11/9/2011
Principles of Chemistry: A Molecular Approach, 1st Ed.
Nivaldo Tro
Chapter 8
Periodic
Properties of
the Elements
Roy Kennedy
Massachusetts Bay Community College
Wellesley Hills, MA
Tro, Principles of Chemistry: A Molecular Approach
Mendeleev
Periodic law
- when the elements are arranged in order of
increasing atomic mass, certain sets of
properties recur periodically
- put elements with similar properties in the same column
- where atomic mass order did not fit other properties, he
reordered by different properties as seen for Te and I
predicting properties
- used patterns to predict properties of undiscovered elements
based on its position on the table
- doesn’t explain why the pattern exists
- quantum mechanics is a theory that explains why the periodic
trends in the properties exist
Tro, Principles of Chemistry: A Molecular Approach
2
Electron Spin
- experiments by Stern and Gerlach showed that a beam of
silver atoms is split in two by a magnetic field.
- the experiment revealed that electrons spin on their axis and
as a result, generate a magnetic field
 Spinning charged particles generate a magnetic field
- if there is an even number of electrons, about half the atoms
will have a net magnetic field pointing “north” and the other
half will have a net magnetic field pointing “south.”
The Property of Electron Spin
- spin is a fundamental property of all electrons with all
electrons having the same amount of spin
- the orientation of the electron spin is quantized—it can only
be in one direction or its opposite often referred to spin up or
spin down
Tro, Principles of Chemistry: A Molecular Approach
3
1
11/9/2011
Quantum Numbers
spin quantum number, ms
- describes how the electron spins on its axis
- spins must cancel in an orbital (must be paired)
with one spin being spin up and the other spin down
- ms can have values of
+½ or −½
Note: - by convention, a half-arrow pointing up is used to
represent an electron in an orbital with spin up
Pauli Exclusion Principle
- No two electrons in an atom may have the same set of
four quantum numbers
- therefore, no orbital may have more than two electrons, and
they must have opposite spins
Tro, Principles of Chemistry: A Molecular Approach
4
Allowed Quantum Numbers
- knowing the number of orbitals in a subshell allows us to
determine the maximum number of electrons in the sublevel




s subshell has one orbital; therefore, it can hold 2 electrons
p subshell has three orbitals; therefore, it can hold 6 electrons
d subshell has five orbitals; therefore, it can hold 10 electrons
f subshell has seven orbitals; therefore, it can hold 14 electrons
Tro, Principles of Chemistry: A Molecular Approach
5
Electron Configurations
ground state of the electron
- the lowest energy orbital it can occupy
electron configuration
- the distribution of electrons into the various orbitals in
an atom in its ground state
He = 1s2
 The number designates the principal energy level
 The letter designates the sublevel and type of orbital
 The superscript designates the number of electrons in that sublevel
- we often represent an orbital as a square and the electrons in
that orbital as arrows.
unoccupied
orbital
Tro, Principles of Chemistry: A Molecular Approach
orbital with
1 electron
orbital with
2 electrons
6
2
11/9/2011
Sublevel Splitting in Multielectron Atoms
degenerate
- orbitals with the same energy
- this is the case with the sublevels in each principal energy
shell of hydrogen (or any single-electron system)
For multielectron atoms
- the energies of the sublevels are split due to electron–
electron repulsion
- the relative energy levels are defined by the value of the l
quantum number with the lower value of the l quantum
number, the less energy the sublevel has
s (l = 0) < p (l = 1) < d (l = 2) < f (l = 3)
Tro, Principles of Chemistry: A Molecular Approach
7
Penetrating and Shielding
Tro, Principles of Chemistry: A Molecular Approach
7s
6s
En
nergy
5s
4s
6d
6p
5f
4f
4d
4p
3d
3p
2p
1s
5d
5p
3s
2s
8
Notice the following:
1. Because of penetration, sublevels within an
energy level are not degenerate.
