Download Effective Nuclear charge

Effective nuclear charge
Effective Nuclear charge
H
• Modern periodic table: arrange elements in order of
increasing atomic number.
1 proton
•Effective nuclear charge (Zeff):
actual charge _________________________
•Main concept of Ch. 7
1 e–
•The Zeff ≠ charge on nucleus
due to _____________________________.
Effective Nuclear charge
In H atom, e– attracted by 1 proton
Effective Nuclear charge
Li
He
Z=3
In He atom, each e– attracted by 2 protons
Effective Nuclear Charge
In Li atom, valence e– attracted by 3 protons, but
repelled by e– sometimes.
What does cross section of e– distribution look like
(probability of finding e– for 3s orbital)?
• Zeff is approx. since orbitals have differing shapes,
sizes and nodes.
• What is full e– configuration for Mg?
1s22s22p63s2
What are the valence electrons?
which ones are closer to nucleus?
Probability
(e– density)
Distance from nucleus
1
Calculate Zeff for:
Z - S = Zeff
Mg
11-10 ∼ 1
Cl
K
Ca
Br
Rb
Sr
I
Na
Zeff ∼ Z – S
Effective Nuclear Charge
summary
Z = atomic number
S = core e–
Sizes of Atoms and Ions
• Outer e– attracted to nucleus,
• core (inner) e– screen valence (outer) e– from the
nuclear charge.
• .
• Distance between two
nuclei is bond distance.
How does Zeff help predict radii of atoms?
Which of the following atoms is larger?
Na
Z - S = Zeff
Mg
Cl
atomic radius (nm)
• Bonding radius is
estimated from known
structures of molecules.
0.17
0.15
0.13
0.11
0.09
0
2
4
Zeff
6
8
HINT: think Zeff , sizes of valence orbitals, # of protons
2
Atomic radius trends
Sizes of Atoms and Ions
Atomic size varies in periodic table.
• Down a group, atoms become _______________
• Across a period, atoms become _______________
Fig 7.6
Trends in the Sizes of Ions
•Cations lose e– from largest orbitals;
___________________ than parent atom.
• Ion size is distance between ions in ionic solid.
• Ion size depends on Zeff, n of valence e– .
and # of e–
•Anions gain e– to largest orbital
___________________ than parent atom.
• Which is bigger, Na or Na+?
• Which is bigger, Cl or Cl–?
Trends in the Sizes of Ions
• Isoelectronic: atoms that have same e– config
He and Li+
• Which is bigger in this iso-electronic series?
O2- F- Na+ Mg2+ Al3+
1.18
• :
Fig 7.7
3
According to electron configurations, from which orbitals are
electrons lost?
(hint: think e– config)
Ionization energy
Li:
• 1st ionization energy, IE, is E to remove 1 e– from
1 atom in gas phase:
Na(g) → Na+(g) + e--
Be:
Element
1st
electron
2nd
electron
Li
• The 2nd IE, is the E is for 2nd e–
Na+(g) → Na2+(g) + e--
B:
3rd
electron
4th
electron
X
X
Be
X
B
• Larger IE = harder to remove
e–.
According to e– configurations, from which orbitals are e– lost?
Li: 1s22s1
Element
Li
Be
Be:
1s22s2
1st
electron
2nd
electron
B: 1s22s22p1
X(g) → X+(g) + e−
Ionization energy
Energies in kJ/mol to remove electrons
Using what you know, talk with your
neighbor to explain the data below:
3rd
electron
4th
electron
Element
X
X
Li
X
Be
B
1st
electron
2nd
electron
3rd
electron
4th
electron
B
Ionization Energy
• Note HUGE increase in IE when a core e– is removed.
Fig 7.10
Table 7.2
4
Electron Affinities
• Electron affinity: opposite of ionization energy.
∆E when an atom(g) gains an e– to form anion (g) :
Cl(g) + e- → Cl– (g) EA = –349 kJ/mol
• EA can either be exo or endothermic:
Ar(g) + e- → Ar–(g) EA = 35 kJ/mol
Fig 7.11
Electron Affinities
+ EA,
Hi IE,
No rxns
Low IE, low –EA,
Lose e–
Hi –EA, Hi IE,
Gain e–
5