Classification of Elements and Periodicity in Properties

Historical development and fundamental concepts of the Periodic Table, including key laws, scientists, and structural organization.

Johann Dobereiner

A German chemist who in the early 1800s noted similarities in physical and chemical properties among groups of three elements, known as Triads.

Law of Triads

The observation that the middle element of a triad has an atomic weight and properties approximately halfway between the other two members of the group.

A.E.B. de Chancourtois

A French geologist who in 1862 arranged known elements in order of increasing atomic weights in a cylindrical table to display periodic recurrence of properties.

Law of Octaves

A principle proposed by John Alexander Newlands in 1865 stating that when elements are arranged by increasing atomic weight, every eighth element has properties similar to the first.

Dmitri Mendeleev

The Russian chemist credited with developing the Periodic Law and table, proposing that properties are a periodic function of atomic weights.

Lothar Meyer

A German chemist who independently proposed a periodic arrangement of elements based on physical properties like atomic volume against atomic weight.

Mendeleev's Periodic Law

The principle stating that the physical and chemical properties of elements are a periodic function of their atomic weights.

Eka-Aluminium

The name given by Mendeleev to the undiscovered element predicted to sit below aluminium, later discovered and named Gallium.

Eka-Silicon

The name given by Mendeleev to the undiscovered element predicted to sit below silicon, later discovered and named Germanium.

Henry Moseley

An English physicist who proved in 1913 that atomic number ($Z$) is a more fundamental property of an element than atomic mass using X-ray spectra.

Modern Periodic Law

The principle that the physical and chemical properties of elements are periodic functions of their atomic numbers.

Atomic Number ($Z$)

A value equal to the nuclear charge (number of protons) or the number of electrons in a neutral atom, used as the basis for modern classification.

Periods

The horizontal rows in the Periodic Table, where the period number corresponds to the highest principal quantum number ($n$) of the elements.

Groups

The vertical columns in the Periodic Table, also called families, containing elements with similar outer electronic configurations.

IUPAC

The International Union of Pure and Applied Chemistry, which recommends the modern numbering of groups from 1 to 18.

Lanthanoids and Actinoids

Two series of 14 elements from the sixth and seventh periods, respectively, that are placed in separate panels at the bottom of the Periodic Table.

Glenn T. Seaborg

A scientist whose work on transuranium elements (94 to 102) led to the reconfiguration of the periodic table; element 106, Seaborgium ($Sg$), is named in his honor.