How to differentiate elements as metals and non metals by looking at their valencies?

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  • Valence shell is the outermost shell of an atom.
  • The electrons present between the valence shell are known as valence electrons.
  • Each & every element always try to become happy by completing its own Octet ie. by keeping 8 electrons in its valence shell.
  • There are some elements who become happy by stealing the electrons of the other elements. Such thieves elements are called Non-Metals.
  • Other innocent elements whose electron get stolen are called Metals.
  • Lets take the favourite example of students ie. formation of NaCl
    The Electronic configuration of Sodium (Na) is,

  • K=2 , L=8, M = 1
  • 1s2,2s2,2p6,3s1
  • While the Chlorine ( Cl ) have Electronic configuration,

  • K=2, L=8, M= 7
  • 1s2,2s2,2p6,3s2,3p5

  • Each & Every element always try to complete its octet by keeping 8 electrons in its valence shell.

  • Sodium hates it own electron from its valence shell because there is only one electron in its valence shell (K=1). Sodium has already completed the octet in previous inner shell of valence shell (L=8). That is why Sodium don't want that 1 one electron situated in valence shell. Sodium is sad now. :(

  • On the other hand, Chlorine ( Cl ) has 7 electron in its valence shell so Chlorine (Cl) want only one Electron in its valence shell to become happy by completing its octet. Hence due to lack of 1 electron Chlorine is also upset. :(

  • That is why Chlorine steals that unwanted electron which was present in valence shell of Na.

  • Now, Outermost shell of Na ie. shell-M disappears because there is no any Electron. It leads to the Inner previous shell-L to be the outermost shell of Na. ie. Valence shell of Na which contains already 8 Electrons. So Now Na is happy! :)

  • Initially Chlorine had 7 electrons in its valence shell, Just now, Chlorine have stolen that 1 unwanted electron of sodium. So now Cl has 7+1=8 Electrons in its valence shell. It means octet is completed. So Cl is also happy now ! :)

  • Sodium (Na) lost its 1 electron, it means Na lost its 1 -ve charge ie. Na got 1 +ve charge. Hence Sodium (Na) become Cation.

  • Chlorine (Cl) steals the electron & hence it got 1 -ve charge ie. Chlorine (Cl) become Anion.

  • Both Na & Cl made happy each other, Na has 1 +ve charge while the Cl have 1 -ve charge. Since Equal & Opposite charges attract each other. So Sodium (Na) & Chlorine (Cl) Sticks together & forms NaCl, Because both made happy each other ! :)

  • Your question asks, "How to know the element whether it is metal or non-metal by using number of valence shell electron?" Right?
    That is why I've given you this funny example, by using this example you can conclude that,

  • Non-Metal :- The element who becomes happy (completes its octet) by stealing the electrons of other element. eg. Mostly gases like Chlorine.
  • Metal :- That innocent element who becomes happy when its electrons got stolen. eg. Na

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