Answer:
An electron is a negatively charged subatomic particle and a proton is a positively charged subatomic particle. Positive charge(s) attract negative charge(s) and vice versa. The proton and neutron stay together and attract one another to give the atom an overall charge of zero (neutral). Which is the charge of an atom. When there is an unequal number of protons and neutrons an ion is formed. If the number of protons are more than the electron, a positively charged ion called cation is formed. On the other hand, if the number of electrons are more than the protons a negatively charged ion called anion is formed. 
 
        
                    
             
        
        
        
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Answer:
 was difficult to place isotopes of elements as they have the same chemical properties but different atomic masses. It was not possible to predict how many elements could be discovered between two heavy elements as the rise in atomic mass is not uniform.