History of the Periodic Table
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2012 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including
CK-12 Curriculum Material) is made available to Users
in accordance with the Creative Commons Attribution/NonCommercial/Share Alike 3.0 Unported (CC BY-NC-SA) License
(http://creativecommons.org/licenses/by-nc-sa/3.0/), as amended
and updated by Creative Commons from time to time (the “CC
License”), which is incorporated herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: January 30, 2013
www.ck12.org
C ONCEPT
Concept 1. History of the Periodic Table
1 History of the Periodic Table
Lesson Objectives
•
•
•
•
•
Explain early attempts at organizing chemical elements
Understand how Mendeleev organized his periodic table
Know the improvements that Moseley made on Mendeleev’s table
Know the periodic law
Describe components of the modern periodic table: periods, groups, metals, nonmetals, and metalloids
Lesson Vocabulary
•
•
•
•
•
•
•
group
metal
metalloid
nonmetal
period
periodic law
periodic table
Early Attempts to Organize Elements
By the year 1700, only a handful of elements had been identified and isolated. Several of these, such as copper
and lead, had been known since ancient times. As scientific methods improved, the rate of discovery dramatically
increased (Figure 1.1).
With the ever-increasing number of elements, chemists recognized that there may be some kind of systematic way
to organize the elements. The question was: how?
A logical way to begin to group elements together was by their chemical properties. In other words, putting elements
in separate groups based on how they reacted with other elements. In 1829, a German chemist, Johann Dobereiner
(1780-1849), placed various groups of three elements into groups called triads. One such triad was lithium, sodium,
and potassium. Triads were based on both physical as well as chemical properties. Dobereiner found that the
atomic masses of these three elements, as well as other triads, formed a pattern. When the atomic masses of lithium
and potassium were averaged together (6.94 + 39.10)/2 = 23.02, it was approximately equal to the atomic mass of
sodium (22.99). These three elements also displayed similar chemical reactions, such as vigorously reacting with
the members of another triad: chlorine, bromine, and iodine (Figure 1.2).
While Dobereiner’s system would pave the way for future ideas, a limitation of the triad system was that not all of
the known elements could be classified in this way.
English chemist John Newlands (1838-1898) ordered the elements in increasing order of atomic mass and noticed
that every eighth element exhibited similar properties. He called this relationship the Law of Octaves. Unfortunately,
there were some elements that were missing and the law did not seem to hold for elements that were heavier than
calcium. Newlands’s work was largely ignored and even ridiculed by the scientific community in his day. It was
1
www.ck12.org
FIGURE 1.1
Dates of discovery of the chemical elements.
FIGURE 1.2
The elements chlorine, bromine, and iodine: Chlorine is a greenish-yellow gas.
Bromine is a dark orange liquid. Iodine
is a shiny blue-black solid. Though different in appearance, they have very similar
chemical properties.
not until years later that another more extensive periodic table effort would gain much greater acceptance and the
pioneering work of John Newlands would be appreciated.
Mendeleev’s Periodic Table
In 1869, Russian chemist and teacher Dmitri Mendeleev (1836-1907) published a periodic table of the elements. The
following year, German chemist Lothar Meyer independently published a very similar table. Mendeleev is generally
given more credit than Meyer because his table was published first and because of several key insights that he made
regarding the table.
Mendeleev was writing a chemistry textbook for his students and wanted to organize all of the known elements at
that time according to their chemical properties. He famously organized the information for each element on to
separate note cards that were then easy to rearrange as needed. He discovered that when he placed them in order
of increasing atomic mass, certain similarities in chemical behavior repeated at regular intervals. This type of a
2
www.ck12.org
Concept 1. History of the Periodic Table
repeating pattern is called periodic. A pendulum that swings back and forth in a given time interval is periodic, as is
the movement of the moon around the Earth. Figure 1.3 shows an early version of Mendeleev’s table.
