There is no ELEMENT with the symbol "DO" however, it would be the molecule which is one atom of deuterium (D) and one atom of oxygen (O). Deuterium is an isotope of hydrogen. This molecule would be called deuterium oxide.
iron
Hydrogen
Chlorine.
there is no such element
lithium
Nitrogen
Rubidium.
Barium
Carbon
Arsenic
No element has the symbol A
there is no element with the symbol A.