The Atom editor is not available via the Ubuntu software repository (i.e. via the
apt tool), so it has to be installed by downloading the
.deb file (e.g.
atom-amd64.deb) and then manually installing. I have had some trouble with the Ubuntu GUI software manager, Ubuntu Software (I am running Ubuntu 16.04) so I prefer the command line when possible.
In order to “manually” install the .deb file I referred to this superuser.com entry, which worked perfectly. My terminal session, from the directory containing the .deb file:
And that’s it. No dependencies, it just installed. See also this unix.stackexchange entry.