Covalent bond

Covalent Bonds Are Formed by Sharing a Pair of Electrons

     Instead of giving up or acquiring electrons to achieve a filled outer shell, an atom can achieve a filled outer shell by sharing a pair of Electrons. For example- two fluorine atoms can each attain a filled second shell by sharing their unpaired valence electrons.

A bond formed as a result of sharing electrons between two nuclei is called a covalent bond.

A covalent bond is commonly shown by a solid line rather than as a Pair of dots.

Two hydrogen atoms can form a covalent bond by sharing electrons. As a result of covalent bonding, each hydrogen acquires a stable, filled first shell

Similarly, hydrogen and chlorine can form a covalent bond by sharing electrons. In doing so, hydrogen fills its only shell, and chlorine achieves an outer shell of eight electrons.

We just saw that because hydrogen has one shell electron and chlorine has seven valence electrons, each can achieve a filled outer shell by forming one covalent bond.

Newlands' Octaves

     

                           Newlands

   British chemist Newlands (1837 - 1898)  was born in London and studied at Royal college of chemistry.  He set up a practice as an analytical chemist  in 1864 and in 1868 became chief chemist in a sugar refinery.  Later he left the refinery  and worked as an analyst. 

   After the failure of Donbereiner's triads, the next attempt to classify elements was done by Newlands.  By this time 56 elements were discovered. Newlands arranged all these elements in an increasing order of their atomic masses.  He found that every eighth element had properties similar to that of the first.  He compared this to the octaves found in music.  Therefore his classification was known as 'Newlands' Octaves'. 

      Newlands law states that  "When the elements are arranged in an increasing order of their atomic masses, the properties of the eighth element are similar to the first." 

     

• Some features of Newlands table 

🔹Newlands could arrange elements only up to calcium out of total 56 elements known.

🔹 After calcium every eighth element did not posses properties similar to that of the first.

🔹 At the time of Newlands only 56 elements were known but later several elements were discovered.

🔹 In order to fit the existing elements Newland. Places two elements in the same position which differed in their properties.

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https://worldofchemistry0909.blogspot.com/2021/04/donbereiners-triads.html

Donbereiner's Triads

   

          Johann Wolfgang Donbereiner

    Johann Wolfgang Donbereiner  a German scientist (1780 - 1849) studied as a pharmacist at Munchberg in Germany and then studied chemistry at  Strasbourg.  He later became professor of chemistry and pharmacy at the University of  Jena.

    In 1829 he found some groups of three elements which showed similar properties. These groups were called as a triads. ln this triads, atomic masses of the middle elements was approximately the mean of the atomic masses of the other two elements. Also these elements showed similarities in properties. The table below shows four triads arranged vertically. 

         In the above table, take the triads of Lithium, Sodium and Potassium. The Atomic mass of sodium (23) is the mean of the atomic masses of Lithium and Potassium.

    Similarly, you can verify the atomic masses of Strontium, Bromine and Selenium from the other triads.

    The triads were known as Donbereiner's Triads. 

    Donbereiner could identify only some triads  from the elements known other triads did not obey  Donbereiner's rule. Hence, the system of Triads was not useful.

⭐ https://worldofchemistry0909.blogspot.com/2021/03/aromatic-compounds.html