The periodic table is a systematic arrangement of elements in groups and periods. Several scientists have attempted to arrange elements in a table. In 1669 German merchant and amateur alchemist Hennig Brand attempted to create a Philosopher’s Stone; an object that supposedly could turn metals into pure gold. He heated residues from boiled urine, and a liquid dropped out and burst into flames. This was the first discovery of phosphorus.
In 1863 English chemist John Newlands divided the then discovered 56 elements into 11 groups, based on characteristics.
In 1869 Russian chemist Dimitri Mendeleev started the development of the periodic table, arranging chemical elements by relative atomic mass . He predicted the discovery of other elements, and left spaces open in his periodic table for them.
In 1894 Sir William Ramsay and Lord Rayleigh discovered the noble gases, which were added to the periodic table as group 0.

The modern periodic law was postulated by Henry Moseley. It states that the physical and chemical properties of elements are a periodic function of their atomic numbers. This law explains that when elements are arranged according to their atomic numbers, their properties appear similar periodically. Elements are arranged in the modern periodic table according to their electronic configuration.

The electron configuration of an element describes how electrons are distributed in its atomic orbitals. For a neutral atom, the atomic number which is the proton number is equal to its electron number.

Electrons which are negatively charged atomic particles are arranged in shells and increasing energy levels from lowest to highest. The shells are arranged from the one closest to the nucleus to those farthest from the nucleus. The maximum number of electrons a shell can accommodate can be calculated using: 2n²

The modern periodic table is divided into eight vertical columns called groups and seven horizontal rows called columns. Groups are vertical columns. There are 8 groups in the modern periodic table numbered I to VIII (group 0). Elements are arranged in groups according to the number of valence electrons in their outermost shell. Hence, elements in the same group have the same number of valence electrons in their outermost shell.

Hydrogen does not fit into any group due to its unique chemical properties. Hydrogen can have properties of metals or non-metals in certain conditions. It is however placed in group 1 for convenience. Helium is also placed in Group 0 because of its chemical properties as it posseses a stable dublet configuration making it chemically unreactive (inert) which is similar to that of noble gases although it has two valence electrons in its outermost shell.

The transition elements which possess variable oxidation states are placed between groups 2 and 3.

There are seven periods in the modern periodic table. Elements are placed in periods according to their number of electron shells. Hence, elements in the same period have the same number of electron shells. The number of valence electrons increases by one across the period from left to right.

The inner transition elements called Actinides and Lanthanides are placed in periods 6 and 7.

Elements are classified in the periodic table according to their chemical properties and electronic configuration.

• Chemical properties

Elements are classified in the periodic table according to chemical properties into three:

1. Metals
These are elements that ionize by loss of electrons. They usually lie between groups 1 to 3.

2. Non metals
These are elements that ionize by gain of electrons. They are usually gases. They usually lie between groups 5 to 0.
3. Metalloids
These are elements that are intermediates between metals and nonmetals.

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• Electronic configuration

Elements can also be classified using subshell electronic configuration. Elements can be classified according to the SPDF configuration.