Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
e
CHEM1002 [Part 2]
A/Prof Adam Bridgeman (Series 1)
Dr Feike Dijkstra (Series 2)
Weeks 8 – 13
Office Hours:
Room:
e-mail:
e-mail:
Monday 2-3, Friday 1-2
543a
adam.bridgeman@sydney.edu.au
feike.dijkstra@sydney.edu.au
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e
Periodic Trends
Lecture 4:
•
Titrations
Lecture 5:
•
Periodic Trends
•
Oxides and Hydroxides
•
Strengths of Acids and Bases
•
Blackman, Chapter 4, Section 4.8-4.9
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Periodicity of Atomic Radii
Rb
Cs
K
Na
Li
Ar
Xe
Kr
Ne
He
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Periodicity of Atomic Radii
• Across each period
 Increase in nuclear charge Z
 Increase in e-/e- repulsion
• Electrons move very fast and can avoid each other
 Increase in effective nuclear charge: Zeff
 Zeff increases across period
 Size decreases across period
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Periodicity of Atomic Radii
• Increase in size down each group
 Increase in principal quantum number, n
 Increase in shell size
• Electrons in inner shells shield charge
• Core charge = number of protons – number of
core electrons
Zeff (Li) ≈ 3 - 2 = 1
Zeff (Na) ≈ 11 - 10 = 1
Zeff (K) ≈ 19 - 18 = 1
• Elements in a group have similar properties
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Trends in Two Atomic Properties
increases
atomic size
decreases
increases
decreases
ionization energy
• Trends in atomic properties are usually either similar to the
trend in the atomic radii or the exact opposite
• Diagonally related elements often have similar properties:
diagonal relationship
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Ionic and Atomic Radii
• Ionization decreases e-/e- repulsion
 Zeff increases
 Cations are smaller than the respective atom
 e.g. Ba - 222 pm but Ba2+ 135 pm
• Adding an electron increases e-/e- repulsion
 Zeff decreases
 Anions are larger than the respective atom
 e.g. O - 73 pm but O2- 140 pm , F 72 pm but F- 133 pm
• Anions are often much larger than cations
 e.g. Ba2+ 135 pm (row 6) but O2- 140 pm (row 2)
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Electronegativity
• Tendency of an atom to attract electrons towards itself in a
molecule
 increases with Zeff
 decreases with atomic size
decreases
increases
electronegavity
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The Pure Elements
• Bonding ranges from metallic for metals (bottom left of the periodic
table) to covalent for non-metals (top right), with semimetals
(metalloids) lying between.
tend to gain
electrons to
form anions
tend to lose
electrons to
form cations
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Oxides and Hydroxides I
• Metal oxides and hydroxides are basic
 Metal oxides and hydroxides are ionic due to large
difference in electronegativity (EN ) between O and metal
• Metal oxides (e.g. Na2O, EN = 2.6) contain the O2- ion,
which combines with H+ to form H2O:
Na2O(s) + H2O(l)  2OH-(aq) + 2 Na+(aq)
• Metal hydroxides (e.g. NaOH) contain the OH- ion
• OH- and (especially) O2- have high affinity for H+ ion
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Oxides and Hydroxides II
• Non-metal oxides and ‘hydroxides’ are acidic
 non-metals form covalent bonds to oxygen due to low
electronegativity difference
 they do not contain O2- or OH-
• Non-metal oxides dissolve in OH- to form a salt + H2O:
e.g. CO2(g) + 2OH-(aq)  CO32-(aq) + H2O(l)
• Non-metal hydroxo compounds (e.g. HNO3, H2SO4) react with
OH- as follows:
e.g. X-O-H(aq) + OH-(aq)  X-O-(aq) + H2O(l)
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Oxides and Hydroxides III
• Semi-metal (metalloid) oxides and ‘hydroxides’ are amphoteric they can act both as acids and bases)
 electronegativity is intermediate between metal and non-metal
 As a base:
Al2O3(s) + 6 HCl(aq)  Al2Cl6(aq) + 3 H2O(l)
 As an acid:
Al2O3(s) + 2 OH-(aq) + 3 H2O(l)  2 Al(OH)4-(aq)
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electronegativity increases
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Acid-Base Behaviour
basic oxide
(ionic)
+ H2O → OH+ acid → salt + water
amphoteric oxide
+ acid → salt + water
+ base → oxyanion + water
acidic oxide
(covalent)
+ base → oxyanion + water
+ acid → H+
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Relative Strengths of Acids and Bases
• Strongest acids lose their protons easily
 more polarised the H-X bond is,
the stronger the acid
 the more X=O groups,
the greater the acid strength
increasing bond length,
increasing acidity
 the larger the X is, the weaker the bond,
the stronger the acid
increasing electronegativity,
increasing acidity
H2O
HF
H2S
HCl
H2Se
HBr
H2Te
HI
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x
Practice Examples
1. Rank the following series of atoms in order of increasing electronegativity.
N O F P As
(a) N < O < F < P < As
(b) F < O < N < P < As
(c) As < P < N < O < F
(d) P < As < N < O < F
(e) F < N < O < As < P
2. Arrange the following atoms in order of increasing atomic radius.
N F Si P
(a) F < N < P < Si
(b) N < F < P < Si
(c) N < F < Si < P
(d) F < N < Si < P
(e) P < Si < N < F
3. Rank H2O(l), H2S(aq) and HF(aq) in order of their Brønsted acid strengths.
Explain your reasoning (2007-N-3)
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