For lighter stable isotopes, the ratio is twice but for heavier stable isotopes the ratio is greater than twice.
Work Step by Step
In case of lighter stable isotopes, the number of protons is nearly equal to the number of neutrons. Consequently, the mass number(combined number of protons and neutrons) becomes twice the atomic number. For stable heavier isotopes, protons are greater in number than neutrons due to which the atomic number and mass number ratio is no longer twice but becomes a higher value.