BECE Science 2026 Paper

Concept:Atoms with the same element symbol but different mass numbers are isotopes.Isotopes have the same number of protons but different numbers of neutrons.Explanation:In the symbol $^{12}_{6}\mathrm{X}$, the bottom number $6$ is the atomic number.The atomic number equals the number of protons and, in a neutral atom, the number of electrons.The top number $12$ is the mass number, which equals protons + neutrons.For $^{12}_{6}\mathrm{X}$: number of neutrons $= 12 - 6 = 6$.For $^{14}_{6}\mathrm{X}$: number of neutrons $= 14 - 6 = 8$.Both atoms have $6$ protons, so they have the same atomic number and same electron number.They differ only in their neutron numbers.Answer:D. neutron numbers.