More to Learn
Periodic Table of the Elements
By the middle of the 1800s, chemists had identified a large number
of elements. To keep track of all the elements, they needed a way
to sort them into categories. Chemists had observed that some
elements shared similar chemical properties. They looked for a way
to arrange the elements according to these properties. The chemists
that are given credit for successfully grouping the elements into a
pattern according to their properties are Dimitri Mendeleev and
Julius Meyer. Individually, they were responsible for arranging the first
version of the Periodic Table of the Elements.
While the first periodic table grouped the elements according to
properties, years later, the arrangement also revealed information
about the structure of the atoms of those elements. For example, the
modern periodic table, which has been updated since Mendeleev’s
time, shows that the elements are arranged according to their
numbers of protons.
The element with the smallest number of protons, one, is hydrogen (H).
You can find hydrogen on the top row in the left column. Another gas,
helium (He), with two protons, is in the same row but in the column on
the right. The pattern goes on like this for the rest of the table. Each
element has one more proton than the element that came before it.
Because each element has a different number of protons, the number
of protons can be used to tell one element from another. The number
of protons in an atom is called its atomic number. The atomic
number of hydrogen is 1, and the atomic number of
helium is 2. Each element has a unique atomic number.
In the periodic table shown at the back of this Unit,
6
the atomic number of each element can be found in
the upper left corner of each square. Remember that
the protons and neutrons make up almost all the mass
of an atom. The sum of the number of protons and
Carbon
neutrons in an atom of an element is the element’s
12
atomic mass. This can be found below the chemical
symbol on each square.
C
AQ 93
Periodic Table
of the Elements:
a table listing
all the known
elements and their
properties.
atomic number:
the number of
protons in an
atom’s nucleus.
atomic mass: the
average number
of total protons
and neutrons in an
atom’s nucleus.
Atomic Number
Chemical Symbol
Name
Atomic Mass
AIR QUALITY
Learning Set 2 • What Is in Air?
chemical family:
group of elements
found in a column
of the periodic
table. These
elements exhibit
similar properties.
noble gases: a
family of elements
with full electron
energy levels.
These elements
do not undergo
chemical reactions
and are found in
column 18 in the
periodic table.
period: a word to
describe something
that repeats in a
regular pattern.
In addition to being arranged in order of atomic number, the elements
of the periodic table are organized into rows and columns. Each column
represents a group or chemical family. The elements in a chemical
family all have similar properties. The reason for this is that the
elements in each column have the same number of outer electrons.
Therefore, they are likely to behave in similar ways to become stable.
The elements in the first column of the periodic table (all the way to the
left) have just one outer electron. They are very likely to combine with
other atoms to become stable. On the contrary, the elements in the last
column of the periodic table (all the way to the right) all have a complete
set of outer electrons. Because they have a complete set of outer
electrons, these elements are already stable and do not combine with
other atoms. These elements are known as the noble gases. Notice
that argon is a noble gas. In your atomic-model kit, the sphere for argon
did not have any holes in it. Now you know why.
The rows of the periodic table are known as periods. Things that are
periodic repeat in a regular pattern, such as time on a clock or ocean
tides. The properties of the elements change in a pattern that repeats
itself with each new row. This is how the table got its name. Mendeleev
and other scientists observed periodic changes in the properties of
the elements when they were arranged in rows and columns. The two
rows of elements at the bottom of the periodic table, elements 57–71
(Lanthanide Series) and 89–103 (Actinide Series), actually fit into the
sixth and seventh rows. They are separated from the rest of the table to
make the table fit on a page. You will learn more about the periodic table
later in this Unit and in future science courses.
Currently, there are more than 10 0 known elements. Each element has
properties that make it unique. Scientists continue to search for new
elements and for ways the periodic table might be improved.
Stop and Think
1. Find oxygen in the periodic table. What is its atomic number? What
is the next element to the right of oxygen? How many protons does
that element have?
2. Find nitrogen in the periodic table. What is its atomic number?
3. Find two other familiar elements in the periodic table. List their
atomic numbers and atomic masses.
Project-Based Inquiry Science
AQ 94
More to Learn
Development of the Periodic Table
Ca-40
Li-7
Cl-35
Sr-88
Na-23
Br-80
Ba-137
K-39
I-127
1829
Numbers after the elements are atomic weights.
English chemist John Newlands arranged the
56 known elements in order of increasing atomic
weight. He observed similarities that repeated after
each set of eight elements. As a result, he proposed
the law of octaves, because an octave describes a
group of eight notes in music.
Li-7
F-19
Cl-35
Br-80
I-127
Na-23
K-39
Rb-85
7Cs-133
Ca-40
Sr-88
Ba-137
German chemist Johann Döbereiner arranged the
55 known elements by atomic weight into groups
of three, called a triad, based on similar properties.
Döbereiner noticed that he could organize elements
into groups of three in such a way that the atomic
weight of one element was an average of the atomic
weights of the other two elements.
1864
An octave of eight musical notes.
1869
Russian chemist Dmitri Mendeleev developed a
table of 63 known and predicted elements in order
of atomic weight but arranged them in columns
based on similar properties. He left spaces for
elements that had not yet been discovered based
on the properties he predicted. Mendeleev’s table
had columns and rows so that the properties of
elements changed from left to right across a row
and then repeated in a similar way in the next row.
German chemist Lothar Meyer also observed
patterns of properties when he arranged the 56
known elements in order of atomic weight and
developed his own periodic table. However, Meyer
did not predict the existence of missing elements.
Numbers after the elements are atomic weights.
Li-3
British chemist Henry Moseley determined the
atomic number of each element, which is the number
of protons in the nucleus. He changed the periodic
table to arrange the 92 known elements in order of
atomic number rather than atomic weight.
Na-11
1914
K-19
Cl-17
Ca-20
Br-35
Sr-38
I-53
Ba-56
Numbers after the elements are atomic numbers.
Pu-94
Am-95
Cm-96
Bk-97
Es-99
Fm-100
Md-101
N0-102
Cf-98
1940
Nine new elements discovered by Seaborg.
Numbers after the elements are atomic numbers.
AQ 95
American chemist Glenn Seaborg discovered nine
new elements after uranium, and his discoveries led
to the current arrangement of the periodic table. He
won the Nobel Prize in chemistry for his work.
AIR QUALITY