Periodic Table: History and Basic Properties

Historical Development

From the beginning of the 19th century, scientists began to work on classifying elements and understanding their relationships. Because known and newly discovered elements were extremely confusing. In 1829, Johann Wolfgang Döbereiner took the first important step by classifying some elements with similar chemical properties into groups of three. In 1864, John Newlands, in his study in which he ranked the elements according to increasing atomic weights, noticed that every eighth element showed similar properties and called this repetition the Octave Law, but these systems were inadequate for some exceptions and newly discovered elements.

In 1869, Dmitri Mendeleyev arranged the 62 elements known at that time in order of increasing atomic weight and grouped those with similar properties one below the other. He left gaps in his table for some elements that had not yet been discovered at that time. In fact, within a few years, Mendeleyev's predictions were confirmed with the discovery of the elements scandium, gallium and germanium.

Mendeleev created the first version of the periodic table we know today by placing elements with similar properties in the same vertical column.

At the same time as Mendeleyev, German chemist Lothar Meyer developed a similar classification of elements, but Mendeleyev's table gained wider acceptance in the scientific world thanks to its accurate predictions.

In the 1890s, William Ramsay and other researchers discovered a new group of elements known as the noble gases (helium, neon, argon, etc.). Argon, discovered in 1895, did not fit into any group in the table at the time, so a new group numbered 0 was created and the other noble gases were placed in this group.

With the understanding of atomic structure in the early 20th century, the scientific principle on which the periodic system is based also became clear: In 1913, Henry Moseley accepted that the size that should be taken as a basis in the ordering of elements is not the atomic weight but the number of protons in the nucleus.

Especially after World War II, under the leadership of Glenn T. Seaborg, elements heavier than uranium were discovered one after another and added as a separate row, thus the periodic table took its current form. Today, there are a total of 118 elements in the table.

Basic Structure of the Periodic System

The periodic table is a system in which elements are arranged according to increasing atomic numbers (proton numbers) and similar properties are repeated at certain intervals.

Horizontal rows are called periods, and vertical columns are called groups. Each period shows the number of shells (energy levels) that the atoms in that period have; for example, all elements in period 3, from sodium to argon, have 3 electron shells. As you move from left to right across a period, the atomic number of the elements increases by one.

Vertical groups bring together elements with similar chemical properties because atoms in the same group have the same number of valence electrons. For example, Group 1 elements (alkali metals) have one valence electron, while Group 17 elements (halogens) have seven valence electrons.

Classification of Elements

The elements in the periodic system can be generally divided into two broad categories, metals and nonmetals, according to their properties, and in more detail, Alkali Metals, Alkaline Earth Metals, Halogens, Noble Gases, Transition Metals, Inner Transition Metals, Lanthanides, Actinides, Nonmetals, Semimetals can be listed.

The table shows not only the individual properties of the elements but also their general trends, making it possible to predict the expected properties of an unknown element or to compare the reactivity of two elements.