Transcript Chapter 7 Periodic Properties of the Elements

Chapter 7
Periodic Properties
of the Elements
Periodic
Properties
of the
Elements
Periodic Trends
• In this chapter, we will rationalize observed trends in
 Atomic radii.
 Ionic radii.
 Ionization energy.
 Electronegativity.
• These trends can be explained using the concepts of:
 Coulomb’s law (we have seen this already).
 Shell model (we have seen this one too).
 Shielding effect.
 Effective nuclear charge.
Periodic
Properties
of the
Elements
Shielding Effect
• Successive shells are larger than the previous
shells.
• Electrons in successive shells spend more of
their time further away from the nucleus.
• Electrons in successive shells are partly
“shielded” from the attractive forces (of protons)
by the electrons in the previous shells.
• This shielding effect (repulsion from inner
electrons) reduces electrostatic attractions
between the outer electrons and the nucleus.
Periodic
Properties
of the
Elements
Effective Nuclear Charge
• In a many-electron atom,
electrons are both
attracted to the nucleus
and repelled by other
electrons (shielding
effect).
• The nuclear charge that
an electron experiences
depends on both factors.
Periodic
Properties
of the
Elements
Effective Nuclear Charge
The effective nuclear
charge, Zeff, is found this
way:
Zeff = Z − σ
where Z is the atomic
number and σ is a
shielding constant, usually
close to the number of
inner electrons.
Periodic
Properties
of the
Elements
Atomic radii
The bonding atomic
radius is defined as
one-half of the
distance between
covalently bonded
nuclei.
Periodic
Properties
of the
Elements
Sizes of Atoms
Bonding atomic radius tends
to…
…decrease from left to right
across a row due to an
increase Zeff that pulls
electrons closer to the
nucleus.
…increase from top to bottom
of a column due to
increasing value of n
Periodic
Properties
of the
Elements
Atomic radius in a group
• Atomic radii increase while moving down
a group, as new shells are added with
larger orbitals, increasing the shielding
effect on the outer electrons.
• A 2s orbital is much larger than a 1s
orbital.
• Increasing the principal quantum number
is like adding layers to an onion.
Periodic
Properties
of the
Elements
Atomic radius down a group
• According to Coulomb’s law, the force of
attraction between two charges decreases as
the distance between the charges (protons and
electrons) increases.
F = k q1 q2
d2
Periodic
Properties
of the
Elements
Atomic radius in a period
• Atomic radii decreases while moving left to right across
a period.
• Valence electrons are in the same shell.
• The shielding effect experienced by the valence
electrons in the same subshell is similar.(σ)
• But the effective nuclear charge increases because the
number of protons increases.
Zeff = Z − σ
• Increasing the number of protons creates a greater force
of attraction on the electrons within a subshell.
• The greater force pulls the electrons closer to the
nucleus, causing the atomic radius to decrease.
Periodic
Properties
of the
Elements
Atomic radius in a period
• According to Coulomb’s law, the force of
attraction between two charges increases as the
charges protons and electrons) increases.
F = k q1 q2
d2
Periodic
Properties
of the
Elements
Recall Ion Formation
• Metals lose electrons to form positive ions
(cations).
• Non-metals gain electrons to form negative
ions (anions).
• Hydrogen (non metal) may lose or gain. It acts
as an anion (hydride) when it gains 1 e(MgH2) and as a cation when it loses 1 e(HCl).
• Elements gain or lose electrons to acquire the
electron configuration of the noble gases.
Periodic
Properties
of the
Elements
Size of species in an Isoelectronic Series
• Isoelectronic species are atoms and ions that share the
same electronic configurations, but have different radii.
S2ClAr
K+
Ca2+
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6
(Z=16)
(Z=17)
(Z=18)
(Z=19)
(Z=20)
• As the number of protons increases (and the number of
electrons remains constant) the force of attraction on those
electrons increases reducing the radii. Coulomb’s law Periodic
Properties
of the
Elements
Sizes of Ions
• Ionic size depends
upon:
 Nuclear charge.
 Number of
electrons. (electron
repulsion)
 Orbitals in which
electrons reside.
