Chemistry - A Molecular Science

Mendeleev’s arrangement of the elements has become known as the

periodic chart

or

the

periodic table

and is the source of a great deal of information about the physical and

chemical properties of the elements. The columns define

groups

, which consist of

elements with similar properties. The rows define

periods

, which contain elements whose

properties change gradually. There has been

some controversy over the numbering of the

groups in the periodic table. The American me

thod, 1A - 8A and 1B - 8B, and the newer

method, which numbers the groups as 1 - 18, are both shown on the periodic tables in this book (Figure 1.8 and the back cover). Both methods are in use today, but the older method is still more common, and we will use the ol

der method in the remainder of this text.

The elements in the groups 1A - 8A, the ‘A’ groups, are the

main group elements

,

and groups 1B - 8B are the

transition metals

. The two rows at the bottom of the periodic

table are the lanthanides and the actinides, which are also known as the

inner transition

metals

.

The periodic table contains a great deal of information about the elements. Each box in

the table presented in Figure 1.8 contains (from top to bottom) the atomic number, the chemical symbol, and the atomic mass of the element. As shown in the margin, the atomic number of hydrogen is 1, its symbol is H, and its atomic mass is 1.0079.

The properties of an element are dictated by

its position in the periodic table. In the

table on the following page, they are also indicated by the background color as follows:

The

metals

occupy the left side of the per

iodic table. They represent

about 75% of the elements. All

metals are solids under ordinary

conditions, with the exception of

mercury (Hg), which is a liquid, and

most are lustrous. Metal atoms lose

electrons to form cations in their

compounds with nonmetals.

The

nonmetals

lie on the right side of the Periodic Table and

represent about 20% of the element

s. Many of the nonmetals occur

as gases (nitrogen, oxygen, fluori

ne, chlorine, and the 8A’s); only

bromine is a liquid at room conditi

ons, and the remaining five (carbon,

phosphorus, sulfur, selenium and iodi

ne) are solids. Nonmetal atoms

gain electrons to become anions in their compounds with metals.

The

metalloids

or semimetals lie on a diagonal between the metals

and nonmetals and have characteri

stics that are intermediate

between metals and nonmetals. Metalloids are shiny like metals, but brittle rather than malleable. Metalloids are neither good conductors nor nonconductors of electricity. Instead they are semiconductors (their conductivity changes with temperature), with silicon being a well-known example.

metal nonmetal

metalloid

(^1) H
1.0079
Atomic Number Atomic Symbol Atomic Mass (Weight)
Chapter 1 The Early Experiments
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