Dmitri Mendeleev
Photo by Magnus Manske
Photo by Magnus Manske
Since scientists have been able to isolate
and study single elements, they have
known that some elements have very
similar physical and chemical properties.
Lithium, Sodium, and Potassium are all silverygrey metals that are soft enough to cut with a
knife. All 3 of them violently react with water.
One of the easiest things to determine about each
element was its atomic mass (also called its atomic
weight). Early chemists noticed that the atomic
weights of elements with similar physical and chemical
properties seemed to show that these elements were
related on a deeper level. Of the 60 or so elements
known by chemists at the time, these similar elements
seemed to come in sets of 3 which they called triads.
Photo by Magnus Manske
When early chemists looked at a group of 3 similar
elements, it seemed like the average mass of the
elements was always very close to the middle element.
(6.9 + 23.0 + 39.1) / 3 = 23.0
This became known as the Law of Triads, and scientists
concluded that this meant these elements were related
at the atomic level.
Photo by Magnus Manske
Calcium
40.0 grams
Strontium
87.6 grams
Barium
137.3 grams
Scientists found many of
these triads. The idea was
so popular some scientists
tried to say things were
triads when they were not.
While lists of elements had existed for a while, the idea that
there might be a natural order to the elements started people on
the quest to find the pattern within the elements.
John Newlands was one of the first to present his idea about
how the elements were related. He noticed that if your place
them in a table together according to increasing atomic masses,
the pattern of similarities repeated after every 8th element.
He called this the Law of Octaves because it reminded
him of the musical scale.
Newlands’ idea was NOT taken very seriously at the time
because of the problems that were apparent in his table.
Cobalt and nickel appear in the same column as chlorine.
Iron appears in the same column as oxygen and sulfur.
Copper is in lithium, sodium and potassium’s column.
It turns out that Newlands’ idea works perfectly,
but only for elements 3-18! Since some of these
elements were still undiscovered and Newlands
used all known elements, his table was obviously
not correct.
Dmitri Mendeleev
A Russian chemist named Dmitri Mendeleev created the
first widely accepted periodic table. He still relied on this
idea of 8 groups, but he understood that many elements
were still missing. He placed gaps in his periodic table
where he thought these elements belonged, and this kept
his groups more properly aligned.
Dmitri Mendeleev
He predicted the properties of these
undiscovered elements and when
one of them (Germanium) was
discovered shortly after, it had
properties almost identical to what
Mendeleev had predicted.
Photo by Jurii
Cobalt has a higher atomic mass than nickel,
but it has a smaller atomic number.
For decades scientists used atomic masses to place elements in
order, but this seemed to throw some of the groups out of
alignment. Henry Moseley, an English physicist, understood the
work being done on the atom by Rutherford and Bohr and realized
that if you order them by atomic number instead of atomic mass,
then the groups fall into their expected order.
The modern periodic table that we use in class is based on our
current knowledge of the atom. We now understand that the
electron energy levels and energy sublevels of atoms are the
reason for the repeating, periodic arrangement of elements.
In science, PERIODIC means repeated, regular intervals or patterns.
There are several things you will need to
memorize about the modern periodic table.
Find the dark jagged line on
the Periodic Table
On the left side of this
line are all of the metals
On the right
side of this line
are all of the
nonmetals
The elements that lay on either side of the
dark, jagged have rather unique properties.
Sometimes they act like metals, and
sometimes they act like nonmetals.
Because of this, they are called Metalloids
The metals in the middle of the periodic table
are known as the transition metals.
Transition Metals
Columns on the periodic
table are known as groups
or families.
Elements in the same
group/family have similar
physical and chemical
properties because they all
have the same number of
valence electrons!
All of the
elements
in group
17 have
7 valence
electrons
Valence electrons are the electrons in the
outer energy level.
How many valence electrons do each of
the following atoms have?
Remember: We can determine the number of valence
electrons in the atom of an element by using the number
attached to the As and Bs at the top of the group.
How many
valence
electrons do
the elements
of group 16
have?
How many
valence
electrons does
phosphorous
have?
Use your periodic table to determine
how many valence electrons are in each
of the elements on your notes.
Use your periodic table to determine the
number of valence electrons for elements
below. Decide which Lewis dot diagram is an
accurate representation.
Which Lewis Dot Diagram below accurately
represents lithium?
Li Li
Li
Which Lewis Dot Diagram below accurately
represents boron?
B
B
B
Which Lewis Dot Diagram below accurately
represents phosphorous?
P
P
P
For each of the examples on your
notes, circle the Lewis Dot Diagram
that shows the correct number of
valence electrons for each element.
Group 1 = Alkali Metals
Group 2 = Alkaline Earth Metals
Group 17 = Halogens
Group 18 = Noble Gases
Halogens
Noble Gases
Alkali Metals
Alkaline Earth Metals
There are several groups/families on the periodic
table whose name you should have memorized.
Decide which group number goes
with the group names found on
your notes.
Rows on the periodic table are
known as periods.
All of the elements in a period have
the same number of energy levels.
The elements whose valence electrons are in the
s and p sublevels are known as the representative
elements. The number of valence electrons of
these elements is followed by an A.
s
p
The elements whose valence electrons are in the
d and f sublevels are known as the transition
elements. The number of valence electrons of
these elements is followed by a B.
d
f
You also need to know that Iron, Cobalt, and Nickel
have special properties such as the ability to be made
into magnets. Together, these three elements are
known as the Iron Triad .
Iron Triad
Which group on the periodic table
undergoes the fewest chemical
reactions?
Group 18
The Noble Gases
They are inert gases. This is because they
have 8 valence electrons and their s and
p sublevels are full.
Inert means they resist
chemical reactions!