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Periodic Trends

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Periodic Trends Still all about + and charges! Electron Affinity Electron Affinity is complementary to ionization energy. Rather than give up an electron, the ... – PowerPoint PPT presentation

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Title: Periodic Trends


1
Periodic Trends
  • Still all about and charges!

2
Which of the following atoms is the largest?
  1. Mg
  2. Ca
  3. Sr
  4. How the fluorine should I know?

3
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4
Which of the following atoms is the largest?
  1. Mg
  2. Al
  3. Cl

5
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6
Periodic for a Reason
  • It is a periodic table because of the periodic
    trends that make it up.
  • We saw that the electron configurations are
    periodic (s-block, p-block, d-block, etc.) and I
    suggested that the Chemistry associated with the
    atoms followed the electron configuration.

7
Other Periodic Properties
  • Electron configuration isnt the only periodic
    trend in the table.
  • The other important trends are all rationalizable
    based on the most important trend atomic size!

8
Atomic Size
  • Suppose we want to compare the atomic radius of
    two atoms on the periodic table, for example Na
    and K.
  • Which would you think is bigger?

9
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10
What does an atom look like?
  • Bohr Model

11
The Bohr Model
Nucleus
e-
n
p
p
n
e-
n
p
n
e-
So, what determines the size of the atom?
12
The size is determined by
  • where the last electron lies.
  • So, which is bigger Na or K?
  • K it is Na with a whole extra shell of
    electrons!

13
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14
Na vs. K
  • Na 1s22s22p63s1
  • K 1s22s22p63s23p64s1
  • This is a general trend. As you go down a column
    in the periodic table, the atomic radius
    increases.

15
What about the rows?
  • Suppose I ask the same question about Na and Mg
    which is larger?

16
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17
Na vs. Mg
  • Na 1s22s22p63s1
  • Mg 1s22s22p63s2
  • Does this help us any?
  • They have the same valence shell (n3). The same
    highest orbital (3s).
  • Does this mean they are the same size? ?

18
Those pesky charges
  • What are the differences between Na and Mg?
  • One extra electron
  • One extra proton
  • Does that help?

19
The Bohr Model - Na
20
The Bohr Model - Mg
21
The Bohr Model Na vs. Mg
22
Na vs. Mg
  • Same outer shell of the electrons (- charge).
  • More charges in nucleus.
  • What do you think happens?
  • Mg is actually slightly smaller than Na due to
    the extra charge in the nucleus pulling the
    electrons in closer!

23
Periodic Trend atomic radius
  • This pair of observations describes the general
    trend of atomic size
  • Larger as you go down a column (large effect)
  • Smaller as you go across the row (small effect)
  • Note There are exceptions due to special cases
    (1/2 full orbitals, etc.)

24
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25
Rank from Largest to smallest
  1. Sn, In, Ga, As
  2. In Sn As Ga
  3. In Sn Ga As
  4. In Ga Sn As

26
Periodic Trend atomic radius
  • Both of these trends are related to the charged
    species
  • Larger as you go down a column (more electron
    shells adding layers)
  • Smaller as you go across the row (stronger
    attraction between more protons and the outer
    electrons)

27
Other Trends
  • Understanding the trend in atomic radius and
    keeping the charge issues in mind make it easy to
    understand and predict some other periodic trends.

28
Ionization Energy
  • Ionization energy is the amount of energy
    required to remove an electron from an atom
  • Na energy ? Na e-
  • (You simply raise the electron from nvalence to
    n8)

29
Ionization Energy
  • Na energy ? Na e-
  • If you are going to remove an electron, what is
    the relevant issue?
  • Charge what a surprise!
  • What Charge?
  • The nuclear charge! - At least in part

30
Na vs. Mg
  • Compare the Ionization Energy of Na to that of
    Mg.
  • Which do you think would be larger?
  • Na or Mg
  • Why?
  • They have the same outer orbital, but Mg has a
    larger nuclear charge (sound familiar) Mg
    should have the larger ionization energy!

31
Larger Ionization Energy
  1. Sodium
  2. Magnesium
  3. I have no frigging idea

32
Na vs. K
  • Compare Na to K, what is relevant?
  • K is bigger than Na why?
  • Because K has more shells and larger radius.
    Electrons are farther from nucleus.
  • Same argument for ionization energy. Electron,
    farther away, less attracted to nucleus, smaller
    ionization energy!

33
Ionization Energy vs. Atomic Radius
  • It is the same arguments, with the same results.
  • Ionization energy has a trend that tracks the
    radius. Smaller atoms, larger ionization energy!

34
Electron Affinity
  • Electron Affinity is complementary to ionization
    energy. Rather than give up an electron, the
    atom receives it
  • Na e- ? Na-
  • What will determine if an atom wants an electron?
  • Attraction for the nucleus so electron affinity
    will also track atomic radius

35
Electron Affinity
  • Na vs. K
  • K will have a smaller electron affinity K is
    larger, a new electron is farther away, less
    attracted to nucleus
  • Na vs. Mg
  • Na will have a smaller electron affinity same
    radius, smaller charge, less attraction for the
    electron.

36
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37
Electronegativity
  • Electronegativity is the ability of an atom to
    attract electrons to itself. (Kind of like
    electron affinity, but on a different scale)
  • Electronegativity is important in predicting
    whether a bond is ionic or covalent.
  • Electronegativity will have the same trend as
    electron affinity.

38
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