The Periodic Table of the Elements
Substances that contain only atoms with the same number of protons are called elements. The
Periodic Table lists all the known elements in order of their atomic number and in columns that
depend on similarities in their chemical and physical properties. The Periodic Table is a useful tool
for both students and professionals to identify the properties of the elements and understand the
properties of molecules.
Become familiar with the organization of the Periodic Table
Appreciate both the diversity and commonalities in the chemical and physical properties of the
Identify groups and periods in the Periodic Table
Use the Periodic Table to provide information about the elements
Activity 02-1: Atoms, Isotopes and Ions
Activity 02-2: Mass Spectrometry and Masses of Atoms
Dmitri Mendeleev (1834 − 1907), a Russian scientist, constructed the ﬁ rst Periodic Table by listing
the elements in horizontal rows in order of increasing atomic mass. He started new rows whenever
necessary to place elements with similar properties in the same vertical column. Mendeleev found
that the correlations in properties between some elements in the columns were not perfect. These
observations led him to predict the existence of undiscovered elements and to wonder how the
table might be better organized. Later H.G.J. Moseley used x-ray spectra to reﬁ ne the ordering and
show that atomic numbers rather than atomic masses should be used to order the elements.
In the Periodic Table, elements with similar properties occur in vertical columns called groups.
Two numbering conventions are used to label the groups. The older convention numbers the groups
using Roman numerals I through VIII followed by a letter A or B; the other convention numbers
each column 1 through 18. The A groups are known as the main group elements. The B groups are
called the transition elements. The group numbers IA through VIIIA in the older convention tells
you how many valence electrons an element has. The valence electrons are the outer electrons that
are most important in determining the chemical bonding and other properties of the element.
The horizontal rows of the table are called periods, and are numbered 1 through 7 starting with the
row that only contains H and He.