Periodic
Properties
of the
Elements
Cations are smaller than their
parent atoms
 The outermost electrons are
removed and electron
repulsions are reduced.
 The amount of positive charge
pulling on each electron
increases.
 Many atoms lose their
outermost shell when they lose
their valence electrons to
become cations. When an atom
lose its outermost shell the
radius decreases dramatically.
Periodic
Properties
of the
Elements
Anions are larger than their
parent atoms
Electrons are added to he
same subshell.
Electron repulsions are
increased.
 This causes the electrons
to move further away
from the nucleus.
Periodic
Properties
of the
Elements
Sizes of Ions in a group
• Ions increase in size as you go
down a column.
 New shells are added with larger
orbitals.
 Valence electrons experience a
greater shielding effect.
 Coulomb’s law
 Same as atomic radius
Periodic
Properties
of the
Elements
Ionization Energy
• Amount of energy required to remove the
least tightly held electron from the ground
state of a gaseous atom or ion.
Endothermic
First ionization energy is that energy required to
remove first electron.
Second ionization energy is that energy required
to remove second electron, etc.
Ionization energy increases with increase
effective nuclear charge and decrease atomic
Periodic
Properties
radius.
of the
Elements
Recall Photoelectron Spectroscopy PES
Ionization Energies in MJ/mol obtained from PES
Element
1s
2s
2p
3s
3p
3d
Na
104
6.84
3.67
0.50
Mg
126
9.07
5.31
0.74
Al
151
12.1
7.79
1.09
0.58
Si
178
15.1
10.3
1.46
0.79
P
208
18.7
13.5
1.95
1.01
S
239
22.7
16.5
2.05
1.00
Cl
273
26.8
20.2
2.44
1.25
Ar
309
31.5
24.1
2.82
1.52
K
347
37.1
29.1
3.93
2.38
0.42
Ca
390
42.7
34.0
4.65
2.90
0.59
Sc
433
48.5
39.2
5.44
3.24
0.77
4s
0.63
Periodic
Properties
of the
Elements
Shielding effect within a shell
• The average distance between the nucleus and
electrons decreases as subsequent subshells
are added within a shell. d3s > d3p > d3d
• From Coulomb’s law we would expect that the
IE for an electron in 3d would be greater than
that for an electron in 3p.
• But PES indicates that the IE3s>IE3p>IE3d
• Due to an increase in the shielding effect.
Periodic
Properties
of the
Elements
First Ionization Energies in a group
• As one goes down a column,
less energy is required to
remove the first electron.
 Valence electrons reside in
a higher orbital, farther
from the nucleus.
 According to Coulomb’s
law the force of attraction
protons-electrons
decreases as the distance
between them increases.
Periodic
Properties
of the
Elements
First Ionization Energies in a period
• Generally, as one goes
across a period, it gets
harder to remove an
electron. (IE increases)
 As you go from left to
right, Zeff increases.
 According to
Coulomb’s law the
force of attraction
protons-electrons
increases as the # of
protons and electrons
increases.
Periodic
Properties
of the
Elements
Trends in First Ionization Energies
However, there are
apparent
discontinuities in this
trend.
Periodic
Properties
of the
Elements
Trends in First Ionization Energies
• The first occurs between Be and
B and between Mg and Al.
Electron removed from p-orbital
rather than s-orbital.
 Harder to remove an electron
from a full subshell (2s or 3s)
that it is to pull an electron
out of a subshell with only
one electron (2p or 3p)
• The second occurs between Zn
and Ga and between Cd and In.
 Harder to remove an electron
from a full subshell (3d or 4d)
that it is to pull an electron
out of a subshell with only
one electron (4p or 5p)
Periodic
Properties
of the
Elements
Trends in First Ionization Energies
• The third occurs between
Groups VA (p3) and VIA (p4).
 Electron removed comes
from doubly occupied
orbital.
 Repulsion between
electrons is reduced
when the electron is
removed helping
stabilizing the ion.
Periodic
Properties
of the
Elements
Ionization Energy
• It requires more energy to remove each successive
electron.
 The radius is reduced upon removal of an electron and
the effective nuclear charge increases.
