Lecture 5.2 - Periodic Properties 2
Transcript of Lecture 5.2 - Periodic Properties 2
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Periodic Properties of the
Elements
Part 2
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Ionization Energy (continued)
1st I.E. = I1
2nd I.E. = I2
3rd I.E. = I3
( ) ( )energy X Xg g e+
+ +
2( ) ( )energy X Xg g e+ +
+ +
2 3( ) ( )energy X Xg g e
+ + + +
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When e- is removed from an atom, therepulsion among remaining e- s
decreases
More energy is required to removesubsequent e-
I1 < I2 < I3 <
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I1 = 580 kJmol-1
I2 = 1817 kJmol-1
I3 = 2740 kJmol-1
( ) ( )Al Alg g e+
+
2
( ) ( )Al Alg g e+ +
+
2 3
( ) ( )Al Alg g e+ +
+
Why?
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Al
Zeff = 13 12 = +1
1+
I1 = 580 kJmol-1
Zeff = 13 10 = +3
I2 = 1817 kJmol-1
2+3+
I3 = 2740 kJmol-1
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Al 1+ Zeff = 13 10 = +3
I2 = 1817 kJmol-1
I3 = 2740 kJmol-1
Why is I3 > I2even though Zeff is same?
e- pair repulsion
2nd e- is easier to remove
2+
I2
= 1817 kJmol-1
3rd e- no longerfeels repulsion
3+
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Al 3+
Zeff = 13 4 = +9Al 3+
I4 = 11,575 kJmol-1
4+
I4 >> I3because e- is removed
from lower energy subshell
What about I4?
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Higher ionization energies Ionization energy
always increasesas you pull offmore electrons.
Ionization energiestake a huge leapwhen we try toremove an electron
from a new innersubshell.
Zeff increases dramatically at each lower subshell,
thus more energy is required to overcome the
attractive force between e- and nucleus
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Higher ionization energies
A: Al3+ has a stable e- configuration andthus more energy (I4) is required to remove
the fourth e-
FAIL
Q: For Al whyis I
4
> I3
?
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Account for
differencesin I1, I2, & I3
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Exception 1: N vs O and P vs S
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Trend: I1 N < I1 O
Reality: I1 N > I1 O
Why?
e- pair repulsion - Less energy required toremove 1st e-
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Without e- repulsion, I2 obeys periodic trend
I2 N I2 O I1 S and I2 P < I2 S for same reason
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Exception 2: Be vs B and Mg vs Al
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Zeff = 4 2 = +2
Zeff = 5 4 = +1
Lower Zeff Lower I1
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Electron Affinity (E.A.) The energy change that occurs when an electron is added to an
atom (or ion) in the gaseous state. X(g) + e- X-(g) kJmol-1
Frequently costs nothing but actually yields energy therefore EAsare usually exothermic
H < 0 because X(g) is less stable (high PE) than X-(g) (lower PE)
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O(g) + e- O-(g)
unstable(high PE)
Less unstable + Energy(lower PE)
unstable stable + Energy
F(g) + e- F-(g)
Thus E.A. F > E.A. O
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E.A. increases (more - )
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Electronegativity (E.N.)
The ability of an atom to attract pairs ofelectrons toward itself in a covalent bond
A property of an atom in a molecule
takes into account atomic radius, mass,
electron affinity
Electron greed; the higher the E.N. the
more greedy an atom is
Atom with higher E.N. will pull shared e-
(electron density) closer to itself
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First proposed by Linus Pauling in 1932asa development ofvalence bond theory
Pauling Scale of E.N.ranges from 0.7 (Fr) to 4.0 (F)
E.N. differences determine
bond polarity
In a molecular formula, the
element with lowest E.N. is
written first; element with
highest E.N. is written last
(except H in acids)
http://upload.wikimedia.org/wikipedia/commons/d/db/Pauling.jpg -
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E.N. increases
Pauling Scale Values for Electronegativity
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Zeff, I.E., E.A., E.N. increases
A.R. , I.R.
decreases
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Break
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