Q:
      
      
         
            
An atom has 7 electrons in its M - shell and contains 18 neutrons in its nucleus. What is its mass number
         
       
      
      
      
          
      
      
          Answer & Explanation
         Answer: C) 35         
         
Explanation: According to Bohr model, Each shell can occupy 2 electrons in their orbits where n=1,2,3.. for K,L,M,..
 
The atom has 2 electrons in k, 8 electrons in L and 7 electrons in M = 2 + 8 + 7 = 17
 
And given that its nucleus has 18 neutrons
 
=> its mass number = number of (electrons + neutrons ) = 17 + 18 = 35
       
      
      
      
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