Which of these pairs of atoms are isotopes?
1. Atom A: 13 protons, 12 neutrons and 12 electrons. Atom B: 12 protons, 12 neutrons and 12 electrons
2. Atom A: 12 protons, 12 neutrons and 12 electrons. Atom B: 12 protons, 12 neutrons and 13 electrons
3. Atom A: 12 protons, 12 neutrons and 12 electrons. Atom B: 12 protons, 13 neutrons and 12 electrons
