Mass number means the total number of protons and total number of neutrons present in an atom. Mass number is written at the top left of the symbol of atom.
Whereas atomic number means the total number of protons present in the atom. Atomic number is written at the bottom left of the symbol of atom.
For example, in , the number 12 denotes mass number of carbon atom whereas 6 denotes the atomic number of carbon atom.
Thus, we can conclude that in the symbol , the superscript 4 is the mass number for helium and the subscript 2 is the atomic number for helium.
The above reaction is least energetic because there is no phosphoanhydride bond present with adenosine mono phosphate.Phospho anhydride bond is an energy rich bond.
As a result hydrolysis of AMP generates very little amount of energy in comparison to the hydrolysis of ATP and ADP.