If a colon or dot but no group name follows the user name, that user is made the owner of the files and the group of the files is changed to that user's login group.
Group is unchanged if missing, but changed to login group if implied by a period.
A colon may replace the period. The full documentation for chown is maintained as a Texinfo manual. If the info and chown programs are properly installed at your site, the command info chown should give you access to the complete manual.