An [atom:]entry element must have a [atom:]title and [atom:]updated child.
Order doesn't matter except that children of the same type must be grouped together.
You can put the whole content of the entry inside a "content" element:
<atom:entry> <atom:title>ATOM is Cool</atom:title> <atom:update>2005-04-13T16:11:00-7:00</atom:updated> <atom:content type="xhtml"> <div xmlns="http://www.w3.org/1999/xhtml"> <p>RSS is nice but ATOM is cool because I can properly embed XHTML. That means I don't need any funky business to process this text.</p> </div> </atom:content> </atom:entry>