FIGURE 1.3
Mendeleev’s first published periodic table shows elements arranged in vertical
columns according to increasing atomic
mass. The atomic mass is the number
which follows each symbol.
Elements
with question marks were unknown at the
time, but were discovered at a later date.
In this table, atomic mass increases from top to bottom of vertical columns, with successive columns going left to
right. As a result, elements that are in the same horizontal row are groups of elements that were known to exhibit
similar chemical properties. One of Mendeleev’s insights is illustrated by the elements tellurium (Te) and iodine (I).
Notice that tellurium is listed before iodine even though its atomic mass is higher. Mendeleev reversed the order
because he knew that the properties of iodine were much more similar to those of fluorine (F), chlorine (Cl), and
bromine (Br) than they were to oxygen (O), sulfur (S), and selenium (Se). He simply assumed that there was an
error in the determination of one or both of the atomic masses. As we will see shortly, this turned out not to be the
case, but Mendeleev was indeed correct to group these two elements as he did.
Notice that there are several places in the table that have no chemical symbol, but are instead labeled with a question
mark. Between zinc (Zn) and arsenic (As) are two such missing elements. Mendeleev believed that elements with
atomic masses of 68 and 70 would eventually be discovered and that they would fit chemically into each of those
spaces. Listed in Table 1.1 are other properties that Mendeleev predicted for the first of these two missing elements,
which he called “eka-aluminum,” compared with the element gallium.
TABLE 1.1: Mendeleev’s Predictions for Eka-Aluminum
Atomic mass
Melting point
Eka-Aluminum (Ea)
68 amu
Low
Gallium (Ga)
69.9 amu
30.15°C
3
www.ck12.org
TABLE 1.1: (continued)
Density
Formula of oxide
Eka-Aluminum (Ea)
5.9 g/cm3
Ea2 O3
Gallium (Ga)
5.94 g/cm3
Ga2 O3
The element gallium was discovered four years after the publication of Mendeleev’s table, and its properties matched
up remarkably well with eka-aluminum, fitting into the table exactly where he had predicted. This was also the case
with the element that followed gallium, which was named eventually named germanium.
Mendeleev’s periodic table gained wide acceptance with the scientific community and earned him credit as the
discoverer of the periodic law. Element number 101, synthesized in 1955, is named mendelevium after the founder
of the periodic table. It would, however, be several years after Mendeleev died before the several discrepancies with
the atomic masses could be explained and before the reasons behind the repetition of chemical properties could be
fully explained.
FIGURE 1.4
A wax sculpture of Mendeleev.
Interactive periodic table websites:
• Periodic History: http://freezeray.com/flashFiles/periodicHistory.htm
• Discovery Dates: http://freezeray.com/flashFiles/discoveryDates.htm
The Periodic Law
Recall that Rutherford’s experiments which resulted in the discovery of the nucleus occurred in 1911, long after
Mendeleev’s periodic table was developed. Just two years later, in 1913, English physicist Henry Moseley (18871915) examined x-ray spectra of a number of chemical elements. His results led to the definition of atomic number
as the number of protons contained in the nucleus of each atom. He then realized that the elements of the periodic
table should be arranged in order of increasing atomic number rather than increasing atomic mass.
When ordered by atomic number, the discrepancies within Mendeleev’s table disappeared. Tellurium has an atomic
number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic
mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with
being most responsible for the modern periodic law: When elements are arranged in order of increasing atomic
number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as
we know it today (Figure 1.5). Each new horizontal row of the periodic table corresponds to the beginning of a new
4
www.ck12.org
Concept 1. History of the Periodic Table
period because a new principal energy level is being filled with electrons. Elements with similar chemical properties
appear at regular intervals, within the vertical columns called groups.
FIGURE 1.5
The periodic table of the elements.
The Modern Periodic Table
The periodic table has undergone extensive changes in the time since it was originally developed by Mendeleev
and Moseley. Many new elements have been discovered, while others have been artificially synthesized. Each fits
properly into a group of elements with similar properties. The periodic table is an arrangement of the elements in
order of their atomic numbers so that elements with similar properties appear in the same vertical column or group.
