The command you want is \sffamily. I don't know why bold is a series (other font weights are too; it's probably a typographical term), but it should be clear why small caps and italics are shapes; sans-serif fonts are a family of fonts, hence the name. (While I understand the logic, it does feel like consistency would be niceā¦.) There's a list in a TeX FAQ answer:
The default set of modal commands offers weights \mdseries and \bfseries, shapes \upshape, \itshape, \scshape and \slshape, and families \rmfamily, \sffamily and \ttfamily.
The commands, in order, activate medium weight, boldface, upright, italic, small caps, slanted, roman (serif), sans serif, and typewriter text (monospaced). To form the \textXX commands, the first two letters of the modal commands are always used.