2. Penetration of the fourth and higher energy
levels is so strong that their s sublevels are
lower in energy than the d sublevels of the
lower energy level.
3. The energy difference between levels
becomes smaller for higher energy levels
(and can cause anomalous electron
configurations for certain elements).
Tro, Principles of Chemistry: A Molecular Approach
9
3
11/9/2011
Filling the Orbitals with Electrons
- energy shells fill from lowest energy to highest.
aufbau principle
- sublevels fill from lowest energy to highest.
» s→p→d→f
- orbitals that are in the same sublevel have the same energy
Pauli exclusion principle
- no more than two electrons per orbital
Hund’s rule
- when filling orbitals that have the same energy, place one
electron in each before completing pairs.
Tro, Principles of Chemistry: A Molecular Approach
10
Ground State Electron Configurations
electron configuration
- a listing of the subshells in order of
filling, with the number of electrons
in that subshell written as a
superscript
1s
condensed electron configuration
- shorthand way of writing an
electron configuration
- use the symbol of the previous
noble gas in brackets to represent
all the inner electrons, then just write
the last set
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
7s
Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1
Tro, Principles of Chemistry: A Molecular Approach
11
=[Kr]5s1
Valence Electrons
valence electrons
- the electrons in all the subshells with the highest principal
energy shell
core electrons
- electrons in lower energy shells
Note: - chemists have observed that one of the most important
factors in the way an atom behaves, both chemically and
physically, is the number of valence electrons.
P = 15 electrons =1s22s22p63s23p3
= [Ne]3s23p3
Tro, Principles of Chemistry: A Molecular Approach
12
4
11/9/2011
Electron Configuration and the Periodic Table
Considerations of Electron Configuration and the periodic table:
1.) the group number corresponds to the number of valence
electrons
2.) the number of columns in each “block” is the maximum
number of electrons that sublevel can hold
3 ) the period number corresponds to the principal energy
3.)
level of the valence electrons
Tro, Principles of Chemistry: A Molecular Approach
13
P = 15 electrons =1s22s22p63s23p3 = [Ne]3s23p3
Tro, Principles of Chemistry: A Molecular Approach
14
P = [Ne]3s23p3
P has five valence electrons.
1A
s1 2A
1
3A 4A 5A 6A 7A
p1
s2
p2
p3
2
3
p4
p5
8A
s2
Ne p6
3s2
d1 d2 d3 d 4 d5 d6 d7 d8
d 9 d 10
4
P
3p
p3
5
6
7
f2
f3 f4
f5 f6
Tro, Principles of Chemistry: A Molecular Approach
f7 f8
f 9 f 10 f 11 f 12 f 13 f 14 f 14 d 1
15
5
11/9/2011
Transition Elements
- for the d block metals, the principal energy level is one less
than the valence shell.
 one less than the period number
 sometimes s electron is “promoted” to d sublevel
Zn
Z = 30, period 4, group 2B
[Ar]4s23d10
4s
3d
- for the f block metals, the principal energy level is two less
than the valence shell.
 two less than the period number they really belong to
 sometimes there is a d electron in the electron configuration
Eu
Z = 63, period 6
[Xe]6s24f 7
Tro, Principles of Chemistry: A Molecular Approach
6s
4f
16
Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 = [Kr]5s1
Tro, Principles of Chemistry: A Molecular Approach
17
As = [Ar]4s23d104p3
As has five valence electrons.
8A
1A
1
2
3
4
5
6
7
3A 4A 5A 6A 7A
2A
3d10
Ar
As
4s2
4p3
Consider:
Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 = [Kr]5s1
Tro, Principles of Chemistry: A Molecular Approach
18
6
11/9/2011
Anomalous Electron Configurations
- we know that because of sublevel splitting, the 4s sublevel is
lower in energy than the 3d, and therefore the 4s fills before
the 3d
- but the difference in energy is not large
- some of the transition metals have anomalous electron
configurations in which the (n)s only partially fills before the
(n−1)d or doesn
doesn’tt fill at all
Expected
• Cr = [Ar]4s23d4
• Cu = [Ar]4s23d9
• Mo = [Kr]5s24d4
• Ru = [Kr]5s24d6
• Pd = [Kr]5s24d8
Tro, Principles of Chemistry: A Molecular Approach
Found experimentally
• Cr = [Ar]4s13d5
• Cu = [Ar]4s13d10
• Mo = [Kr]5s14d5
• Ru = [Kr]5s14d7
• Pd = [Kr]5s04d10
19
Properties and Electron Configuration
- elements in the same column have similar chemical and
physical properties because they have the same number of
valence electrons in the same kinds of orbitals.
