The Periodic Table
of Elements
History
• In 1869, Dmitri Mendeleev created the
first periodic table.
• He organized elements into rows and
groups based on their properties.
• The picture to
the left shows
his early table.
• How is this table
different from
today’s?
GROUPS
• Vertical columns
• Same number of valence electrons
(electrons in outermost energy level)
• Share similar properties
Memory trick: Group has the word “up” in it, and groups go up and down!
GROUPS
• Groups are also called
families since they share
similar properties.
• Look at the Bohr Models of
elements to the right… can
you figure out what they
have in common?
GROUPS
• All the elements have the same
number of electrons in their
outer shells: 1.
• Electrons in the outer shell are
called valence electrons.
• The number of valence electrons
determine the chemical
reactivity of the element.
Group 1: Alkali Metals
• 1 valence electron (1 electron
in outer shell)
• Have shared properties:






React with water and oxygen
Good conductors
Ductile, malleable and soft
Silvery luster
Low density
Low melting point
PERIODS
• Horizontal Rows
• Have the same number of energy levels
Periods
• Look at the Bohr Models of elements below…
can you figure out the pattern?
Periods
• As you go down the period to the right, the number of
Protons increases
• The number of protons is called the Atomic Number
• This means the periods are arranged by increasing
Atomic Number.
Periods
• You may think that the atoms get bigger as they go
across the period, but they don’t!
• The atoms get smaller as they go across. Why do
you think this is?
• HINT: As you go along the row there are more
protons and electrons. Think about the charges of
these particles.
Periods
• As you move to the right along each period, there
is a stronger positive and negative charge, since
there are more protons and electrons.
• This pulls the shell closer together, causing it to be
smaller.
Practice:
Which two elements are in the same group?
Which two elements are in the same period?
Which two elements have the most similar chemical
properties?
Find another element that has the same properties as C.
A
B
C
D
Central Questions:
Consider these questions about the periodic table:
1. What is the significance of patterns among the
elements in the periodic table?
2. Can the theory of the atom explain the
organization of the periodic table?
3. What does the periodic table allow us to predict
about the elements?
Conclusion:
1. The patterns of the elements in the periodic
table allow scientists to better understand the
properties of each element.
2. The structure of the atom (atomic theory) plays a
key role in how the periodic table is structure,
since groups are arranged by outer valence
electrons and periods by increasing atomic
number
3. The periodic table allows us to predict the
properties, outer valence shell, atomic number
and size of unknown elements.
Patterns Across and Down the Table
• The physical and chemical properties of the
elements change in a predictable way.
• Many nonmetals are gases and most metals
and metalloids are solids.
Patterns Across and Down the Table
• Density of the elements increases as you
move down a group.
• The melting points of metals decrease as you
move down a group.
• The melting points of nonmetals increases as
you move down a group.
Patterns Across and Down the Table
• The ease at which a metal reacts with other
substances decreases from the left to right
across a period.
• The elements in Group 1 are highly reactive,
but the elements in Group 11 are slow to
react.
Patterns Across and Down the Table
• Nonmetals in Group 18 almost never react.
• Nonmetals in Groups 13-17 are more reactive
from left to right.
Why the Periodic Table Works
• The periodic table works because it is based on the
structures of atoms, especially the valence electrons.
• Each element in a period has one more electron than
the element to its left.
• Elements in a column or group have the same number
of valence electrons, which is why these elements
react in similar ways.