1 many electron atoms and the periodic table. 2 objectives v explain the scientific basis for the...
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Objectives Explain the scientific basis for the Periodic Table Apply the Aufbau principle, the Pauli Exclusion
principle and Hund’s rule to electrons in an atom Explain the concept of energy levels in an atom and
the order of filling these levels Write the electronic configuration of the first 20
elements Draw and explain a block diagram of the Periodic Table Explain the meaning and position of the transition
elements Explain the periodic variations of atomic size, ionisation
energy, electron affinity and electronegativity
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The Periodic Law
The physical and chemical properties of the elements are a function of the electronic configuration of the their atoms which vary with increasing atomic number in a periodic manner
hence the Periodic Table elements are grouped according to their electronic
structure provides information on chemical properties
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Many Electron Atoms
Describe in terms of hydrogen orbitals Same quantum numbers and shapes energies different
For Hydrogen: Depend on n
3s, 3p, 3d all the same energy
For many e- atom: Different subshells at different energies
2s < 2p depend on n and l
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Orbital Energies
Ene
rgy
1s 1s
n=1
n=3
n=2
2s 2p
3s 3p 3d
0
n=1
n=3
n=2
2s2p
3s
3p
3d
0
Ene
rgy
n=4
4s
Hydrogen Many e- Atoms
Need to consider Effect of increased nuclear charge
Repulsions between electrons
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Effective Nuclear Charge
Z
e- of interest
3 e- found in sphere
Eff. Nuclear charge = 5 - 3 = 2
Assume 5
Net positive charge from nucleus attracting an electron electron shielded by inner e-
effective nuclear chargeZeff = Z - SS = No. of e- between atom and nucleus
hence outer shell e- experience less + charge
effect of “screening” depends on e- distribution need to consider orbital shape
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Orbital shape and Energy
s-orbital e- can be close to nucleus p-orbital further away than an “s” e-
d-orbital further from nucleus than “p” e-
therefore, “s” e- has least screening by other e-
so a larger effective nuclear charge and is more tightly bound(lower energy than “p” ie. more stable)
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need to consider another property of electrons to determine how electrons populate orbitals
envisage electron as spinning on own axis quantized only 2 spin states
distinguished by the spin -magnetic quantum number ms
Electron Spin
+12
12
-
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Electron Spin
Stern-Gerlach experiment - when a beam of ground state H atoms (1s) is passed through a magnetic field, the beam splits into two beams
REMIND!
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Pauli and Hund
Pauli Exclusion Principle no two electrons in an atom can have the same
4 quantum numbers there are only 2 values of ms
hence, an orbital can only hold 2 electrons and they must have opposite spins
Hund’s Rule If orbitals have the same energy, add electrons
singly with spins parallel first
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The Aufbau Principle
Building up
fill available orbitals with available electrons starting with lowest energy orbitals (most stable)
this gives ground state
Note
don’t forget Pauli and Hund!!
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Aufbau Principle The aufbau
principle shows how orbitals are filled: in the order to the left. Two extra rules are needed.
shell 1 shell 2 shell 4shell 3
3d
1s
2s
2p
3s
3p4s
4p
4d
Increasingenergy
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Aufbau Principle-2
Hund’s rule states that when filling a set of orbitals at the same energy (sunshell), one electron is placed in each orbital before pairing occurs.
Pauli’s principle tells us that when placing a second electron in an orbital, its spin must be opposite to the electron already in the orbital. (Spin is usually represented by an arrow in an orbital box.)
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Aufbau Principle-3a “Box” or “orbital diagrams for electron configurations.
H (1e)1S
He (2e)1S
The 1s orbital is filled.Second electron is paired. (Pauli)
Start at 1s
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Aufbau Principle-3b
Li (3e)1S 2S 1s2, 2s1
C (6e)1S 2S 2p
Hund’s rule applies to p subshell
1s2, 2s2, 2p2
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Aufbau Principle-3c
In 2p subshell, Hund’s rule! Next electron follows Pauli principle
O (8e)1S 2S 2p 1s2, 2s2, 2p4
Ti (22e)(22e)
4S 3d
1s2, 2s2, 2p6, 3s2,3p6, 4s2, 3d2
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Electrons and the Periodic Table
Electron fit logically into the periodic table. The s block elements (see next slide) start filling at level 1, the p block at level 2, and the d block at level 3.
