electricity is a flow of electrons. As electricity (electrons) enters one end of a piece of metal, an equal number of electrons exit the other end.

129. Alloys- solutions of solids in solids

130. Substitutional alloy- atoms of one metal are substituted for atoms of a similar-sized metal in a metallic crystal. Ex. brass, sterling silver, pewter Interstitial alloy- smaller metal atoms fit into holes in the crystal structure of a metal with larger atoms Ex. steel (carbon in iron)

132. Amalgam- alloy which contains mercury

133. Covalent Bonding

134. Hydrogen and nonmetals of Groups 4,5,6 & 7 often become stable and gain noble gas electron configurations by sharing electrons to form covalent bonds. Atoms will usually share electrons to follow the octet rule (eight electrons, like most noble gases) or the duet rule (2 electrons, like helium).

135. When atoms share one pair of electrons to form a covalent bond, it is called a single covalent bond. The electrons shared between the atoms are a “shared pair”. A dash can be used instead of two dots to represent the shared pair. Any other electrons on the atoms are “unshared pairs” or “lone pairs”.

136. Ex. H2 H-H Cl2 Cl-Cl HCl H-Cl

137. Atoms must sometimes share more than one pair of electrons to become stable. When two pair of electrons are shared between two atoms, it is called a double bond. If three pair are shared, it is a triple bond. Ex. O N2 O = O N  N

138. Rules for Writing Lewis Structures (electron dot structures): (Use pencil!) 1.Add up the valence electrons from all the atoms. Don’t worry about keeping track of which electrons come from which atoms. If you are working with an ion, you must add or subtract electrons to equal the charge. 2.Use a pair of electrons to form a bond between each pair of bound atoms.

139. 3.Arrange the remaining electrons to satisfy the duet rule for hydrogen and the octet rule for everything else. 4.If necessary, change bonds to double or triple. 5.Remember, we cannot create or destroy electrons!

140. H2O
8 electrons
H-O-H

141. NH3
8 electrons
H-N-H H

142. NH4+
9-1 = 8 electrons H
H-N-H H

143. CO2
16 electrons
O-C-O
This used 20 electrons! BAD!!!

144. CO2
O=C=O

145. 32 electrons
CCl4

146. CCl4
32 electrons Cl
Cl-C-Cl

147. CN-
9 + 1 = 10 electrons
C-N
BAD!

148. CN-
9 + 1 = 10 electrons
CN

149. SO42-
32 electrons O
O S O O

150. CO electrons 2- O
O C O
O C O 2-

151. Coordinate Covalent Bond- Bond in which both electrons came from the same atom. This bond is not really any different than any other single bond.

152. Exceptions to the Octet Rule: A few compounds are stable with less than an octet. They include beryllium or boron. These electron deficient compounds are very reactive.

153. Ex BF BeCl2
24 electrons electrons
F Cl-Be-Cl
F-B-F

154. Elements in the third period and below can exceed the octet rule
Elements in the third period and below can exceed the octet rule. They can place extra electrons in empty d orbitals. Elements in the second period can not exceed the octet rule because there is no 2d orbital for the extra electrons to go into. If it is necessary to exceed the octet rule, place the extra electrons on the central atom.

155. Ex PCl SF6
Cl Cl F F F
P S
Cl Cl Cl F F F

156. I3-
22 electrons
[ I-I-I ]-

157. More practice: NF3

158. F F - N - F

159. OF2 20 electrons

160. F - O - F

161. KrF4 36 electrons

162. F F- Kr - F F

163. BeH electrons

164. H - Be - H

165. SO electrons

166. 2-
O O - S - O

167. NO3- 24 electrons

168. O O - N - O

169. H2O2 14 electrons

170. H - O - O - H

171. Resonance occurs when more than one valid Lewis structure can be written for a molecule. The actual structure is an average of all of the resonance structures.

172. Ex. NO3- O O O O - N - O O - N - O O - N - O

173. In nitrate, the experimental bond length is in-between that of a single bond and a double bond. It acts like a 1 1/3 bond.

174. Ex. Benzene, C6H6

176. VSEPR

177. Lewis structures can be used to determine the shapes of molecules
Lewis structures can be used to determine the shapes of molecules. Their shapes will tell us a lot about their chemical behavior.

178. The valence shell electron pair repulsion (VSEPR) theory tells us that valence electrons on the central atom repel each other. They are arranged as far apart as possible around the central atom so that repulsions among them are as small as possible. When we are using VSEPR to determine molecular shape, we are really looking for regions of electron density. Double and triple bonds count the same as single bonds in determining molecular shape.

179. In CO2, there are only two regions of electron density (effective electron pairs) around the central atom. These regions arrange themselves as far apart as possible, making the bond angle 180o and the molecular shape linear O = C = O

180. In CO32-, there are three effective electron pairs around the central atom. The bond angle will be 120o and the shape will be trigonal planar O C O O

181. In CH4, there are four effective electron pairs
In CH4, there are four effective electron pairs. We might expect the bond angle to be 90o. Actually, since molecules are three-dimensional, the electron pairs are 109.5o apart (further than 90o) and take a tetrahedral arrangement H H C H H

183. In NH3, there are four effective electron pairs
In NH3, there are four effective electron pairs. Three are shared but one is unshared. Unshared pairs of electrons take up more space than shared pair because they are pulled closer to the nucleus. The presence of the unshared pair distorts the other bond angles, making them less than 109.5o and the shape is called trigonal pyramidal. (The bond angles in ammonia are about 107o.) H - N - H H

185. In H2O, there are four effective electron pairs, also
In H2O, there are four effective electron pairs, also. Two are shared and two are unshared. Since the unshared pairs repel more than shared pair, the bond angle is less than 109.5o (actually 104.5o for H2O) and the shape is bent H – O – H

188. In PF5, there are five effective pairs, all shared
In PF5, there are five effective pairs, all shared. The bond angles are 90o and 120o and the shape is called trigonal bipyramidal. It is like two trigonal pyramids with their bases touching F F F - P - F F

190. In SF6, there are six effective pairs, all shared
In SF6, there are six effective pairs, all shared. The bond angles are 90o and the shape is called octahedral. It is like two square pyramids with their bases touching.