Noble
Gases
Alkali
metalls
Halogens
Tro, Principles of Chemistry: A Molecular Approach
20
Eight Valence Electrons
Eight-valence electrons
- quantum-mechanical calculations show that an atom with
eight valence electrons should be unreactive and stable
He has two valence electrons, but that fills its valence
shell.
Elements having other than eight-valence electrons
- elements that have either one more or one less electron
should be very reactive
1.) halogen atoms
- have seven valence electrons and are the most
reactive nonmetals
2.) alkali metals
- have one more electron than a noble gas and
are the most reactive metals (as a group)
Tro, Principles of Chemistry: A Molecular Approach
21
7
11/9/2011
Properties and Electron Configuration
Noble gases
- have eight valence electrons (except for He, which has only
two electrons)
- are especially nonreactive
- nonreactive due to the electron configuration of the noble
gases is especially stable
Alkali metals
- have one more electron than the previous noble gas
- in reactions, the alkali metals tend to lose their extra
electron, resulting in the same electron configuration as a
noble gas.
 forming a cation with a 1+ charge
Tro, Principles of Chemistry: A Molecular Approach
22
Properties and Electron Configuration
Halogens
- have one fewer electron than the next noble gas
- in reactions with metals, the halogens tend to gain
an electron and attain the electron configuration of
the next noble gas forming an anion with charge 1−
- iin reactions
ti
with
ith nonmetals,
t l th
they ttend
d tto share
h
electrons with the other nonmetal so that each
attains the electron configuration of a noble gas
Tro, Principles of Chemistry: A Molecular Approach
23
Trend in Atomic Radius - Main Group
Atomic radius
- an average radius of an atom based on measuring large
numbers of elements and compounds
Methods for measuring atomic radius:
i.) van der Waals radius = nonbonding radius
ii.) covalent radius = bonding radius
Tro, Principles of Chemistry: A Molecular Approach
24
8
11/9/2011
Periodic trends in atomic radius:
i.) atomic radius increases down group
 valence shell farther from nucleus
effective nuclear charge fairly close
ii.) atomic radius decreases across period (left to right)
adding electrons to same valence shell
effective nuclear charge increases
valence shell held closer
Note:
Atomic radii of d block
metals (transition metals)
- roughly the same size
across the d block
Tro, Principles of Chemistry: A Molecular Approach
25
Shielding and Effective Nuclear Charge
- in a multielectron system, electrons are simultaneously
attracted to the nucleus and repelled by each other
- outer electrons are shielded (screening effect) from the
nucleus by the core electrons.
 Note: Outer electrons do not effectively screen for each other.
- because of this shielding, the outer electrons do not
experience
i
th
the ffullll strength
t
th off th
the nuclear
l
charge.
h
effective nuclear charge
- a net positive charge that is attracting a particular electron
Zeffective = Z − S
where Z = the nuclear charge;
S = the charge due to electrons in lower energy
levels where the trend is s > p > d > f
Tro, Principles of Chemistry: A Molecular Approach
26
Screening and Effective Nuclear Charge
Tro, Principles of Chemistry: A Molecular Approach
27
9
11/9/2011
Electron Configuration and Ion Charge
- we have seen that many metals and nonmetals form one ion,
and that the charge on that ion is predictable based on its
position on the periodic table
group 1A = 1+, group 2A = 2+,
group 7A = 1−, group 6A = 2−, etc.
