Bangkok full official ceremonial name is " Krungthep Mahanakorn Amorn Ratanakosin Mahinthara Ayuthaya Mahadilokphop Noparajrachathani Burirom Udomrajanives Mahasathan Amornpimarn Avartarnsthit Sakathathiya Vishnukraam Prasith " therefore both Rattanakosin and Krungthep are part of the full name
As for Bangkok it was originate from the Thai name of the old village in existence since Ayuthya period. Thais pronounce the old name 'Baarng Ma Gork' (_long vowel at Baaaaarng and Gooork). It means the place of which Makork is plentiful_ Makork is some kind of fruit tree, Barng Ma Gork in time became shorten to BarngKork, Barng means a place of which _ _ _ _, you still can find many places, towns or cities in Thailand begin with Barng like in Bangkok you have BarngRaak, Barng Lumpoo, Barng Sue etc
Of course, farangs can not pronounce bangkok the way the Thais pronounce so it became Bangkok, _a place where one can bang his kock
I used to try to correct you a couple time, turned out I was a joker myself, but for this instance I am 100% sure of what I typed