10/28/2011 1

Chem 105 Friday 28 Oct 2011

1) Electron configurations of transition metal atoms2) Electron configurations of ions3) Trends in Periodic Table: atomic radius4) Ion size5) 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 e-

1s22s22p63s23p63d104s24p64d55s2

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.)

Noble gas notation = [Kr] 4d55s2

2

10

18

30

20

36

38

43

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Box notation for technecium (Tc) atom

[Kr]

5s4d

What is one correct set of four quantum numbers (n, l, ml, ms) for this electron?

n = 4l = 2 (it’s in a d-orbital)m

l= +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 ofwhich subshell contains the atom’s outermost electrons.

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21 Sc [Ar]3d14s2

22 Ti [Ar]3d24s2

23 V [Ar]3d34s2

24 Cr [Ar]3d54s1

25 Mn [Ar]3d54s2

26 Fe [Ar]3d64s2

27 Co [Ar]3d74s2

28 Ni [Ar]3d84s2

29 Cu [Ar]3d104s1

30 Zn [Ar]3d104s2

This is due to stabilization offered by a half-filled or full d-subshell.

Table 7.3

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Please learn and remember the electron configurations of all main group atoms plus 1st row of transition metals

Lower d-block and f-block atoms contain more slight inconsistencies with straight aufbau filling due to very close orbital energies.

Exceptions

Cr Cu

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The “valence shell” of an atom contains all the electronswith highest n-value.

How many electrons are in the valence shell of a silicon (Si) atom?

one

two

thre

e

four

six

2

57

7

3429

1. one

2. two

3. three

4. four

5. six

1 2 3 4 5 6 7 8 9 10 11 12 13

1

14

27

40

53

66

79

92

105

118

131

144

157

170

183

196

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1. 12. 23. 34. 45. 6

[Ne]3s23p2

4 electrons in Si atom have n = 3, which is the highest n-value (of electrons in this atom).

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?

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Electron configuration in ions

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Monatomic ion charges (review)

1+ 2+ 3+ 1-2-3-

2+variable

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Electron configurations of main cations are just the smaller noble gas..

Ba atom is [Xe] 6s2

Ba2+ is [Xe]

O atom is [He] 2s22p4

O2- ion is [Ne]

Electron configurations of main anions are just the larger noble gas..

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Transition metal atoms always lose outer shell s electrons 1st.

Fe atom is [Ar] 3d6 4s2

3d 4s

Fe3+ ion is [Ar] 3d5

3d 4s

[Ar]

[Ar]

These go 1st.Then 1 of these (to maximize spin).

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The apparent paradox of “filling 4s first when making atoms”, but then “removing highest n-value 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

3d 4s

Fe2+ ion is [Ar] 3d6

3d 4s

[Ar]

[Ar]

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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3d 4s

[Ar]

[Ar]

[Ar]

[Ar]

Calculated Fe2+ ion energies (gas phase)

123

64

94

0

Relative energy (kcal/mol)

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 mor 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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0.0001 e-/Å3 contour

0.0448 e-/Å3

We define the “calculated atomic radius” = distance from nucleus out to electron density ~ 0.05 e-/Å3

Electron distribution in H atom/H2 molecule system

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H-H dist = 74 pm

H covalent radius = 74÷÷÷÷2=37 pm

H2 molecule

H atom

Electron distribution in H atom/H2 molecule system

H atom radius = 44 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.

4+2e-

--

Berylium atom(1s2 2s2)

12+2e-

--

8e-

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.

e- in 2s orbitale- in 1s orbital

2e- in 3s orbital

8e- in 2s,2p orbitals

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12+2e--

8e-

Magnesium atomr = 145 pm

- 13+2e-

-

8e-

Aluminum atomr = 118 pm

-

-

14+2e-

-

8e-

Silicon atomr = 111 pm

-

--

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.

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Place the following atoms in order of increasingatomic radii: S, Se, Cl , As

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Cl <

S <

Se

< ...

S <

Cl <

As <

...

S <

Cl <

Se <

...

As <

Se<

S < C

... S

e <

As <

S < ..

.

88

18

387

1. Cl < S < Se < As2. S < Cl < As < Se3. S < Cl < Se < As4. As < Se< S < Cl5. Se < As < S < Cl

1 2 3 4 5 6 7 8 9 10 11 12 13

1

14

27

40

53

66

79

92

105

118

131

144

157

170

183

196

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ClS

SeAs

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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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Atom and Common Anion Size Comparison

These 3 have 10 e- �

(isoelectronic)