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AP Chemistry
Unit 3 - Elements
Lesson 12 – Periodic Trends
Book Section: 7.2-7.5, 8.3
Effective Nuclear Charge


In a many-electron
atom, electrons are
both attracted to the
nucleus and repelled
by other electrons.
The nuclear charge
that an electron
experiences depends
on both factors.
Effective Nuclear Charge

The effective nuclear
charge, Zeff, is found this
way:


Zeff = Z – S
Where Z is the atomic
number and S is a
screening constant,
usually close to the
number of inner electrons.
(Assume this, don’t count
valence electrons)
What Is the Size of an Atom?


The bonding atomic
radius is defined as
one-half of the
distance between
bonded nuclei.
This is how we
measure atomic
radius.
Sizes of Atoms

Atomic radius tends
to…


…decrease from left to
right across a row
 due to increasing Zeff
…increase from top to
bottom of a column
 due to increasing
value of n
1984 MC #43

A)
B)
C)
D)
E)
The elements in which of the following have most
nearly nearly the same atomic radius?
Be, B, C, N
Ne, Ar, Kr, Xe
Mg, Ca, Sr, Ba
C, P, S, I
Cr, Mn, Fe, Co
1984 MC #43

A)
B)
C)
D)
E)
The elements in which of the following have most
nearly nearly the same atomic radius?
Be, B, C, N
Ne, Ar, Kr, Xe
Mg, Ca, Sr, Ba
C, P, S, I
Cr, Mn, Fe, Co – 48% correct, medium
1999 MC #50

A)
B)
C)
D)
E)
In the periodic table, as the atomic number
increases from 11 to 17, what happens to the
atomic radius?
It remains constant.
It increases only.
It increases, then decreases.
It decreases only.
It decreases, then increases.
1999 MC #50

A)
B)
C)
D)
E)
In the periodic table, as the atomic number
increases from 11 to 17, what happens to the
atomic radius?
It remains constant.
It increases only.
It increases, then decreases.
It decreases only. – 62% correct, easy
It decreases, then increases.
Sizes of Ions

Ionic size depends
upon:



The nuclear charge
The number of
electrons
The orbitals in which
electrons reside
Sizes of Ions

Cations are smaller
than their parent
atoms.

The outermost electron
is removed and
repulsions between
electrons are reduced.
Sizes of Ions

Anions are larger than
their parent atoms.

Electrons are added
and repulsions between
electrons are increased.
Sizes of Ions

Ions increase in size
as you go down a
column.

This is due to
increasing value of n.
Sizes of Ions


In an isoelectronic series, ions have the same
number of electrons.
Ionic size decreases with an increasing nuclear
charge.
Ionization Energy

The ionization energy (I) is the amount of energy
required to remove an electron from the ground
state of a gaseous atom or ion.


The first ionization energy (I1) is the energy required to
remove the first electron.
The second ionization energy (I2) is the energy required
to remove the second electron, etc…
Ionization Energy


It requires more energy to remove each
successive electron.
When all valence electrons have been removed,
the ionization energy takes a quantum leap.
Trends in First Ionization
Energies

As one goes down a
column, less energy is
required to remove the
first electron.

For atoms in the same
group, Zeff is essentially
the same, but the
valence electrons are
farther from the nucleus
(Coulomb’s Law –
physics)
Trends in First Ionization
Energies

Generally, as one
goes across a row, it
gets harder to remove
an electron.

As you go from left to
right, Zeff increases.
Trends in First Ionization
Energies

However, there are
two apparent
discontinuities in this
trend.
Trends in First Ionization
Energies


The first occurs
between groups IIA
and IIIA.
In this case a full
s-orbital is preferred,
and it is harder to
remove the electron
from IIA than from IIIA.
Trends in First Ionization
Energies

The second occurs
between groups VA
and VIA.

In this case, half-filled
p orbitals are especially
stable.
1999 MC #37
Ionization Energies for Element X (kJ mol-1)
First
Second
Third
Fourth
Fifth
580
1,815
2,740
11,600
14,800
The ionization energies for element X are listed in the table
above. On the basis of the data, element X is most likely to
be
A) Na
B) Mg
C) Al
D) Si
E) P
1999 MC #37
Ionization Energies for Element X (kJ mol-1)
First
Second
Third
Fourth
Fifth
580
1,815
2,740
11,600
14,800
The ionization energies for element X are listed in the table
above. On the basis of the data, element X is most likely to
be
A) Na
B) Mg
C) Al – 35% correct, hard
D) Si
E) P
Electron Affinity


Electron affinity is the energy change
accompanying the addition of an electron to a
gaseous atom:
Cl + e-  Cl-
Trends in Electron Affinity

In general, electron
affinity becomes
larger as you go
left to right across a
row.
Trends in Electron Affinity

There are, again,
however, two
discontinuities in
this trend.
Trends in Electron Affinity


The first is between
IA and IIA.
IIA already has a
full s-orbital, so
adding another
electron to the psublevel is not
favorable.
Trends in Electron Affinity


The first is between
IVA and VA.
IVA already has a
half-full psublevel, so
adding another
electron is not
favorable.
Electronegativity


Electronegativity is
the ability of atoms to
attract electrons to
themselves.
On the periodic table,
electronegativity
increases as you
go…


From left to right
across a row
From the bottom to the
top of a column.
1989 MC #1, 3
O
B) La
C) Rb
D) Mg
E) N
1) What is the most electronegative element?
3) Which of the elements above has the smallest ionic
radius for its most commonly found ion?
A)
1989 MC #1, 3
O
B) La
C) Rb
D) Mg
E) N
1) What is the most electronegative element? A, 81%
correct, very easy
3) Which of the elements above has the smallest ionic
radius for its most commonly found ion? D, 33%
correct, hard
A)
HW: 7.16, 19, 22, 24, 26, 28, 36,
42, 44, 54, 8.36, 38

This Week:




Friday – Radioactive Decay & Nuclear Equations
10/18 – Gravimetric Analysis of a Chloride
Salt Due
10/20 – Elements Exam
10/21 – Problem Set 2 Due
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