The Periodic Table

The periodic table of elements is an important reference in chemistry for organizing the elements. The table has seven rows, or periods, numbered top to bottom, and eighteen columns called groups or families.

While the modern numbering system recommended by the International Union of Pure and Applied Chemistry or IUPAC, numbers the groups from 1 to 18, the Chemical Abstracts Services uses roman numerals and letters to designate the groups. Here, the main group elements are designated with letter A and the transition elements are designated with letter B.

The elements in a periodic table are ordered from left to right and top to bottom by increasing atomic number. Those with similar properties fall in the same group.

Some of the groups have common names. The elements in group 1, except for hydrogen, are known as alkali metals and the elements in group 2 as alkaline earth metals. All of these metals are very reactive and are found in nature combined with other elements.

Highly reactive, nonmetallic elements in group 17 are known as halogens. The nonreactive, nonmetallic elements of group 18 are referred to as noble or inert gases.

The elements can be divided into three broad classes: metals, nonmetals, and metalloids.

Metals are grouped toward the left and middle of the periodic table and are the vast majority of the elements. They are good conductors of electricity and heat. All are solid at room temperature, except for mercury, which is liquid.

Metals are shiny, and they can be hammered into sheets and drawn into wires. Metals, like potassium, tend to lose electrons to form cations, positively charged ions, when they react with other elements.

Nonmetals are grouped on the right side of the periodic table and usually poor conductors of heat and electricity. At room temperature, some are solid, while others are liquids or gases. Nonmetals tend to gain electrons and form anions, negatively charged ions, when they react with other elements.

Metalloids are elements that are found along the divide between the metals and non-metals. They are the smallest class of elements and exhibit mixed properties.

All metalloids exist as solids at room temperature. Some of them, for example, silicon and germanium, act as semiconductors, meaning that unlike metals, they get better at conducting electricity as the temperature increases.

Elements in the periodic table are also divided into three categories according to their location: the main-group elements, the transition metals, and the inner transition metals also known as the lanthanides and the actinides for the first element in the series in each row.