Chapter 5
The Periodic Law
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History of the Periodic Table
1869 – Dmitri Mendeleev published his
periodic table. He had arranged it by
grouping together the elements that had
similar properties and
by increasing atomic
masses. His periodic
table left empty spaces
for new elements that
would be discovered.
Mendeleev’s List of elements in Russian Circa 1869
Periodic Table circa 1898
History of the Periodic Table

1911 – Henry Moseley (a student of Ernest
Rutherford) rearranged a few elements on
the periodic table so that elements were
arranged by increasing atomic
number rather than by atomic
mass.
History of the Periodic Table

1944 – Glenn T. Seaborg rearranged the
periodic table to make it look like it does
today. He moved the Actinide Series and
the Lanthanide Series
elements to the bottom
of the periodic table.
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Modern Periodic Table
The Periodic Law
The
physical and chemical
properties of the elements are
periodic functions of their
atomic numbers
What are the trends among the
elements for atomic radius?
The
atomic radius of an element is one
half of the distance between the nuclei of
two atoms of the same element when the
atoms are joined.
– Atomic radii are often measured in
picometers (pm).
1. Interpret Diagrams Which element in the figure above has the largest
atomic radius?
Summary: trends in atomic radius
6. Compare If a halogen and an alkali metal are in the same period, which
element will have the larger atomic radius?
Group trends
Atomic radius
increase as you
move down
because each
atom has another
energy level, so
the atoms get
bigger.
H
Li
Na
K
Rb
Periodic Trends
 As
you go across a period, the radius gets
smaller.
 Electrons are in same energy level.
 Increasing
number of positive protons
within the nuclei pull on the valence
electrons.
Na
Mg
Al
Si
P
S Cl Ar
Learning Check
According to their positions on the
Periodic Table, which of the
following neutral atoms has the
largest atomic radius?
A
Beryllium
B
C
D
Magnesium
Calcium
Strontium
CATIONS
 Cations
are positively charged ions
 Cations form by losing electrons.
 Cations are smaller than the atom they
come from.
 Metals form cations.
 Cations of representative elements
have noble gas configuration.
ANIONS
 Anions
are negatively charged ions
 Anions form by gaining electrons.
 Anions are bigger than the atom they
come from.
 Nonmetals form anions.
 Anions of ‘main’ groups elements have
noble gas configuration.
Group trends
Adding energy level
 Ions get bigger as you
go down.

Li1+
Na1+
K1+
Rb1+
Cs1+
Ionic Radius
A
greater number of protons will cause
an ion to become smaller due to
attractive forces between the nucleus
and the valence electrons
 greater number of electrons will cause
an ion to become larger due to electron
repulsion.
N31O2Li1+
F
B3+
Be2+
C4+
Period trends in ionic radius
Learning Check


2 Which of the following statements is true about the trend
for ionic radii?
A When electrons are lost forming positive ions, the ionic
radius is larger than its corresponding neutral atom.

B When electrons are lost forming positive ions, the ionic
radius is smaller than its corresponding neutral atom.

C When electrons are gained forming positive ions, the
ionic radius is larger than its corresponding neutral atom.

D When electrons are gained forming positive ions, the
ionic radius is smaller than it’s corresponding neutral atom.
What are the trends among the
elements for ionization energy?
The
energy required to remove an electron
from an atom is called ionization energy.
– This energy is measured when an element is in
its gaseous state.
– The energy required to remove the first electron
from an atom is called the first ionization
energy.
Ionization Energy



Decreases as we move down the periodic table.
The outermost energy level is farther from the nucleus.
Therefore, it is easier to remove an electron.
Ionization Energy
– Increases as you move across the periodic table
– The nuclear charge(atomic #) increases across
the period because you have more protons and
your atomic radius is getting smaller. As a
result, there is an increase in the attraction of
the nucleus for an electron.
– The outermost electrons are just a little closer to
the nucleus and so they are harder to remove.
Summary: trends in
ionization energy
11. Compare Which element would have a larger first ionization energy:
an alkali metal in Period 2 or an alkali metal in Period 4?
Learning Check
3 Which element in group 17 (group 7A),
the halogens, has the highest ionization
energy?
 A Fluorine
 B Chlorine
 C Bromine
 D Iodine

Learning Check
4 Based on the periodic trend for ionization
energy, which element in Period 3 has the
greatest tendency to lose an electron?
 A Sodium
 B Aluminum
 C Phosphorus
 D Argon

Atoms with large ionization energy values are
A. more likely to form
positive ions
B. less likely to form
positive ions
C. most likely to lose
their outer electrons
D. lacking valence
electrons
Electronegativity
The tendency for an atom to attract
electrons to itself when it is chemically
combined with another element.
 High electronegativity means it pulls the
electron toward it.

Electronegativity
 The
further down a group, the
farther the electron is away, and
the more electrons an atom has.
 More willing to share.
 Low electronegativity.
Electronegativity


Metals let their
electrons go easily
resulting in a low
electronegativity
Nonmetals want more
electrons and take
them away from
others resulting in a
high electronegativity.
Electronegativity
Electronegativity Values for Selected Elements
H
2.1
He
0
Li
1.0
Be
1.5
B
2.0
C
2.5
N
3.0
O
3.5
F
4.0
Ne
0
Na
0.9
Mg
1.2
Al
1.5
Si
1.8
P
2.1
S
2.5
Cl
3.0
Ar
0
K
0.8
Ca
1.0
Ga
1.6
Ge
1.8
As
2.0
Se
2.4
Br
2.8
Kr
0
Rb
0.8
Sr
1.0
In
1.7
Sn
1.8
Sb
1.9
Te
2.1
I
2.5
Xe
0
Cs
0.7
Ba
0.9
Tl
1.8
Pb
1.9
Bi
1.9
Learning Check

Which diagram below correctly depicts the
increase in electronegativity across periods
and down groups?
Summary: periodic trends
15. Explain Periodic Trends In general, how can the periodic trends exhibited
by the elements be explained?
Reactivity
Reactivity increases as you go
down the columns of metallic
elements.
 Reactivity decreases as you go
down the columns of nonmetallic elements.

– Flourine is the most reactive
nonmetal
Valence Electrons
The electrons available to be lost,
gained, or shared in the formation
of compounds.
 The electrons in the highest energy
level
 Label valence electrons on periodic
table

Extra Slides
0
1+
3+
44+
3-
2-
1-
He
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Ga
Ge
As
Se
Br
Kr
Rb
Sr
In
Sn
Sb
Te
I
Xe
Cs
Ba
Tl
Pb
Bi
Po
At
Rn
Fr
Ra
H
2+
Li