Section 4.5—Periodicity
Objectives:
•Define periodic trend
•Use periodic trends in atomic radius,
ionization energy, and electron affinity to
organize elements
Periodic Trends
•Periodicity is the presence of a predictable
pattern in the properties of elements.
•If the properties of an element change in a
predictable way, we call it a “trend”.
•In the periodic table, there are trends
observed within a group (from top to
bottom) and across a period (from left to
right).
• Note that there are some exceptions to
these periodic trends!!
Trend 1: Atomic Radii
Periodic Trends
Atomic radius is
defined as half the
distance between the
nuclei of identical
atoms that are
chemically bonded
together.
Atomic radius is a
measure of atomic
size.
Distance between nuclei
Atomic radius of atom
Atomic Radii Trends
Period Trend:
Decreases
Group
Trend:
Increases
Why do atomic radii
decrease across a period?
Moving left to right, the number of protons, neutrons
and electrons all increase.
e
e
e
Move across the
periodic table
n
p
n p
n p
Lithium atom
e
Radius decreases
e
e
pn
p p
p
n nn
e
Beryllium atom
As the # of protons/electrons increase, the attraction
between the positive nucleus and negative electron cloud
increases. This attraction “pulls” in on the electrons.
Why do atomic radii increase
down a group?
Protons, neutrons and electrons are also added as
Sodium atom
you move down a group.
e
e
e
Move down the
periodic table
e
e
e
+
Radius increases
e
e
e
e
+
e
e
e
e
Lithium atom
However, the electrons are added in new
energy levels. The inner electrons act like a “shield”
for the outer electrons, so they do not feel the pull of
the nucleus so strongly.
Trend 2: Ionization Energy
What is Ionization Energy?
•An ion is a particle that has a positive or
negative charge.
•It got this charge when the particle lost
electrons (it becomes positively-charged +)
or gained electrons (it becomes negativelycharged -)
•The energy that is needed to remove ONE
of the outermost electrons is called the
ionization energy.
Ionization Energy Trends
Period Trend: INCREASES
Group Trend:
DECREASES
Why does Ionization Energy
increase across a period?
Recall that, as you move left to right across a period,
the radius of the atom decreases.
Move across the
periodic table
e
e
n
p
n p
n p
e
e
Radius decreases
IE increases
Lithium atom
e
e
pn
p p
p
n nn
e
Beryllium atom
When an atom is smaller, the electrons are closer to
the nucleus, and therefore feel the pull more strongly.
It is harder to pull electrons away from the atom.
Why does ionization energy
decrease down a group?
Recall that, as you move down a group, the radius
Sodium atom
increases.
e
e
+
e
Move down the
periodic table
e
e
e
e
e
e
IE decreases
e
+
Radius increases
e
e
e
e
Lithium atom
As the outer electrons are farther
from the nucleus, they will feel the “pull” of the
nucleus less. It is easier to remove them from the
Trend #3: Electron Affinity
What is Electron Affinity?
•Recall that to form negative ions, an
atom gains electrons.
•The energy released when an electron
is added to an atom is called the
electron affinity.
Electron Affinity Trends
Period Trend: Increases
Group
Trend:
Decreases
Why does Electron Affinity
increase across a period?
Again, recall that atomic radius decreases as you
move across a period.
Move across the
periodic table
e
e
n
p
n p
n p
Lithium atom
e
Radius decreases
EA increases
e
e
e
pn
p p
p
n nn
e
Beryllium atom
Recall that in smaller atoms, the electrons feel the
pull of the protons more strongly. Such an atom can
handle an extra electron more easily as it can be
more “controlled” by the closer nucleus.
Why does electron affinity
decrease down a group?
As you move down a group, the radius increases.
Sodium atom
e
Move down the
periodic table
e
+
e
e
e
e
e
e
Radius increases
e
e
+
EA decreases
Lithium atom
e
e
e
e
As the size of the atom increases, the
outer electrons feel the “pull” of the nucleus less and
less. A larger atom is less able to “control” a new
electron added.
Ionic Charge & Radii
Review
There are 2 kinds of charged particles.
A cation is positively charged ion. It
results from an atom losing electrons.
An anion is a negatively charged ion. It
results from an atom gaining electrons.
Ionic Radii—Cations
Since atoms lose electrons to create cations, the
atom ends up having more protons than electrons.
Each proton, therefore, has a greater pull on each
electron. A cation is smaller than its parent atom!
e
e
e
Creating a cation,
losing electrons
+
e
e
+
Radius decreases
Lithium atom
Li+ ion
Ionic Radii—Anions
Since atoms gain electrons to create negative ions,
the atom ends up having more electrons than
protons. In this case, each proton has a weaker
pull on each electron. An anion is bigger than it
parent atom.
e
e
e
e
e
e
e
Creating an anion,
gaining electrons
e
+
e
e
e
+
e
e
e
Radius increases
e
e
e
e
Oxygen atom
O2- ion
Let’s Practice
Example:
List Li, Cs
and K in
order of
increasing
Atomic radii
Example:
List Li, N
and C in
order of
increasing
Atomic radii
Ionization Energy
Electron Affinity
Ionization Energy
Electron Affinity
Let’s Practice
Example:
List Li, Cs
and K in
order of
increasing
Atomic radii
Li, K, Cs
Ionization Energy
Cs, K, Li
Electron Affinity
Cs, K, Li
Example:
List Li, N
and C in
order of
increasing
Atomic radii
N, C, Li
Ionization Energy
Li, C, N
Electron Affinity
Li, C, N