Transcript Trends & the Periodic Table

Trends & the Periodic Table
Trends
• More than 20 properties change in
predictable way based on location of
elements in PT
• Ex: density, melting point, atomic radius,
ionization energy, electronegativity
• Atomic radius
• Ionization energy
• Electronegativity
Atomic Radius
• Atomic radius = 1/2 distance between
neighboring nuclei in molecule or crystal
• “size” varies bit from substance to
substance
Cannot measure electron cloud:
X-ray diffraction pinpoints nuclei to measure distance
Trends:
Atoms get larger as go
down a column
– ↑ principal energy levels
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
↑ energy levels as go down, so
makes sense that atoms get larger
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Li: Group 1 Period 2
Cs: Group 1 Period 6
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
You are still adding electrons – shouldn’t they get larger?! Atoms
actually get a bit smaller as you go across a row What’s going on?
What do you remember about
charge?
• opposites attract/like charges repel
• valence electrons are pulled into
atom by (+) charge of nucleus
• the greater the (+) charge, the more
pulling power
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as go L to R across row the size ↓ a bit because of
greater “proton pulling power (PPP)”
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size  as you go  &
size  as you go 
Ionization Energy
• amount energy required to remove
electron from an atom
• Ionization energy = energy required to
remove most loosely held valence electron
Trends in ionization energy
• What do you think happens to the
ionization energy as you go down a
column of the periodic table?
• As you go across a row?
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Same group:
Cs’ valence electron farther away from nucleus so electrostatic
attraction is much weaker (easier to steal electron away from
Cs)
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Same row:
•easier to steal electron from Li than Ne
•Li: less PPP than Ne
Trends in ionization energy
• Ionization energy ↓ as go down column
– easier to remove valence electron as
gets farther away (↑ # electron levels)
• Ionization energy ↑ as go across row
– it’s more difficult to remove valence
electron due to ↑ PPP
Electronegativity
• Ability of atom to attract electrons in a bond
• Noble gases do not form bonds
– are inactive
– don’t have electronegativity values
• Unit = Pauling
– Fluorine: most electronegative element (4.0 Paulings)
Trends in electronegativity
• Related to “proton pulling power (PPP)”
• ↑left to right across a row
• ↓top to bottom of a column
Electronegativity increases in direction of arrow
from Fr towards F (most electronegative element)
Reactivity of Metals
• metals are losers!
• judge reactivity of metals by how easily
they give up electrons
• most active metals: Fr (#1) and Cs (#2)
• Reactivity metals: ↑ as ionization energy ↓
Trends for Reactivity of Metals
(AKA: Metallic Character)
• Increases as go down column
–Easier to lose electrons!
• Decreases as go across row
–Harder to lose electrons!
Reactivity of Non-metals
• non-metals are winners!
• judge reactivity of non-metals by how
easily they gain electrons
• most active non-metal: fluorine
• reactivity non-metals: ↑ as electronegativity ↑
Trend for Reactivity of
Non-metals depends on PPP
• ↑ as go across row (left to right)
• ↓ as go down column (top to bottom)
– shielded by more inner-shell electrons
Ionic Size Relative to Parent Atom
• Depends on if (+) ion or (-) ion
• How do you create a positive ion?
Remove electrons
• How do you create a negative ion?
Add electrons
How do you know if an atom gains
or loses electrons?
Octet rule: magic # 8
• Metals have 1, 2, or 3 valence electrons
– easier to lose them
• Nonmetals have 5, 6, or 7 valence
electrons
– easier to gain more
• Noble gases: have 8 so don’t form ions
Positive ions or cations
• Cations always smaller than parent
atom
– lost electrons therefore smaller in size
Negative ions or anions
• Anions always larger than parent
atom
– gained electrons therefore larger in size