- these atoms form ions that will result in an electron
configuration that is the same as the nearest noble gas
Tro, Principles of Chemistry: A Molecular Approach
28
Electron Configuration of Anions in Their
Ground State
- anions are formed when atoms gain enough electrons to have
eight valence electrons.
 filling the s and p sublevels of the valence shell
Consider S and S2-:
p63s23p
p4
S atom = 1s22s22p
= [Ne]3s23p4
(6 valence)
S2− anion = 1s22s22p63s23p6
= [Ne]3s23p6
Tro, Principles of Chemistry: A Molecular Approach
(8 valence)
29
Electron Configuration of Cations in Their
Ground State
- cations are formed when an atom loses all its valence
electrons.
 resulting in a new lower energy level valence shell
 However, the process is always endothermic
Consider Mg and Mg 2+:
Mg atom = 1s22s22p63s2 (2 valence)
= [Ar]3s2
Mg2+ cation = 1s22s22p6 (loses valence electrons)
= [Ar]
Tro, Principles of Chemistry: A Molecular Approach
30
10
11/9/2011
Electron Configuration of Cations in Their
Ground State
transition metals cations:
- electrons may also be removed from the sublevel
closest to the valence shell
Al atom = 1s22s22p63s23p1
Al3+ ion = 1s22s22p6
Fe atom =1s22s22p63s23p64s23d6
Fe2+ ion = 1s22s22p63s23p63d6
Fe3+ ion = 1s22s22p63s23p63d5
Cu atom =1s22s22p63s23p64s13d10
Cu+ ion = 1s22s22p63s23p63d10
Tro, Principles of Chemistry: A Molecular Approach
31
Magnetic Properties of
Transition Metal Atoms and Ions
paramagnetism
- electron configurations that result in unpaired electrons
mean that the atom or ion will have a net magnetic field
 will be attracted to a magnetic field
diamagnetism
- electron configurations that result in all paired electrons
mean that the atom or ion will have no magnetic
 slightly repelled by a magnetic field
Consider Zn atoms and Zn2+:
Zn atoms—[Ar]4s23d10 (diamagnetic)
Zn2+ ions—[Ar]4s03d10 (diamagnetic)
Ag forms both Ag+ ions and Ag2+ (rarely)
Ag atoms—[Kr]5s14d10 (paramagnetic)
Ag+ ions—[Kr]4d10 (diamagnetic)
Ag2+ ions—[Kr]4d9 (paramagnetic)
Tro, Principles of Chemistry: A Molecular Approach
32
Trends in Ionic Radius
- ions in same group have same charge
- ion size increases down the group due to higher valence shell
- cations are smaller than
- anions are bigger than
the neutral atom
the neutral atom
Tro, Principles of Chemistry: A Molecular Approach
33
11
11/9/2011
- larger positive charge = smaller cation
- larger negative charge = larger anion
Note: the above holds for isoelectronic species
1A
-1
+1
H
2A
+1
3A
+2
4A
+3
Li 0.68 Be 0.31 B 0.23
+1
+1
K
+2
1.33 Ca 0.99
+1
Rb 1.47 Sr
+1
+2
1.13
+2
Cs 1.69 Ba 1.35
-4
Al 0.51 Si
Na 0.97
g 0.66
0 66
0 97 Mg
0.62 +3
+1
Ga
-3
6A
7A
-3
-4
-3
0.71 +4
+2
Sn
0.95 +3
+1
Tl
0.84 +4
+2
Pb
-1
Cl 1.81
-2
As 2.22 Se 1.98
0.81 +3
+1
In
F 1.33
-2
P 2.12 S 1.84
1 84
Ge
-1
-2
N 1.71 O 1.40
C
+3
+2
5A
-4
-1
Br 1.96
-2
Sb
Te 2.21
-1
I
2.20
- cations smaller than anions
Tro, Principles of Chemistry: A Molecular Approach
Ionization Energy
ionization energy
- minimum energy needed to remove an electron from an
atom
 gas state
 endothermic process
 valence electron easiest to remove, lowest IE
First ionization energy (IE)
- energy to remove electron from neutral atom
M(g) + IE1  M+(g) + 1 e−
Second ionization energy (IE)
- energy to remove electron from 1+ ion
M+(g) + IE2  M2+(g) + 1 e−
Tro, Principles of Chemistry: A Molecular Approach
35
General Trends in First Ionization Energy
- the larger the effective nuclear charge on the electron, the
more energy it takes to remove it
- the farther the most probable distance of the electron is from
the nucleus, the less energy it takes to remove it
General trends:
1.) First IE decreases
d
down
th
the group since
i
valence electron farther
from nucleus
2.) First IE generally
increases across the
period since effective
nuclear charge increases
Tro, Principles of Chemistry: A Molecular Approach
36
12
11/9/2011
Irregularities in the Trend
- ionization energy generally increases from left to right across
a period.
