TOPIC: Trends in the Periodic
Table: Atomic Radius
Do Now: p.8
Trends
• more than 20 properties change in predictable
way based location of elements on PT
• some properties:
- anyone know where we can find these numbers?!
– Density
– melting point/boiling point
– atomic radius
– ionization energy
– electronegativity
When you’re
done it will
look like this so
leave room for
writing!
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
increasing # energy levels as go down
Increasing number
of energy levels
Atomic Radius
• Atomic radius: defined as ½ distance
between neighboring nuclei in molecule or
crystal
• Affected by
1. # of energy
levels
2. Proton Pulling
Power
Increasing Atomic
Radius
Increasing number of energy levels
Cs has more energy levels, so it’s bigger
previous | index | next
Li: Group 1 Period 2
Cs: Group 1 Period 6
As we go across, elements gain electrons, but they
are getting smaller!
Family
IA or 1
IIA or 2
IIIA or 13
IVA or 14
VA or 15
VIA or 16
VIIA or 17
VIIIA or 18
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
Increasing number of energy levels
Increasing Atomic Radius
Decreasing
Atomic
Radius
previous | index | next
Why does this happen..
• As you go from left to right, you again
more protons (the atomic number
increases)
• You have greater “proton pulling
power”
– Remember the nucleus is + and the electrons
are - so they get pulled towards the nucleus
• The more protons your have, the more Proton
Pulling Power
previous | index | next
as go across row size tends to decrease a bit
because of greater PPP “proton pulling power”
We can “measure” the Proton
Pulling Power by determining
the Effective nuclear charge
• It is the charge actually felt by valence electrons
• The equation
Nuclear charge - # inner shell electrons
(doesn’t include valance e-)
previous | index | next
+7
+1
Calculate “effective nuclear charge”
• # protons minus # inner electrons
What the inner electrons do….
They Shield the charge felt by the valance electrons.
previous | index | next
H and He:
only elements
whose valence
electrons feel
full nuclear
charge (pull)
NOTHING
TO
SHIELD
THEM
Increasing number of energy levels
Increasing Atomic Radius
Decreasing Atomic Radius
Increased Electron
Shielding
Look at all the shielding Francium's one valance
electron has. It barely feels the proton pull from the
nucleus. No wonder it will lose it’s one electron the
easiest. No wonder it’s the most reactive metal
Positive ions (cations)
• Formed by loss of electrons
• Cations always smaller than parent
atom
2e
8e
8e
2e
Ca
Ca
8e
8e
2e
Ca+2
Negative ions or (anions)
• Formed by gain of electrons
• Anions always larger than parent
atom