ch101 lecture 4
Post on 10-Apr-2015
311 Views
Preview:
TRANSCRIPT
Chemical Periodicity
OBJECTIVE:• Interpret group trends in atomic
radii, ionic radii, ionization energies, and electronegativities.
Periodic TrendsPeriodic Trends
What nuclear charge is felt by the outermost electrons of a magnesium atom?
1s2 2s2 2p6 3s2
Slater’s Rules
How much energy is needed to remove thelast electron from a magnesium atom?
1s2 2s2 2p6 3s2 → 1s2 2s2 2p6 3s1 + e-
22
422n
hnemZ2E π
−=
Slater’s Rules
1s2 2s2 2p6 3s2
blocks 85% of outside3s electron
blocks 100%of outside 3s
electron
Z* = Zeff = Z - S
= 12 - [2(1.00) + 8 (0.85) + 1 (0.35)] = 2.85
blocks 35% of theother 3s electron
Slater’s Rules
22
422n
hnemZ2E π
−=
2
21
2
2H
nZ)molMJ312.1(
nZR ⋅⋅−=⋅−= −
1s2 2s2 2p6 3s2 → 1s2 2s2 2p6 3s1 + e-
IE is the energy difference betweenthe total electron energy of these two configurations.
Slater’s Rules
1s2 2s2 2p6 3s2 → 1s2 2s2 2p6 3s1 + e-
DE = Σ (2E1f+8E2f+1E3f) - Σ (2E1i+8E2i+2E3i)
DE = 1E3f - 2E3i
= RH [-3.202/32] - 2 · RH [-2.852/32]
= 0.667 RH = 0.875 MJ/mol
The energy of inner e- areunaffected by outer e- shielding
Actual IE = 0.738 MJ/mol, 19% error
The basis of Slater’s ruless and p orbitals have better “penetration” to the nucleus than d (or f) orbitals for any given value of n
i.e. there is a greater probability of s and p electrons being near the nucleus
This means:
1. ns and np orbitals completely shield nd orbitals2. (n-1) s and p orbitals don’t completely shield n s and
p orbitals
Slater’s rules are only approximate and can give poor predictions. For example:
They ignore the differences in penetration between s and p orbitals. Real s and p orbitals do not have the same energy.
They assume that all electrons in lower shells shield outer electrons equally effectively.
Z* can be used to estimate ionization energy
Slater Calculation of Effective Nuclear Charge
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0 5 10 15 20 25
Atomic Number
Zeff
Trends in Ionization EnergyThe amount of energy required to completely remove an electron from a gaseous atom.Removing one electron makes a 1+ ion.The energy required to remove the first electron is called the first ionization energy.
Ionization EnergyThe second ionization energy is the energy required to remove the second electron.Always greater than first IE.The third IE is the energy required to remove a third electron.Greater than 1st or 2nd IE.
Symbol First Second ThirdHHeLiBeBCNOF Ne
1312 2731 520 900 800 1086 1402 1314 1681 2080
5247 7297 1757 2430 2352 2857 3391 3375 3963
11810 14840 3569 4619 4577 5301 6045 6276
Symbol First Second ThirdHHeLiBeBCNOF Ne
1312 2731 520 900 800 1086 1402 1314 1681 2080
5247 7297 1757 2430 2352 2857 3391 3375 3963
11810 14840 3569 4619 4577 5301 6045 6276
What determines IEThe greater the nuclear charge, the greater IE.Greater distance from nucleus decreases IEFilled and half-filled orbitalshave lower energy, so achieving them is easier, lower IE.Shielding effect
Group trends
As you go down a group, first IE decreases because...The electron is further away.More shielding.
Periodic trendsAll the atoms in the same period have the same energy level.Same shielding.But, increasing nuclear chargeSo IE generally increases from left to right.Exceptions at full and 1/2 full orbitals.