Figure 1.5 shows the most commonly used form of the periodic table. Each square shows the chemical symbol of
the element along with its name. Notice that several of the symbols seem to be unrelated to the name of the element:
Fe for iron, Pb for lead, etc. Most of these are the elements that have been known since ancient times and have
symbols based on their Latin names. The atomic number of each element is written above the symbol. Each square
on this version of the periodic table also shows the average atomic mass of the element.
A period is a horizontal row of the periodic table. There are seven periods in the periodic table, with each one
beginning at the far left. A new period begins when a new principal energy level begins filling with electrons. Period
1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements. Periods 4 and 5 have 18
5
www.ck12.org
elements. Periods 6 and 7 have 32 elements because the two bottom rows that are separated from the rest of the table
belong to those periods. They are pulled out in order to make the table itself fit more easily onto a single page.
A group is a vertical column of the periodic table. There are a total of 18 groups. There are two different numbering
systems that are commonly used to designate groups and you should be familiar with both. The traditional system
used in the United States involves the use of the letters A and B. The first two groups are 1A and 2A, while the last
six groups are 3A through 8A. The middle groups use B in their titles. Unfortunately, there was a slightly different
system in place in Europe. To eliminate confusion the International Union of Pure and Applied Chemistry (IUPAC)
decided that the official system for numbering groups would be a simple 1 through 18 from left to right. Many
periodic tables show both systems simultaneously
Metals, Nonmetals, and Metalloids
Elements can be classified in a number of different ways. Classifying by period and/or group is important because
it is based on their electron configuration. Another way is to classify elements based on physical properties. Three
broad classes of elements are metals, nonmetals, and metalloids.
A metal is an element that is a good conductor of heat and electricity. Metals are also malleable, which means
that they can be hammered into very thin sheets without breaking. They are ductile, which means that they can be
drawn into wires. When a fresh surface of any metal is exposed, it will be very shiny because it reflects light well.
This is called luster. All metals are solid at room temperature with the exception of mercury (Hg), which is a liquid.
Melting points of metals display a very wide variance. The melting point of mercury is −39°C, while the highest
melting metal is tungsten (W), with a melting point of 3422°C. On the periodic table of Figure 1.5, the metals are
blue and are located to the left of the bold stair-step line. About 80 percent of the elements are metals (see examples
in Figure 1.6).
FIGURE 1.6
The elements mercury, gold, and copper
show the properties of metals. Mercury
(left) is the only liquid metal, but has high
luster.
Gold (middle) is malleable and
can be formed into very thin sheets called
gold leaf. Copper is very ductile and a
good conductor. Copper (right) is used
extensively in electrical wiring.
A nonmetal is an element that is generally a poor conductor of heat and electricity. Most properties of nonmetals
are the opposite of metals. There is a wider variation in properties among the nonmetals than among the metals,
as seen in Figure 1.7. Nonmetals exist in all three states of matter. The majority are gases, such as nitrogen and
oxygen. Bromine is a liquid. A few are solids, such as carbon and sulfur. In the solid state, nonmetals are brittle,
meaning that they will shatter if struck with a hammer. The solids are not lustrous. Melting points are generally
much lower than those of metals. On the periodic table, the nonmetals are the green squares and are located to the
right of the stair-step line.
A metalloid is an element that has properties that are intermediate between those of metals and nonmetals. Silicon is
a typical metalloid (Figure 1.8). It has luster like a metal, but is brittle like a nonmetal. Silicon is used extensively in
computer chips and other electronics because its electrical conductivity is in between that of a metal and a nonmetal.
Metalloids can also be called semimetals. On the periodic table, the elements colored red, which generally border
the stair-step line, are considered to be metalloids. Notice that aluminum borders the line, but it is considered to be
6
www.ck12.org
Concept 1. History of the Periodic Table
FIGURE 1.7
Nonmetals have properties that are unlike
those of metals.
