The atomic radius of a chemical element is a measure of the size of its atoms, usually the distance from the nucleus to the boundary of the surrounding cloud of electrons.

    

    
Explaining periodic trends in atomic radius. See the section on factors affecting the valence shell above.
     
    

        

            
trend
            
valence

            n
            
Z
            
core

            electrons
            
net effect on atomic radius
        
        

            
going right across main group rows...
            
no change
            
increases
            
no change
            
the increase in Z causes a decrease in radius
        
        

            
going right across transition series...
            
no change
            
increases
            
increases
            
the increase in Z causes a decrease in radius, but the increase in the number of core electrons causes an increase. The two competing effects cause a small decrease, then small increase!
        
        

            
going down groups...
            
increases
            
increases
            
increases
            
three competing effects; but n is strongest, so radius increases.
        
    

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Z causes decrease in rAdius and electrons causes increase..which one is dominating??