Firs
t Ion
izat
ion
ener
gy
Atomic number
He
He has a greater IE than H.same shielding greater nuclear charge
H
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li has lower IE than Hmore shielding further awayoutweighs greater nuclear charge
Li
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Be has higher IE than Lisame shielding greater nuclear charge
Li
Be
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
HeB has lower IE than Besame shielding greater nuclear chargeBy removing an electron we make s orbital filled
Li
Be
B
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
N
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
N
O
Breaks the pattern, because removing an electron leaves 1/2 filled p orbital
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
N
O
F
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
N
O
F
NeNe has a lower IE than HeBoth are full,Ne has more shieldingGreater distance
Firs
t Ion
izat
ion
ener
gy
Atomic number
H
He
Li
Be
B
C
N
O
F
NeNa has a lower IE than LiBoth are s1
Na has more shieldingGreater distance
Na
Firs
t Ion
izat
ion
ener
gy
Atomic number
Driving Force
Full Energy Levels require lots of energy to remove their electrons.Noble Gases have full orbitals.Atoms behave in ways to achieve noble gas configuration.
2nd Ionization Energy
For elements that reach a filled or half-filled orbital by removing 2 electrons, 2nd IE is lower than expected.True for s2
Alkaline earth metals form 2+ ions.
3rd IE
Using the same logic s2p1 atoms have an low 3rd IE.Atoms in the aluminum family form 3+ ions.2nd IE and 3rd IE are always higher than 1st IE!!!
Electron Affinity
The tendency of an atom or anion to pick upexcess electron density is a measure of theelectron affinity of that atom.
F(g) + e– F– (g)
Electron AffinityAtoms which need a few electrons to attainan outer shell inert gas electronic configuration have high electron affinity.
Electron AffinityAtoms which tend to lose electrons to attainouter shell inert gas electronic configurationhave low electron affinity.
Trends in Electron AffinityEasiest to add to group 7A.
Gets them to full energy level.
Increase from left to right: atoms become smaller, with greater nuclear charge.
Decrease as we go down a group.
Trends in Atomic SizeFirst problem: Where do you start measuring from?The electron cloud doesn’t have a definite edge.They get around this by measuring more than 1 atom at a time.
Atomic Size
Atomic Radius = half the distance between two nuclei of a diatomic molecule.
Radius
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 in
closer.3. Shielding effect
Group trendsAs we go down a group...each atom has another energy level,so the atoms get bigger.
HLi
Na
K
Rb
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
Overall
Atomic Number
Ato
mic
Rad
ius
(nm
)
H
Li
Ne
Ar
10
Na
K
Kr
Rb
Trends in Ionic SizeCations form by losing electrons.Cations are smaller than the atom they come from.Metals form cations.Cations of representative elements have noble gas configuration.
Ionic sizeAnions form by gainingelectrons.Anions are bigger that the atom they come from.Nonmetals form anions.Anions of representative elements have noble gas configuration.
Configuration of IonsIons always have noble gas configuration.Na is: 1s22s22p63s1
Forms a 1+ ion: 1s22s22p6
Same configuration as neon.Metals form ions with the configuration of the noble gas before them - they lose electrons.
Configuration of Ions
Non-metals form ions by gaining electrons to achieve noble gas configuration.They end up with the configuration of the noble gas after them.
Periodic TrendsAcross the period, nuclear charge increases so they get smaller.Energy level changes between anions and cations.
Li1+
Be2+
B3+
C4+
N3-O2- F1-
Group trendsAdding energy levelIons get bigger as you go down.
Li1+
Na1+
K1+
Rb1+
Cs1+
Size of Isoelectronic ionsIso- means the sameIso electronic ions have the same # of electronsAl3+ Mg2+ Na1+ Ne F1- O2- and N3-
all have 10 electronsall have the configuration: 1s22s22p6
Size of Isoelectronic ionsPositive ions that have more protons would be smaller.
Al3+
Mg2+
Na1+ Ne F1- O2- N3-
ElectronegativityThe tendency for an atom to attract electrons to itself when it ischemically combined with another element.How fair is the sharing?Big electronegativity means it pulls the electron toward it.Atoms with large negative electron affinity have larger electronegativity.
Group Trend
The further down a group, the farther the electron is away, and the more electrons an atom has.More willing to share.Low electronegativity.
Periodic TrendMetals are at the left of the table.They let their electrons go easilyLow electronegativityAt the right end are the nonmetals.They want more electrons.Try to take them away from othersHigh electronegativity.
Ionization energy, Electronegativity, and Electron Affinity INCREASE
Atomic size increases
Ionic size increases
107:Bh 108: Hs 109: Mt 110: Ds107:Bh 108: Hs 109: Mt 110: Ds
top related