192. Molecules that exceed the octet rule and have unshared electrons can have more complex shapes such as T-shaped, see-saw, and square pyramidal.

193. Practice Determining Molecular Shape:

194. H2S H - S - H
bent, <109.5o

195. CCl4
tetrahedral, 109.5o Cl
Cl-C-Cl

196. tetrahedral, 109.5o
NH H H - N - H H

197. trigonal planar, 120o
BF F B F F

198. NO O - N = O
bent, <120o

199. PF F F F P F F F
octahedral, 90o

200. SbCl Cl Cl Cl Sb Cl Cl
trigonal bipyramidal, 90o and 120o

201. SO O - S = O
bent, <120o

202. POLAR AND NONPOLAR MOLECULES Covalent bonds within a molecule can be polar (electrons are shared unequally) or nonpolar (electrons are shared equally). We can predict polar bonds by looking up the electronegativity values of each element in the bond and subtracting the smaller value from the larger value to determine the electronegativity difference.

203. If the electronegativity difference (EN) is zero, the bond is nonpolar covalent. If the EN is between 0 and 2.0, we can predict that the bond is polar covalent. If the EN is 2.0 or greater, the bond is usually considered to be ionic.

204. There are no strict dividing lines between covalent and ionic bonds
There are no strict dividing lines between covalent and ionic bonds. Physical properties are used to help determine whether something is covalent or ionic. Some covalent substances are so polar that they ionize partially or completely in water.

206. Polarity of a bond or molecule can be represented by arrows or lowercase delta () symbols to show a partial charge. Ex H F + - This shows that the hydrogen end of the molecule is more positive (less electronegative) and the fluorine end is more negative (more electronegative). A charge difference such as this is called a dipole.

207. An entire molecule is polar if it has polar bonds that do not cancel
An entire molecule is polar if it has polar bonds that do not cancel. This happens in nonsymmetrical molecules. Molecules with all nonpolar bonds such as H2, O2, and Cl2, are always nonpolar. This means that there are no positive and negative ends on the molecule.

208. Heteronuclear diatomic molecules such as HCl, BrCl, or HF are always polar because the polar bonds can not cancel each other out.

209. Linear triatomic molecules are polar only if the two outside legs are different. CO2 is nonpolar because the two C=O bonds cancel each other out Bent molecules are polar.

210. Trigonal planar molecules are nonpolar if the three legs are the same
Trigonal planar molecules are nonpolar if the three legs are the same. Trigonal pyramidal molecules are polar. Tetrahedral molecules are nonpolar if the four legs are all the same. Trigonal bipyramidal and octahedral molecules are nonpolar if all legs are the same element.

211. Practice: Polar or Nonpolar? NH3
1st: Draw dot structure
2nd: Determine shape
3rd: Symmetrical = Nonpolar
Nonsymmetrical = Polar H
H - N - H
Polar

212. SO2
O = S - O
Polar

213. H2O
H - O - H
Polar

214. BF3 F
F - B - F
Nonpolar

215. CH4 H
H - C - H
Nonpolar

216. INTERMOLECULAR FORCES

217. Intramolecular bonding- sharing electrons Intermolecular bonding- interactions between particles (atoms, molecules or ions)

218. Changes in state are due to changes in intermolecular bonding, not intramolecular bonding.

219. Dipole-dipole attraction- attraction of polar molecules for each other
Dipole-dipole attraction- attraction of polar molecules for each other. (negative-positive) *approx. 1% as strong as covalent or ionic bonds. *molecules orient themselves to minimize repulsion and maximize attractions

221. Hydrogen bonding- unusually strong dipole-dipole attractions involving hydrogen atoms which are covalently bonded to a very electronegative element and a very electronegative atom (F,O,N only) with unshared electrons.

223. Two reasons for strength of hydrogen bonds: 1
Two reasons for strength of hydrogen bonds: 1. small size of H atom allows closeness 2. great polarity Substances with much H bonding have high boiling points compared to similar substances. Ex. H2O, NH3, HF

225. London dispersion forces (LDFs)- relatively weak forces (usually) that exist between noble gas atoms and nonpolar molecules. LDFs also exist in compounds that have dipole-dipole and/or hydrogen bonding. LDFs may be the most important force in large molecules of these types. LDFs occur because of momentary electron imbalance (temporary dipole) which can induce the same to occur in adjacent molecules.

227. This force is often very weak, thus the low freezing point of noble gases. The freezing point of noble gases increases going down the group because heavier atoms have more electrons and an increased chance of temporary dipoles. They also have a lower velocity and have more opportunity for attractions. This causes London dispersion forces to increase going down a group on the periodic table.

228. Covalent network solid- Group 4 substances such as diamond, Si, SiC, and Ge form extensive covalent bonds and result in giant molecules. They have an atom at each lattice point and are held together by covalent bonds. These substances have the strongest attractions.

229. General trends in strength of attraction: (weakest) LDF dipole-dipole hydrogen-bonding metallic bonding ionic bonding covalent network bonding (strongest)