 except from 2A to 3A, 5A to 6A
Which is easier to remove an electron from B or Be? Why?
Be  
Be+  
1s 2s
2p
1s 2s
2p
To ionize Be, you must break up a full sublevel; costs extra energy.
B

1s

2s
B+ 
1s

2p

2s
2p
When you ionize B, you get a full sublevel; costs less energy.
Which is easier to remove an electron from, N or O? Why?
Tro, Principles of Chemistry: A Molecular Approach
37
Trends in Successive Ionization Energies
- removal of each successive electron
costs more energy.
 shrinkage in size due to having
more protons than electrons
 outer electrons closer to the
nucleus; therefore
therefore, harder to
remove
- regular increase in energy for each
successive valence electron
Tro, Principles of Chemistry: A Molecular Approach
38
Trends in Successive Ionization Energies
- large increase in energy when start removing core electrons
Tro, Principles of Chemistry: A Molecular Approach
39
13
11/9/2011
Electron Affinity
electron affinity
- energy released when an neutral atom gains an electron
 gas state
M(g) + 1 e−  M−(g) + EA
- defined as exothermic (−), but may actually be
endothermic (+)
( )
 Some alkali earth metals and all noble gases are endothermic.
Why?
- the more energy that is released, the larger the electron
affinity of the atom
 The more negative the number, the larger the EA.
Tro, Principles of Chemistry: A Molecular Approach
40
Trends in Electron Affinity
General Trends:
1.) alkali metals have decreasing EA down the column
 but not all groups do
 generally, anomalous increase in EA from second period to third
period
2.) generally, EA increases across period
 becomes more negative from left to right
 not absolute
b l
 highest EA in period = halogen
 Group 5A generally lower EA than
expected because extra electron
must pair
 Groups 2A and 8A generally have
very low EA because added
electron goes into higher energy
level or sublevel
Tro, Principles of Chemistry: A Molecular Approach
41
Properties of Metals and Nonmetals
Metals
 malleable and ductile
 shiny, lustrous, reflect light
 conduct heat and electricity
 most oxides are basic and ionic
 form cations in solution
 lose electrons in reactions—oxidized
Nonmetals
 brittle in solid state
 dull, nonreflective solid surface
 electrical and thermal insulators
 most oxides are acidic and molecular
 form anions and polyatomic anions
 gain electrons in reactions—reduced
Tro, Principles of Chemistry: A Molecular Approach
42
14
11/9/2011
Metallic Character
Metallic character
- how closely an element's properties match the ideal properties
of a metal
 more malleable and ductile, better conductors, and easier to
ionize
General Trends in Periodic table:
1) M
1.)
Metallic
lli character
h
d
decreases
f
from
l f to right
left
i h across a
period
Metals are found at the left of the period and nonmetals
are to the right.
2.) Metallic character increases down the column
Nonmetals are found at the top of the middle main- group
elements and metals are found at the bottom.
Tro, Principles of Chemistry: A Molecular Approach
43
Tro, Principles of Chemistry: A Molecular Approach
44
15