BECE Science 2011 Paper

Concept:In nuclear notation ${}^{A}_{Z}\mathrm{X}$, the superscript $A$ is the mass number and the subscript $Z$ is the atomic number.Explanation:For ${}^{25}_{12}\mathrm{X}$, mass number $A = 25$ and atomic number $Z = 12$.The atomic number $Z$ gives the number of protons in the nucleus.Therefore, number of protons $= 12$.Mass number is the sum of protons and neutrons:$A = \text{protons} + \text{neutrons}$$25 = 12 + \text{neutrons}$$\text{neutrons} = 25 - 12 = 13$Thus, the atom has 13 neutrons and 12 protons.Answer:13 neutrons and 12 protons.