6.1. History of the Periodic Table http://www.ck12.org
6.1 History of the Periodic Table
Lesson Objectives
- Describe some of the early attempts to organize the chemical elements.
- Understand how Mendeleev organized his periodic table.
- Know the improvements that Moseley made to Mendeleev’s table.
- Know the periodic law.
- Describe various components of the modern periodic table, including periods, groups, metals, nonmetals, and
metalloids.
Lesson Vocabulary
- group
- metal
- metalloid
- nonmetal
- period
- periodic law
- periodic table
Early Attempts to Organize the 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 (Figure6.1).
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 named 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 and chemical properties. Dobereiner found
that the atomic masses of these three elements, as well as those of 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 reactivity, such as vigorously
reacting with the members of another triad: chlorine, bromine, and iodine (Figure6.2).
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) arranged the elements in increasing order of atomic mass and noticed