/PERIODIC_TABLE/ATOM is an XPath expression that selects all ATOM elements.
/PERIODIC_TABLE/ATOM[1] is an XPath expression that selects the first ATOM element.
/PERIODIC_TABLE/ATOM/SYMBOL is an XPath expression that selects all SYMBOL elements.
/PERIODIC_TABLE/ATOM[ATOMIC_NUMBER="3"] is an XPath expression that selects Lithium.
//ATOM[SYMBOL="He"]/DENSITY/attribute::UNITS is an XPath expression that selects the units in which the density of Helium is given.
/child::PERIODIC_TABLE/child::ATOM/child::* is an XPath expression that selects all the child elements of ATOM elements.
round(number(/PERIODIC_TABLE/ATOM[SYMBOL="Fe"]/ATOMIC_WEIGHT) - round(number(/PERIODIC_TABLE/ATOM[SYMBOL="Fe"]/ATOMIC_NUMBER))
is an XPath expression that returns the approximate number of neutrons in iron.
/descendant::* is an XPath expression that selects all the elements in the document.
/descendant-or-self::node() is an XPath expression that selects all non-attribute, non-namespace nodes in the
document.
/descendant-or-self::node() | /descendant::*/attribute:* | /descendant::*/namespace:* is an XPath expression that selects all
nodes in the
document.