Sulfur (left) is brittle
and its distinctive yellow color lacks luster.
Bromine (center ) is the only liquid nonmetal and must be carefully handled due
to its toxicity. Helium (right), a colorless
and unreactive gas, is lighter than air and
thus is used in blimps.
a metal since all of its properties are like those of metals.
FIGURE 1.8
Silicon is an example of a metalloid, having some properties of a metal and some
of a nonmetal.
Explore some interactive periodic tables at:
• Periodic Table: http://chemistry.about.com/library/blperiodictable.htm?nl=1
• It’s Elemental (NOVA): http://www.pbs.org/wgbh/nova/physics/periodic-table.html
• It’s Elemental! Jefferson Lab: http://education.jlab.org/itselemental/index.html
Watch this Khan Academy video on Groups of the Periodic Table (11:51):
MEDIA
Click image to the left for more content.
7
www.ck12.org
Lesson Summary
• Mendeleev arranged chemical elements into a periodic table by arranging them in order of increasing atomic
mass.
• According to the periodic law, physical and chemical properties of elements are periodic functions of their
atomic numbers.
• In the modern periodic table, elements are arranged into periods and groups. Elements in the same group have
similar properties.
• Elements can be classified into the categories of metals, nonmetals, or metalloids.
Lesson Review Questions
Reviewing Concepts
1. Compare Dobereiner’s triads to the modern periodic table. What aspects of his idea agreed with the modern
table and what aspects did not?
2. Why did Mendeleev leave empty spaces in his periodic table?
3. Explain the significance of the discovery of gallium to Mendeleev’s periodic table.
4. What discovery occurred after Mendeleev’s periodic table was published that allowed Moseley to improve it?
5. Identify each physical property as more characteristic of a metal or of a nonmetal.
a.
b.
c.
d.
e.
luster
malleability
brittle
lower melting points
good conductor of electric current
Problems
6. Use Figure 1.1 to answer the following.
a. How many elements were known by 1700?
b. How many elements were discovered between 1735 and 1843?
7. The atomic mass of beryllium (Be) is 9.01 amu. The atomic mass of calcium (Ca), which is in the same group
as beryllium, is 40.08 amu. What would you predict to be the atomic mass of magnesium (Mg), which is also
in the same group but is between Be and Ca?
8. Use the periodic table to put the following elements into pairs that would be expected to have similar chemical
properties: Ba, S, Br, Ca, K, F, Se, Na
9. Identify the elements below as a metal, nonmetal, or metalloid.
a.
b.
c.
d.
e.
phosphorus
boron
cesium
xenon
bismuth
10. Write the name and symbol of the element located in each position on the periodic table.
a. period 2 and group 16
b. period 5 and group 9
8
www.ck12.org
Concept 1. History of the Periodic Table
c. period 4 and group 5A
d. period 6 and group 12
Further Reading / Supplemental Links
Games:
•
•
•
•
•
•
Element Concentration: http://education.jlab.org/elementconcentration/index.html
Element Crossword Puzzles: http://education.jlab.org/elementcrossword/index.html
Element Flash Cards: http://education.jlab.org/elementflashcards/index.html
Element Hangman: http://education.jlab.org/elementhangman/index.html
Element Matching Game: http://education.jlab.org/elementmatching/index.html
Element Word Scramble: http://education.jlab.org/elementwordscramble/index.html
Interactive web sites:
• Did you know you have rocks in your kitchen? Visit Rocks In Your Cabinet at http://www.oercommons.or
g/courses/kids-in-the-hall-of-planet-earth/view.
• Create an Atomic Mobile: http://www.amnh.org/ology/einstein#features/stufftodo_einstein/atommobile.php?TB
_iframe=true&height=550&width=740.
Tutorial and video:
• Islands of Stability: http://www.pbs.org/wgbh/nova/physics/stability-elements.html
9