Periodic Trends

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Periodic Trends

• Still all about and charges!

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Which of the following atoms is the largest?

1. Mg
2. Ca
3. Sr
4. How the fluorine should I know?

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Which of the following atoms is the largest?

1. Mg
2. Al
3. Cl

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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.

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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!

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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?

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What does an atom look like?

• Bohr Model

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The Bohr Model
Nucleus
e-
n
p
p
n
e-
n
p
n
e-
So, what determines the size of the atom?
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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!

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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.

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What about the rows?

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

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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? ?

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Those pesky charges

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

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The Bohr Model - Na
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The Bohr Model - Mg
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The Bohr Model Na vs. Mg
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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!

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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.)

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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

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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)

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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.

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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)

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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

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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!

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Larger Ionization Energy

1. Sodium
2. Magnesium
3. I have no frigging idea

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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!

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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!

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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

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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.

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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.

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