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
Chlorine.
Hydrogen
lithium
there is no such element
Nitrogen
Rubidium.
Carbon
Barium
Arsenic
No element has the symbol A
there is no element with the symbol A.