Periodic Table Definition in Chemistry

The periodic table is a tabular arrangement of the chemical elements by increasing atomic number, which displays the elements so that one may see trends in their properties.

The Russian scientist Dmitri Mendeleev is most often credited with inventing the periodic table in 1869. The modern table is derived from Mendeleev's periodic table but with one significant difference: Mendeleev's table ordered the elements according to increasing atomic weight rather than atomic number. However, his table illustrated recurring trends or periodicity in the element properties. The periodic table is sometimes called the periodic chart or periodic table of chemical elements.

How Is the Periodic Table Organized?

The structure of the periodic table makes it possible to see relationships between elements at a glance and predict properties of unfamiliar, newly discovered, or undiscovered elements.

Periods

There are seven rows of the periodic table, which are called periods. Element atomic number increases moving from left to right across a period. Elements toward the left side of a period are metals, while those on the right side are nonmetals. Moving down a period on the table adds a new electron shell.

Groups

The columns of elements are called groups or families. Groups are numbered from 1 (the alkali metals) to 18 (the noble gases). Elements with a group share a valence electron configuration. Elements within a group display a pattern with respect to atomic radius, electronegativity, and ionization energy. Atomic radius increases moving down a group, as successive elements gain an electron energy level. Electronegativity decreases moving down a group because adding an electron shell pushes the valence electrons further from the nucleus. Moving down a group, elements have successively lower ionization energies because it becomes easier to remove an electron from the outermost shell.

Blocks

Blocks are sections of the periodic table that indicate the outer electron subshell of the atom. The s-block includes the first two groups (the alkali metals and the alkaline earths), hydrogen, and helium. The p-block includes groups 13 to 18. The d-block includes groups 3 to 12, which are transition metals. The f-block consists of the two periods below the main body of the periodic table (the lanthanides and actinides).

The Types of Elements: Metals, Metalloids, and Nonmetals

The three broad categories of elements are metals, metalloids or semimetals, and nonmetals. Metallic character is highest at the bottom left-hand corner of the periodic table, while most nonmetallic elements are in the upper right-hand corner.

The majority of chemical elements are metals. Metals tend to be shiny (metallic luster), hard, conductive, and capable of forming alloys. Nonmetals tend to be soft, colored, insulators, and capable of forming compounds with metals. Metalloids display properties intermediate between those of metals and nonmetals. Toward the right side of the periodic table, the metals transition into nonmetals. There is a rough staircase pattern—starting at boron and going through silicon, germanium, arsenic, antimony, tellurium, and polonium—that identifies the metalloids. However, chemists increasingly categorize other elements as metalloids, including carbon, phosphorus, gallium, and others.

History of the Periodic Table

Dmitri Mendeleev and Julius Lothar Meyer independently published periodic tables in 1869 and 1870, respectively. However, Meyer had already published an earlier version in 1864. Both Mendeleev and Meyer organized elements by increasing atomic weight and organized elements according to repeating characteristics.

Several other earlier tables were produced. Antoine Lavoisier organized elements into metals, nonmetals, and gases in 1789. In 1862, Alexandre-Emile Béguyer de Chancourtois published a periodic table called the telluric helix or screw. This table was probably the first to organize elements by periodic properties.