The isotopes of hydrogen are often referred to by their common names. Write the chemical formula for the isotope deuterium. Include the mass number and the atomic number. - Write the answer in the form AX using the atomic symbol button in the toolbar, where A is the mass number, Z is the atomic number, and X is the chemical symbol for the unknown element. The isotopes of hydrogen are often referred to by their common names. Write the chemical formula for the isotope deuterium. Include the mass number and the atomic number. - Write the answer in the form AX using the atomic symbol button in the toolbar, where A is the mass number, Z is the atomic number, and X is the chemical symbol for the unknown element