The periodic table- Definition and classifications

Published by Daniel Ozori on

The periodic table

The periodic table is the most important classification in chemistry. it is the tabular display of the arrangement of elements in order of increasing atomic number

so that elements with similar atomic structures and chemical properties appear in vertical columns.

The atomic number of an element is calculated by the number of protons present in the atom.

In an atom the number of protons is equal to the number electrons making it electrically neutral.

The basic assumption behind it is the “periodic law” which states that “the properties of the elements are a function of their atomic numbers“.

The periodic table was invented in 1898 by Mendeléev a Russian scientist,

He pointed out that elements with similar properties kept recurring at regular intervals or periods.

Mendeléev’s table had many gaps that he predicted were undiscovered elements which could fill the gaps and stated their possible properties.


How the periodic table works

The key to how elements are placed in the periodic table is in the electronic configuration of their atoms.

As said in other articles on this blog, atoms are made up of a positively charged nucleus  ( the core of the atom containing the protons and neutrons ) with negatively charged electrons revolving around it at given shell or energy levels located at different distances from the nucleus.

These shells are named K, L, M, N, O…….and these correspond with the levels numbered 1, 2, 3, 4, 5……..starting from the shell closest to the nucleus.

Electrons in the shell closets to the nucleus possess less energy compared to the nucleus to the ones in the outermost shell but every electron in the same shell has the same amount of energy.

The maximum possible number of electrons in a given shell is calculated with the formula 2n2, where n is the level number of the given shell.

Example: K has the shell level of 1 giving 2(1)2 = 2, so the maximum number of electrons that can be found in the first shell is 2.

The modern periodic table is divided into Groups and Periods.

Groups are the vertical columns of elements numbered from 0 t 7. Elements in the same group have the same number of electrons in the outermost shell of their atoms, i.e. they have the same number of valence electrons.

Periods are the horizontal rows of element numbered from one to seven, elements in the same period have the same number of electron shells K, L, M, N, O……


Classifications of the periodic table

Alkali metals: The alkali metals make up most of Group 1, the table’s first column. They react with cold water vigorously to liberate hydrogen gas and form liberate hydrogen gas and form alkalis; so they are known as the alkali metals. The oxides of alkili metals also dissolve in water to form very strong alkilis.

Alkaline-earth metals: 



Transition metals: 

Post-transition metals: 




Noble gases: 





1 Comment

How to memorize the periodic table | Easy Chemistry · November 24, 2019 at 8:31 pm

[…] all got to admit that memorizing the periodic table has proven challenging, even just the first 20 elements isn’t so easy for most […]

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