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10/28/2011
Chem 105
Friday 28 Oct 2011
1)
2)
3)
4)
5)
Electron configurations of transition metal atoms
Electron configurations of ions
Trends in Periodic Table: atomic radius
Ion size
Ionization potential
6) Chap 8: Lewis formulas
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What is the electron
configuration of a
technecium atom (Tc)?
Atomic # = 43, therefore 43 e1s22s22p63s23p63d104s24p64d55s2
43
38
Check sum electrons:
2+2+6+2+6+2+10+6+5+2=43
The “spdf” notation for
electron configuration follows
the convention of writing
orbitals in order of increasing n.
(This is not necessarily the
filling order.)
36
30
20
18
10
Noble gas notation = [Kr] 4d55s2
2
3
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Box notation for technecium (Tc) atom
[Kr]
4d
5s
What is one correct set of four quantum numbers (n, l, ml, ms) for this electron?
n=4
l = 2 (it’s in a d-orbital)
ml = +2 (or +1, 0, -1, -2. There is no necessary label on each box)
ms = +1/2 (or -1/2. Up or down are arbitrary for a given single electron)
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Periodic table is organized on the basis of
which subshell contains the atom’s outermost electrons.
Table 7.3
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This is due to stabilization
offered by a half-filled or
full d-subshell.
21
22
23
24
25
26
27
28
29
30
Sc [Ar]3d14s2
Ti [Ar]3d24s2
V [Ar]3d34s2
Cr [Ar]3d54s1
Mn [Ar]3d54s2
Fe [Ar]3d64s2
Co [Ar]3d74s2
Ni [Ar]3d84s2
Cu [Ar]3d104s1
Zn [Ar]3d104s2
5
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Please learn and remember the electron configurations of
all main group atoms plus 1st row of transition metals
Exceptions
Cr
Cu
Lower d-block and f-block atoms contain more slight
inconsistencies with straight aufbau filling due to very close
orbital energies.
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The “valence shell” of an atom contains all the electrons with highest n-value.
How many electrons are in the valence shell of a silicon (Si) atom?
1
1
14
27
40
53
66
79
92
105
118
131
144
157
170
183
196
2
3
4
5
6
7
8
9
10
11
12
si
x
ur
fo
re
e
th
5.
7
2
o
4.
34
29
tw
3.
57
e
2.
one
two
three
four
six
on
1.
13
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The “valence shell” of an atom contains all the electrons with
highest n-value. How many electrons are in the valence shell of a
silicon (Si) atom?
1.
2.
3.
4.
5.
1
2
3
4
6
[Ne]3s23p2
4 electrons in Si atom have n =
3, which is the highest n-value
(of electrons in this atom).
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Electron configuration in ions
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Monatomic ion charges (review)
1+ 2+
3+
variable
2+
3-
2-
1-
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Electron configurations of main cations are
just the smaller noble gas..
Ba atom is
Ba2+ is
[Xe] 6s2
[Xe]
Electron configurations of main anions are
just the larger noble gas..
O atom is
O2- ion is
[He] 2s22p4
[Ne]
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Transition metal atoms always lose outer shell s electrons 1st.
Fe atom is
[Ar] 3d6 4s2
[Ar]
3d
Then 1 of these (to maximize spin).
Fe3+ ion is
[Ar] 3d5
4s
These go 1st.
[Ar]
3d
4s
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The apparent paradox of “filling 4s first when making atoms”, but then “removing highest nvalue first when making ions” is really due to the different effects the higher-charged
nucleus has on orbitals in the ION vs orbitals in the ATOM. For example:
Cr atom is
[Ar] 3d5 4s1
[Ar]
3d
Fe2+ ion is
4s
[Ar] 3d6
[Ar]
3d
4s
These two particles both have 24 electrons, but Fe has 2 more protons in the nucleus, which
changes the orbital energies in a way that disfavors the 4s orbital.
These different configurations can be compared computationally using quantum theory. In one
calculation, the [Ar]3d6 configuration (4 unpaired e) of Fe2+ ion is 94 kcal/mol more stable than the
[Ar]3d54s1 configuration (6 unpaired e).
