What is an Element ? It's History, Classification and Various theories given by different Scientists to arrange the elements in the periodic table...!!

What is an Element ? It's History, Classification and Various theories given by different Scientists to arrange the elements in the Periodic Table.

The study of Elements is really a very vast and fascinating topic.

What is an Element ?

An element is a substance that is entirely made up of one type of atoms. Everything around us whether living or non-living is made up of elements.Elements are distinguished on the basis of their atomic number , which is the number of protons in the nucleus of their atoms . Elements are classified as Metals , Non-Metals and Metalloids. Atoms of the same element have different numbers of neutrons in their nuclei. That means that atoms of the same element can also vary in mass.

Short History of Elements :-

In the Ancient times People were aware of only a few basic elements like gold, silver and copper, as these elements were easily discovered in nature and were relatively simple to mine with their primitive tools. 

Aristotle Theory for defining a Element :-

Nearly around at the end of 329 B.C Aristotle a Greek Philosopher proposed that everything around us is made up of a mixture of one or more of four "roots". According to him these four roots were earth, water, air and fire. These roots were later renamed by another very famous Greek philosopher Plato as Elements . Both of these Aristotle and Plato further did nothing to discover more about elements. Thus up to 1 A.D. Only the elements like Gold, Silver, Copper, Iron, Lead, Tin, Mercury, Sulfur, Carbon Were discovered. After this up to 16th century other scientists discovered a few more elements like Phosphorus, Arsenic and Zinc.

Boyle Theory for defining a Element :-

In 1661 an English scientist Boyle Objected that it was impossible to combine the four Greek elements to form any substance and it was equally impossible to extract these elements from a substance. He therefore proposed a new definition of an element which said that an element is a substance that cannot be broken down into a simpler substance by a chemical reaction. This defination was followed for more than 300 years and even today is taught in a chemistry lecture. After this one after the other different Scientists discovered many new elements like cobalt, platinum, nickel, bismuth, hydrogen, nitrogen, oxygen, chlorine, manganese, tungsten, uranium, chromium, potassium, sodium, lithium, boron, magnesium, iodine and many more. 

After the discovery of so many elements now the Scientists were very curious to become the first to classify the elements.

Johann Wolfgang Dobereiner Theory for classification of elements :-

Thus in 1829 Johann Wolfgang Dobereiner, a German Scientist found a some groups of 3 elements which showed similar properties. He called these groups as Triads In these triads Atomic mass of the middle element was approximately the mean of the atomic masses of other two elements. Also these elements showed similar properties .

Eg. :- Lithium  Li 69

          Sodium  Na 23

       Potassium K 39

In the above triad made by Dobereiner the atomic mass of sodium is the mean of the atomic masses of lithium and potassium. These triads were known as Dobereiner's Triads. But Dobereiner could identify only a few triads from the elements. Other triads do not obey Dobereiner's rule. Hence the system of triads was not useful to classify the elements. By 1870 nearly 63 elements were discovered,due to this many Scientists were attracted towards the ways of classifying the elements

John Newlands Theory for classification of elements :-

After Dobereiner John Newlands an English chemist in 1865 arranged all these elements in an increasing order of their Atomic masses. He found that " Every 8 th element had properties similar to that of the first " .

Eg. :-

H    Li    Be   B   C   N   O

F    Na   Mg  Al  Si   P    S

Cl   K    Ca   Cr  Ti  Mn Fe

Here H is the first element and F is the Eighth element for it. Hence the properties of H and F were similar. Similarly Li is the first element and Na is the Eighth element for it. Hence they both have similar properties. But in the above table after Calcium every eighth element did not posses properties similar to that of the first. Also this periodic table did not include Inert gases, Noble gases etc.

Hence this theory was also not usefull. 

Dmitri Mendeleev Theory for classification of elements :-

In 1866 after Newland, A Russian chemist Dmitri Mendeleev examined the relation between the atomic masses of the elements and their physical and chemical properties. By analysing all the elements Mendeleev believed that atomic mass of the elements was the most fundamental property in classifying the elements . He arranged the elements in the increasing order of their atomic masses and found that the physical and chemical properties of the elements showed repetition after certain intervals. Proceding in this manner, he was able to arrange all the known elements according to their properties and thus created the first periodic table containing 63 elements.

Main Features of Mendeleev's Periodic table :-

1. Mendeleev was the first to successfully classify all the known elements.

2. Mendeleev's Periodic Law :-

The physical and chemical properties of elements are the periodic functions of their atomic masses.

3. Mendeleev kept some blank spaces in his periodic table.This vacant spaces were for elements that were yet to be discovered.

Demerits of Mendeleev's Periodic table :-

1. Hydrogen resembles alkali metals as well as halogens. Therefore no fixed position could be given to hydrogen in the periodic table.

2.There was no place for the isotopes of the various elements, which were discovered later.

3.Mendeleev's table did not include any of the noble gases.

However in 1913 Sir Henry Moseley,an English Physicist discovered that atomic number is the most fundamental property of an element and not it's atomic mass. This discovery changed the whole perspective about elements and their properties. Also the Mendeleev 's periodic law was modified to Modern Periodic Law.

Modern Periodic Law :- The chemical and physical properties of elements are the periodic function of their Atomic numbers.

The next post would give you more knowledge about Modern Periodic Table and Classification of elements in it..!!

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