Transcript Trends in Atomic Radii

Trends & the Periodic Table
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
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.
Use X-ray
diffraction to
pinpoint the
nuclei and
measure the
distance
between
them.
Cannot
measure the
electron cloud.
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.”
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.
But why do the atoms get smaller as
you go across the periodic table?
• You are still adding electrons!
Going across row 2:
Family
IA or 1
IIA or 2
IIIA or 13
Element
Li
Be
B
Configuration
2-1
2-2
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
Atoms actually get a bit smaller as you go across a
row. What’s going on?
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?
Well, almost . . .
• Except for H and He, the valence
electrons don’t 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.
Effective nuclear charge
• = Atomic Number - # of inner shell
electrons.
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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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.”
Row 2 elements: Effective nuclear
charge
Element
Li
Be
B
C
N
O
F
Ne
Configuration
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
Nuc. Chg
3
4
5
6
7
8
9
10
Eff. Nuc. Ch
3–2=1
4–2=2
5–2=3
6–2=4
7–2=5
8–2=6
9–2=7
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
IONS
• An ion is a charged particle formed by the
gain or loss of electrons.
– CATION has a positive charge
– ANION has a negative charge
Ionic Size Relative to Parent Atom
• Depends on if it’s a positive ion or a
negative ion
• How do you make a positive ion?
Remove electrons
• How do you make a negative ion?
Add electrons
Oxidation Number
• Oxidation Number (State) – number
assigned to keep track of electron gain or
loss in redox reactions.
– Positive number means electrons LOST
– Negative number means electrons GAINED