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Calculated Fe2+ ion energies (gas phase)
3d
4s
Relative energy (kcal/mol)
[Ar]
123
[Ar]
[Ar]
[Ar]
64
0
94
B3LYP/6-31G*
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Measuring “magnetic susceptibility”. Paramagnetism depends on number of
unpaired electrons. This is an experimental test for # of unpaired electrons.
Fe3+ is more magnetic than Fe2+.
Fe3+ ions contain 5 unpaired electrons, whereas Fe2+ ions contain 4 unpaired
electrons.
(This ignores the possibility of
“ferromagnetism” which iron metal and
several other transition metals sometimes
achieve – and is way stronger than
paramagnetism.)
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Periodic table trends
♦ Atom sizes
♦ Ion sizes
♦ Ionization Potential
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Atomic Radius
- Measured in picometers (pm) 1 pm = 10-12 m
or Angstroms (Å)
1 Å = 100 pm = 10-8 cm
- Generally increase going down a group (down a column)
and decrease going across a period (L-to-R in a row)
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Group 1 Alkali metals
Group 8A Noble Gases
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“Atomic Radius” values depend somewhat on method used.
- “covalent radius” = half distance between bonded atoms
or
- “calculated radius” = distance out to arbitrary electron density
based on quantum mechanics calculation (Schrödinger equation)
or
- “experimental” based on crystal of metal atoms = ½
interatomic distance
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Electron distribution in H atom/H2 molecule system
0.0448 e-/Å3
0.0001 e-/Å3 contour
We define the “calculated atomic radius” =
distance from nucleus out to electron density ~ 0.05 e-/Å3
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Electron distribution in H atom/H2 molecule system
H atom
H atom radius = 44 pm
H covalent radius = 74÷
÷2=37 pm
H2 molecule
H-H dist = 74 pm
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Covalent radius is always
smaller because electrons are
pulled in by the extra
attractive force of the second
nucleus in the molecule.
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Group 1 Alkali metals
Sodium: 184 pm
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Crystal structure (experimental) of metallic sodium.
Na-Na distance = 365 pm; so, Na radius = 365/2 = 183 pm
365 pm
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Going from atom to atom DOWN a group, you add a
complete shell of electrons plus the same number of
protons in the nucleus.
e- in 1s orbital
2e- in 3s orbital
e- in 2s orbital
4+
- 2e- -
Berylium atom
(1s2 2s2)
-
12+
2e8e-
8e- in 2s,2p orbitals
Magnesium atom
(1s2 2s2 2p6 3s2)
The nuclear charge increases by 8+, but this is counteracted by
the complete inner shell of 8 electrons.
The extra shell shields the outer shell electrons from the
increased positive charge.
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The situation is different going ACROSS a row Left-to-Right.
In this case, electrons are added to the same shell - on the periphery of
the atom, and the # of inner-shell electrons is constant.
The outer-shell electrons DO NOT shield each other from the increasing
nuclear charge because they are spread out with approximately same
average distance from the nucleus.
12+
2e-
8e-
Magnesium atom
r = 145 pm
-
-
-
13+
2e8e-
-
Aluminum atom
r = 118 pm
14+
2e8e-
Silicon atom
r = 111 pm
-
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Place the following atoms in order of increasing
atomic radii: S, Se, Cl , As
Cl < S < Se < As
S < Cl < As < Se
S < Cl < Se < As
As < Se< S < Cl
Se < As < S < Cl
1
1
14
27
40
53
66
79
92
105
118
131
144
157
170
183
196
2
3
4
6
7
8
9
10
11
..
...
3
S<
<A
s<
Se
Se
<
S<
Se
<
<
Cl
12
As
<
As
<
Cl
C.
...
<.
..
...
5
S<
5.
Se
<
4.
8
7
S<
3.
18
S<
2.
Cl
<
1.
88
13
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S
As
Se
Cl
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Sizes of Ions
Cations (remember ca + ion)
always SMALLER than corresponding atom (you’re removing
electrons – usually a whole shell - without changing nuclear
charge)
Anions
Always LARGER than corresponding atom (you’re adding
electrons – to complete a shell usually – without increasing the
nuclear charge.)
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10/28/2011
Atom and Common Anion Size Comparison
These 3 have
10 e
(isoelectronic)