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Periodic Trends
OBJECTIVES:
Interpret group trends in atomic
radii, ionic radii, ionization
energies, m.p., b.p.,
electronegativity and chemical
properties
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Trends in Atomic Size
First problem: Where do youstart measuring from?
The electron cloud doesnt havea definite edge.
They get around this by
measuring more than 1 atom at atime.
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Atomic Size
Atomic Radius = half the distance betweentwo nuclei of a diatomic molecule.
}
Radius
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Trends in Atomic Size
Influenced by three factors:1. Energy Level
Higher energy level is further
away.
2. Charge on nucleus
More charge pulls electrons incloser.
3. Shielding effect e e repulsion
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Group trends
As we go downa group...
each atom hasanother energylevel,
so the atomsget bigger.
HLi
Na
K
Rb
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Periodic TrendsAs you go across a period, the
radius gets smaller.
Electrons are in same energy level.
More nuclear charge.Outermost electrons are closer.
Na Mg Al Si P S Cl Ar
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Overall
Atomic Number
AtomicRadius(nm)
H
Li
Ne
Ar
10
Na
K
Kr
Rb
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Trends in Ionization Energy
The amount of energy requiredto completely remove a mole ofelectrons from a mole of
gaseous atoms.Removing an electron makes a
+1 ion.
The energy required to remove (1mole of) the first electron iscalled the first ionization energy.
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Ionization Energy
The second ionization energy isthe energy required to remove (1mole of) the second electron(s).
Always greater than first IE.
The third IE is the energy
required to remove a thirdelectron.
Greater than 1st or 2nd IE.
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Symbol First Second Third
HHe
Li
BeB
C
N
OF
Ne
13122731
520
900
800
1086
1402
13141681
2080
5247
7297
1757
2430
2352
2857
33913375
3963
11810
14840
3569
4619
4577
53016045
6276
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Symbol First Second Third
HHe
Li
BeB
C
N
OF
Ne
13122731
520
900
800
1086
1402
13141681
2080
5247
7297
1757
2430
2352
2857
33913375
3963
11810
14840
3569
4619
4577
53016045
6276
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What determines IE
The greater the nuclear charge,the greater IE.
Greater distance from nucleusdecreases IE
Filled and half-filled orbitals have
lower energy, so achieving themis easier, lower IE.
Shielding effect
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ShieldingThe electron in the
outermost energylevel experiences
more inter-electronrepulsion (shielding).
Second electron has
same shielding, if itis in the same period
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Periodic trends
All the atoms in the same periodhave the same energy level.
Same shielding.But, increasing nuclear charge
So IE generally increases from
left to right.Exceptions at full and 1/2 full
orbitals.
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FirstIonizationenergy
Atomic number
He
He has a greater IEthan H.
same shielding
greater nuclearcharge
H
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FirstIonizationenergy
Atomic number
H
He
Li has lower IEthan H
Outer electron
further away outweighs greater
nuclear charge
Li
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FirstIonizationenergy
Atomic number
H
He
Be has higher IEthan Li
same shielding
greater nuclearcharge
Li
Be
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FirstIonizationenergy
Atomic number
H
HeB has lower IE
than Be
same shielding
greater nuclearcharge
p orbital is slightly
more diffuse and itselectron easier to
remove
Li
Be
B
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FirstIonizationenergy
Atomic number
H
He
Li
Be
B
C
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FirstIonizationenergy
Atomic number
H
He
Li
Be
B
C
N
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FirstIonizationenergy
Atomic number
H
He
Li
Be
B
C
N
O
Breaks thepattern, becausethe outer electronis paired in a p
orbital andexperiences inter-electron
repulsion.
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FirstIonizationenergy
Atomic number
H
He
Li
Be
B
C
N
O
F
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FirstIonizationenergy
Atomic number
H
He
Li
Be
B
C
N
O
F
Ne
Ne has a lower
IE than He
Both are full,
Ne has moreshielding
Greater
distance
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FirstIoniza
tionenergy
Atomic number
H
He
Li
Be
B
C
N
O
F
Ne
Na has a lower
IE than Li
Both are s1
Na has moreshielding
Greater
distanceNa
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FirstIoniza
tionenergy
Atomic number
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Driving Force
Full Energy Levels require lots ofenergy to remove their electrons.
Noble Gases have full orbitals.
Atoms behave in ways toachieve noble gas configuration.
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Trends in Electron Affinity
The energy change associated withadding an electron (mole ofelectrons) to a (mole of) gaseousatom(s).
Easiest to add to group 7A.
Gets them to full energy level.
Increase from left to right: atomsbecome smaller, with greater nuclearcharge.
Decrease as we go down a group.
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Ionic size
Anions form by gainingelectrons.
Anions are bigger that the atomthey come from.
Nonmetals form anions.
Anions of A groups elementshave noble gas configuration.
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Configuration of Ions
Ions have noble gas configurations(not transition metals).
Na is: 1s22s22p63s1
Forms a 1+ ion: 1s22s22p6
Same configuration as neon.
Metals form ions with theconfiguration of the noble gasbefore them - they lose electrons.
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Group trends
Adding energy level
Ions get bigger asyou go down.
Li1+
Na1+
K1+
Rb1+
Cs1+
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Periodic Trends
Across the period, nuclearcharge increases so they getsmaller.
Energy level changes betweenanions and cations.
Li1+
Be2+
B3+
C4+
N3-
O2- F1-
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Size of Isoelectronic ions
Iso- means the same
Iso electronic ions have thesame # of electrons
Al3+ Mg2+ Na1+ Ne F1- O2- and N3-
all have 10 electrons
all have the configuration:1s22s22p6
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Size of Isoelectronic ions
Positive ions that have moreprotons would be smaller.
Al3+
Mg2+
Na1+Ne F
1- O2- N
3-
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Group Trend
The further down a group, thefarther the electron is away, andthe more electrons an atom has.
More willing to share.
Low electronegativity.
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Ionization energy, Electronegativity,
and Electron Affinity INCREASE
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Atomic size increases,
shielding constant
Ionic size increases
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