Question 10.5: Adenosine triphosphate (ATP) is sometimes called the “univer...
Adenosine triphosphate (ATP) is sometimes called the “universal energy carrier” or “molecular energy currency.” It contains two high-energy bonds (shown in red) that, when hydrolyzed (broken by the addition of water), release the energy necessary for cell function. Resonance stabilization of the hydrogen phosphate ion is one of the reasons the breakdown of ATP releases energy.
Draw all the possible resonance structures for the hydrogen phosphate ion (HPO_{4}^{2-}). Use curved arrows to indicate how electrons are repositioned, and determine the position(s) of the negative charges.
Strategy Draw a valid Lewis structure for HPO_{4}^{2-}, and determine whether and where electrons can be repositioned to produce one or more additional structures. Indicate the movement of electrons with curved arrows, and draw all possible resonance structures. Calculate the formal charge on each atom to determine the placement of charges.
Setup A valid Lewis structure for the hydrogen phosphate ion is
For the purpose of determining formal charges, P and O have five and six valence electrons, respectively.
A lone pair can be moved from one of the oxygen atoms to create a double bond to the phosphorus, and a pair of electrons from the original double bond can be moved onto that oxygen atom. The net result is simply a repositioning of the double bond by moving two electron pairs. This can be done once more, giving a total of three resonance structures for HPO_{4}^{2-}. In each of the resonance structures, the formal charge on phosphorus is [5 – (5)] = 0. The formal charge on each singly bonded oxygen is [6 – (1 + 6)] = -1, and the formal charge on the doubly bonded oxygen is [6 – (2 + 4)] = 0.
