History: A nucleus of hydrogen always has 1 proton, but various forms of hydrogen – called isotopes – are possible. For example, hydrogen-2 (or H-2, otherwise known as deuterium) has 1 neutron. What is the mass number A for this isotope of hydrogen?

A nucleus of hydrogen always has 1 proton, but various forms of hydrogen – called isotopes – are possible. For example, hydrogen-2 (or H-2, otherwise known as deuterium) has 1 neutron. What is the mass number A for this isotope of hydrogen?

• 3
  A = N+Z = 1+1 = 2
• 2
  Correct! A = N+Z = 1+1 = 2
• 0
  A = N+Z = 1+1 = 2
• 1
  A = N+Z = 1+1 = 2