• When all valence electrons have been removed, the
ionization energy takes a quantum leap.
 Electron configuration drops a quantum number and the
radius decreases
Periodic
Properties
of the
Elements
Electronegativity
• An element ‘s ability to attract electrons in
a chemical bond.
• Noble gases do not tend to form chemical
bonds, thus, they do not have
electronegativity values.
• It increases as the atomic radius
decreases.
• F, O, N are the most electronegative
Periodic
elements.
Properties
of the
Elements
Properties of Metal, Nonmetals,
and Metalloids
Periodic
Properties
of the
Elements
Metals versus Nonmetals
Differences between metals and nonmetals
tend to revolve around these properties.
Periodic
Properties
of the
Elements
Metals versus Nonmetals
• Metals tend to form cations.
• Transition metals form colored compounds.
• Nonmetals tend to form anions.
Periodic
Properties
of the
Elements
Metals
• Compounds formed between metals and
nonmetals tend to be ionic.
• Metal oxides tend to be basic.
CaO + H2O  Ca2+ + 2 OH-
Periodic
Properties
of the
Elements
Nonmetals
• Substances containing only nonmetals are
molecular compounds.
• Most nonmetal oxides are acidic.
SO3 + H2O  2H+ + SO42-
Periodic
Properties
of the
Elements
Metalloids
• Have some
characteristics of
metals, some of
nonmetals.
• For instance, silicon
looks shiny, but is
brittle and fairly poor
conductor.
Periodic
Properties
of the
Elements
Group Trends
Periodic
Properties
of the
Elements
Alkali Metals
• Soft, metallic solids.
• Name comes from
Arabic word for
ashes.
Periodic
Properties
of the
Elements
Alkali Metals
• Found only as compounds in nature.
• Have low densities and melting points.
• Also have low ionization energies.
Periodic
Properties
of the
Elements
Alkali Metals
Their reactions with water are famously exothermic.
See video
Periodic
Properties
of the
Elements
Alkali Metals
Flame test
Periodic
Properties
of the
Elements
Alkaline Earth Metals
• Be does not react
with water, Mg
reacts only with
steam, but others
react readily with
water.
• Reactivity tends to
increase as go down
group.
• See video
Periodic
Properties
of the
Elements
Group 6A: Calcogens
• Oxygen, sulfur, and selenium are nonmetals.
• Tellurium is a metalloid.
• The radioactive polonium is a metal.
Periodic
Properties
of the
Elements
Oxygen
• Two allotropes:
 O2
 O3, ozone
• Three anions:
 O2−, oxide
 O22−, peroxide
 O21−, superoxide
• Tends to take electrons
from other elements
(oxidation)
Periodic
Properties
of the
Elements
Sulfur
• Weaker oxidizing
agent than oxygen.
• Most stable
allotrope is S8, a
ringed molecule.
• See video.
Periodic
Properties
of the
Elements
Group VIIA: Halogens
• Some physical properties.
• Name comes from the Greek halos and
gennao: “salt formers”
Periodic
Properties
of the
Elements
Group VIIA: Halogens
• Large, negative electron affinities
 Therefore, tend to oxidize other
elements easily
• React directly with metals to form
metal halides
• Chlorine added to water supplies
to serve as disinfectant.
• Fluorine is the most reactive
• Reactivity decreases as the Periodic
Properties
atomic number increases.
of the
Elements
Group VIIIA: Noble Gases
• Astronomical ionization energies
• Positive electron affinities
 Therefore, relatively unreactive
• Monatomic gases
Periodic
Properties
of the
Elements
Group VIIIA: Noble Gases
• Xe forms three
compounds:
 XeF2
 XeF4 (at right)
 XeF6
• Kr forms only one stable
compound:
 KrF2
• The unstable HArF was
synthesized in 2000.
Periodic
Properties
of the
Elements
Periodicity and the design of new
molecules
• Since elements in the same group have similar
physical and chemical properties, the prediction
of properties of elements and compounds is
possible.
• Formulas of compounds can also be predicted.
• Example: SiO2 can be a ceramic, so SnO2 is
expected to be as well.
Periodic
Properties
of the
Elements