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Excited State Atoms
Occur when energy has been supplied to raise e- energy
Ne
1s22s22p6 Ground state
1s22s22p5….5s1 High energy excited state
1s22s22p5….3p1 Low energy excited state
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Transition metals
Consider the elements in the 4th period(after Ar 1s22s22p63s23p6)
after 3p natural sequence would be 3d but 4s has (slightly) lower energy than3d according to Aufbau must fill 4s before 3d
ground state for K is [Ar]4s1 and Ca is [Ar]4s2
as charge increases the energy of 3d decreases in Sc 3d < 4s Sc+ [Ar]3d14s1 Sc2+[Ar]3d1
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Transition metals
energy drop in 3d continues through to Zn
consequences for elements Cu Zn oxidation state 1+ or 2+
beyond Zn 3d electrons have no chemical role
elements from Sc to Zn called d-block elements filling up the d orbital
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Transition metals
anomalies Cr [Ar]3d54s1 expect [Ar]3d44s2
Cu [Ar]3d104s1 expect [Ar]3d94s2
similar occur in fifth period also Ru [Kr]4d75s1 expect [Kr]4d65s2
sixth period filling is erratic energies of 4f, 5d & 6s comparable
seventh period all are radioactive
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The Periodic TableOrganisation of the elements
electronic configurations related to position of element
elements grouped according to type of orbital theouter shell electrons are in
BLOCK: Named for last subshell occupied
GROUP: the columns all elements have same outer orbital e- configuration similar chemical properties
PERIODS: rows all elements same shell
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The Periodic Table Subshell
orbitals with same energy eg. 2p Shell
orbitals with similar energy eg. 2s, 2p Valence Electrons
occupy outermost shell Core Electrons
occupy filled inner shells Cl 1s22s22p6 3s23p5
Ne core valence Closed Shell Atoms
full outer shell - very stable - noble gases
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Periodic Properties
Predicted by considering e- configurations
Sizes of atoms and ions
Ionisation energies
Electron affinities
Electronegativities
Polarising powers and polarisabilities
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Sizes Of Atoms and Ions
Atoms do not have sharply defined boundaries
Hence, need to define atomic size
Atomic size depends on chemical environment
ie. Bonding etc
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This shielding means that each valence electron in effect only “feels” a +1 charge form the nucleus; this occurs for an highly excited valence electron. Otherwise the shielding makes the “seen” charge is higher than +1
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Defining Atomic and Ionic Size
2r
estimating size atomic radius = half thedistance between nearest atomsin element (in condensed phases)
for ions, base ionic radii on interatomicdistance in ionic crystals. (depends on charge...)
Cu Cu Cu+ Cu+2
atom covalent bonding
1.28 1.17 0.96 0.69 Ao
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Sizes of Atoms and Ions
Decrease
Increase
Why?Consider:1. Principle Quantum number (shell)2. Effective nuclear charge
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Increase nuclear charge
but no. of core electrons stay the same
so effective nuclear charge increases while shell remains the same
hence electrons drawn closer to nucleus
hence decrease atomic size
eg. Na 1s22s22p63s1 1.91Ao
Mg 1s22s22p63s2 1.60 Ao
Across a Period…….
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Down A Group….
More distant electron shell occupied while effective nuclear charge the same
hence atomic size increases
eg. Li 1s22s1 1.57 A0
Na 1s22s22p63s1 1.91 Ao
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Radius of Ions
Cation < Atomeg. Na+ < Na
0.96 Ao 1.91 Ao
1s22s22p6 1s22s22p63s1
lost an e-
core electrons exposed
more tightly bound
Decreases across a period eg. Na+ > Mg2+
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Radius of Ions
Atom < Anioneg. Cl < Cl
0.99 Ao 1.81 Ao
[Ne]3s23p5 [Ne]3s23p6
gained an e-
electron cloud greater
decreases nuclear pull by each electron
Decreases across a period e.g. S2- > Cl-
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Ionisation Energy
The energy required to remove an electron from a ground state atom
X(g) X+(g)
+ e- E = IE1
Measure of stability of outer shell electron configuration Depends on
size of the atom effective nuclear charge screening effect of inner electrons type of electron
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Ionisation Energy
Increase
Decrease Why?Consider1. Effective nuclear charge2. Distance of e- from nucleus
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Across a Period….
Increase in effective nuclear chargeDecrease in radius
hence increase attraction between e- and nucleus
hence increase IE
Exceptions: “p” less stable than “s” (B < Be)orbitals “singly occupied” more stable
than “doubly occupied” (O < N)
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Down a Group…...
Increase radius whileeffective nuclear charge the same
hence Decrease attraction between e- and nucleus
hence decrease IE
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Electron Affinity
The energy released when an e- added to atom to form anion
eg. F(g) + e- F-(g)
EA = 328 kJ/mol
a small EA means e- must be forced to stick measure of ability of atom to accept e-
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Electron Affinities
Same as IEWhy?Consider1. Size2. Effective Nuclear charge
Increase
Decrease
Low for Noble gases
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Electron Affinities
1. F + e- F- EA = 328.0 kJ mol-1
1s2 2s2 2p6
- stable closed shell = Ne
2. Ne + e- Ne- EA = negative
1s2 2s2 2p6 3s1
- new shell, further from nucleus- almost totally screened from nuclear charge
- so unstable
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Electron Affinities
Be 1s22s2 Low EA
Filled s subshellNext e- higher energy level
so need energy to add e-
N 1s22s22p3 Low EA
Half filled “p”Adding another e- will cause e- repulsion
hence unfavourable
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Electronegativity
The ability of an atom to draw e- to itself in a chemical bond
useful for predicting extent of chargetransfer between atoms
eg. “Covalent” “Ionic”
H—HC—HN—HNaCl
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Electronegativity
Related to EA and IE
Cs and F IE1 EA Cs low small F high large
(Cs gives up e- easily, while F accepts e- easily.)
Electron acceptorElectronegative
Electron donorElectropositive
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Electronegativity and Bond Type
Numerical scale of electronegativities developed
Paulings electronegativity scale caesium =0.79
fluorine =3.98
For two bonded atoms
(), is a measure of the bond polarity
with the more electronegative atom having more of the electron density