Trends

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Trends the Periodic Table
Atomic size Effective Nuclear Charge
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Source Conceptual Chemistry by John Suchocki

What the periodic table really looks like!
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Source Conceptual Chemistry by John Suchocki

Space-saver.
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How is the periodic table arranged?

• By Atomic Number!

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Trends

• More than 20 properties change in a predictable
  way based on location of element in P.T.
• Include density, melting point, atomic radius,
  ionization energy, electronegativity

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Atomic Radius

• Atomic radius defined as half the distance
  between neighboring nuclei in a molecule or
  crystal.
• size varies a bit from substance to substance.

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Use X-ray diffraction to pinpoint the nuclei and
measure the distance between them. Cannot
measure the electron cloud.
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Trends Atoms get larger as you go down a column
more principal energy levels Atoms get smaller
as you move across a series more proton
pulling power.
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Going down column 1
Period Element Configuration
1 H 1
2 Li 2-1
3 Na 2-8-1
4 K 2-8-8-1
5 Rb 2-8-18-8-1
6 Cs 2-8-18-18-8-1
7 Fr 2-8-18-32-18-8-1
More principal energy levels as you go down, so
it makes sense that the atoms get larger.
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Source Conceptual Chemistry by John Suchocki

Li Group 1 Period 2. Cs Group 1 Period 6.
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But why do the atoms get smaller as you go across
the periodic table?

• You are still adding electrons!

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Going across row 2
Family Element Configuration
IA or 1 Li 2-1
IIA or 2 Be 2-2
IIIA or 13 B 2-3
IVA or 14 C 2-4
VA or 15 N 2-5
VIA or 16 O 2-6
VIIA or 17 F 2-7
VIIIA or 18 Ne 2-8
Biggest
Atoms actually get a bit smaller as you go across
a row. Whats going on?
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What do you remember about charge?

• Opposites attract and like charges repel.
• The valence electrons are pulled into the atom by
  the positive charge on the nucleus.
• The greater the positive charge, the more pulling
  power, right?

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Well, almost . . .

• Except for H and He, the valence electrons dont
  feel the full effect of all the protons in the
  nucleus.
• For most atoms, the inner shell electrons
  shield the valence electrons from the nucleus.

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Effective nuclear charge

• Charge actually felt by valence electrons.
• Atomic Number - of inner shell electrons.
• Not usually the same as the nuclear charge or
  number of protons in the nucleus.
• Charge felt by valence electrons is attenuated or
  shielded by inner shell electrons.

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Effective nuclear charge

• Atomic Number - of inner shell electrons.

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H and He are the only elements whose valence
electrons feel the full nuclear charge.

Source Conceptual Chemistry by John Suchocki
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Effective nuclear charge 1.
Source Conceptual Chemistry by John Suchocki
The valence electron in Li feels an effective
nuclear charge of 3 2 1.

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Source Conceptual Chemistry by John Suchocki

Calculating effective nuclear charge.
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Row 2 elements Effective nuclear charge
Element Configuration Nuc. Chg Eff. Nuc. Ch
Li 2-1 3 3 2 1
Be 2-2 4 4 2 2
B 2-3 5 5 2 3
C 2-4 6 6 2 4
N 2-5 7 7 2 5
O 2-6 8 8 2 6
F 2-7 9 9 2 7
Ne 2-8 10 10 2 8
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So as you go across a row the size tends to
decrease a bit because of greater proton pulling
power.

Source Conceptual Chemistry by John Suchocki
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Size ? as you go ? and size ? as you go ?

Source Conceptual Chemistry by John Suchocki