I know 1 atom of uranium-238 isn't critical mass, but theoretically, if you could split 1 atom of uranium-238, how much energy would be produced? 

~ee

Are you sure you don't mean U-235, as U-238 isn't normally fissionable (no amount of it will produce a critical mass)

for U-235, 215 million electron-volts is the average energy released per nucleon.

You don't need critical mass for fission of individual nucleons to occur.  Critical mass is only needed to sustain a chain reaction, where the neutrons produced by the fission of nucleons cause fission in further nucleons at an ever increasing rate (for weapon grade) or a evenly sustainable rate (for nuclear fuel grade).

Janus said most of it, but U238 is fissionable by means of (fast) neutrons.

Quote

Wiki

Uranium-238 (²³⁸U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239.

Does this help or do you want to expand on what you were thinking of?

Sorry, i meant uranium-235, and I found my answer:

 

The fission of one atom of uranium-235 generates 202.5 MeV = 3.24E−11 J, which translates to 83.14 TJ/kg.

This is around 2.5 million times more than the energy released from burning coal.

https://en.m.wikipedia.org/wiki/Uranium-235#Fission

Edited June 10, 20187 yr by Elite Engineer

On 8.06.2018 at 8:47 PM, Elite Engineer said:

I know 1 atom of uranium-238 isn't critical mass, but theoretically, if you could split 1 atom of uranium-238, how much energy would be produced?

Uranium-238 has two decay modes.

U-238 -> Th-234 + He-4 + 4.27 MeV

and rarely known even by scientists double beta decay minus:

U-238 -> Pu-238 + e- + e- + Ve + Ve + 1.144 MeV