The increase in the number of atoms allows the strong positive charge of the nucleus to increase. Thus, due to the number of positive protons increasing in the nucleus, the positive charge increases. On the negatively charged electron cloud, the high positive charge of the nucleus has a strong tug.
So the atomic number increases with each element. The atomic mass is the sum of the protons and neutrons. As the atomic number increases it causes an increase in the atomic mass. However sometimes there is not a corresponding increase in the number of neutrons...
