Chemistry - A Molecular Science

Chapter 1 The Early Experiments

electricity. These are the typical properties of

metals

. The elements on the right side of a

period tend to be gases or dull, brittle solids that

are poor conductors of electricity or heat.

These are the typical properties of

nonmetals

. Thus, the properties of the elements

gradually go from metallic on the left side of a period to nonmetallic on the right side of a period. Those elements that have some properties of both metals and nonmetals and lie between the two broader classes are called

metalloids

.

Mendeleev published his observations as the

periodic law in 1869: The elements, if

arranged in an order that

closely approximates that of their atomic masses

, exhibit an

obvious periodicity in their properties. The phrase in italics was used because Mendeleev had so much confidence in his periodic law that he reversed the order of some elements in order to maintain the periodicity even though it meant reversing the order of their atomic masses. Thus, tellurium with an atomic mass of 128 was placed before iodine, which has an atomic mass of 127. He did not understand why the properties of the elements did not exactly follow the order of the atomic masses,

but he was confident that the concept of

periodicity was correct!

Had Mendeleev blindly used a rigid atomic

mass ordering of the elements, he also

would have placed arsenic (atomic mass = 75) next to zinc (atomic mass = 65) because gallium and germanium had not yet been disc

overed. This would have placed arsenic

directly under aluminum, yet he knew their

properties were quite different. Instead, he

placed arsenic directly under phosphorus b

ecause arsenic and phosphorus have similar

properties. Thus, he left two holes in his

chart, which he correctly predicted would

someday be filled by two new elements. He even

went so far as to predict the properties of

these two new elements. While other scientists

were defining similar ordering schemes for

the elements, it was Mendeleev’s faith in the u

nderlying principle of periodicity that made

him leave room in his table for undiscovere

d elements. The confidence displayed by

Mendeleev set him above others who were trying to construc

t similar relationships. For

this reason, he is given credit for the periodic

law.

We now realize that the parameter that characterizes an atom is its atomic number (Z),

but Mendeleev did not know about electrons

or protons, so he had no atomic number to

reference. He had only the atomic mass. We now understand why tellurium comes before iodine in the periodic order. Although tellurium is heavier than iodine, its atomic number is smaller. Today, the

periodic law

is stated as:

Arranged in the order of their atomic numbers, the elements exhibit periodicity in their chemical and physical properties.

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