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>