The set that represent a pair of isotope is 9B AND 11B. Isotopes are chemical element which have two or more forms, these forms have the same number of electrons but different number of neutrons. In an atom of an element, the nucleus contains the neutron and the proton. For a neutral atom, the number of proton and neutron are always the same, while the number of neutron differ. Thus, for the correct option, B stands for the element Boron, which has atomic number of 5. This means that 9B has 5 electrons, 5 protons and 4 neutrons. For 11B, the isotope has 5 protons, 5 electrons and 6 neutrons. 9 and 11 are the mass number of the isotopes; mass number is the summation of number of proton and neutron.
