A covalent bond between 2 phosphate groups contains much more energy than a bond between a sugar and a phosphate.
If you write out the structural formula for ATP, you'll find that the bonds between the 1st and 2nd phosphates and the bond between the 2nd and 3rd phosphates are between 2 double-bonded oxygens (which are bonded to the phosphorous atoms). This is a situation that requires a lot of energy to create (because those double-bonded oxygen atoms don't really want to be that close to each other, so to speak), and a lot of the energy used to jam them together is still there in the covalent bond. That large amount of energy is released when the bond is broken.
The bond between the sugar and the 1st phosphate group requires less energy to form, so it contains less energy.
If you write out the structural formula for ATP, you'll find that the bonds between the 1st and 2nd phosphates and the bond between the 2nd and 3rd phosphates are between 2 double-bonded oxygens (which are bonded to the phosphorous atoms). This is a situation that requires a lot of energy to create (because those double-bonded oxygen atoms don't really want to be that close to each other, so to speak), and a lot of the energy used to jam them together is still there in the covalent bond. That large amount of energy is released when the bond is broken.
The bond between the sugar and the 1st phosphate group requires less energy to form, so it contains less energy.
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Breaking off one of the three phosphate groups to to form ADP releases A LOT of energy.
It is exergonic/exothermic/catabolic action so it releases free energy.
It is exergonic/exothermic/catabolic action so it releases free energy.