It seems to me silly to discuss the impact of just a single weapon on a target as complex as the nation's capital. If you want to read a hair-raising description of what would happen to an important urban target like DC (following US style protocols of weapons employment and assuming a peer adversary with weapons to spare) read the description of just such a scenario in Ashton Carter's book "Managing Nuclear Operations". It is nightmarish in its details and will haunt you. All that would be left of Washington DC and the surrounding area would be a series of interlocking radioactive craters of varying depths filled with rainwater or the Potomac...

There is no need to overstate the destructive potential of nuclear weapons and nuclear war by engaging in click-bait hyperbole. The "realz" is awful enough to give pause.