Electron Shells: Energetic Orbits within Atomic structures:

Electrons, the subatomic particles with negative charge, orbit the nucleus of an atom in distinct energy levels known as electron shells. These shells, labeled as K, L, M, and so on, are arranged in increasing order of energy. Charge carriers fill these orbits sequentially, with the innermost shell holding the lowest energy charge carriers and subsequent shells accommodating higher energy negative charge carriers. 

• Bohr and Bury suggested the distribution of electrons into different orbits of an atom. 

What rules govern Electron distribution in Atomic shells, and what is the maximum capacity in each orbit?

• The following rules are followed for writing the number of negative charge carrier in different energy levels or shells: 
  • The maximum number of negative charge carrier present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1, 2, 3,…. 
    • Hence the maximum number of negative charge carrier in different shells are as follows: 

• first orbit or K-shell = 2 × 12 = 2, 

• second orbit or L-shell will be = 2 × 22 = 8, 

• third orbit or M-shell will be = 2 × 32 = 18, 

• fourth orbit or N-shell will be = 2 × 42 = 32, and so on. 

• Maximum Electrons: Outer Orbits and Inner Shells: The maximum number of negative charge carriers that can be accommodated in the outermost orbit is 8. 
• Subatomic particles are not accommodated in a given shell, unless the inner shells are filled.

Valency in Atoms: Stability through Electron interaction

• Valency in Atoms: Electron Stability and Chemical Reactivity: The subatomic particles present in the outermost shell of an atom are known as the valence electrons. 
  • It was observed that the atoms of elements, completely filled with 8 charge carriers in the outermost shell show little chemical activity i.e their combining capacity or valency is zero. 
  • Of these inert elements, the helium atom has two charge carriers in its outermost shell and all other elements have atoms with eight negative charge carriers in the outermost shell.
• The combining capacity of the atoms of elements, i.e their tendency to react and form molecules with atoms of the same or different elements, was thus explained as an attempt to attain a fully-filled outermost shell. 
  • An outermost-shell, which had 8 negative charge carriers was said to possess an octet. 
  • Atoms would thus react, so as to achieve an octet in the outermost shell by sharing, gaining or losing negative charge carriers. 
• The number of negative charge carriers gained, lost or shared so as to make the octet of the negative charge carrier in the outermost shell, gives the combining capacity of the element, that is the valency. 
  • Example: Hydrogen/ Lithium/Sodium atoms contain 1 negative charge carrier each in their outermost shell, therefore each one of them can lose one negative charge carrier. 
    • So, they are said to have a valency of 1. 
    • Similarly valency of magnesium and aluminum is 2 and 3, respectively. 
• If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way.
  • Example: The fluorine atom has 7 negative charge carriers in the outermost shell and it is easier to gain one negative charge carrier instead of losing seven negative charge carriers. 
  • Hence valency of fluorine is 1.

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