If a stable neutral atom has a mass number of 31, the number of electrons and neutrons respectively are

Analyze the question: We are given only the mass number (31) of a stable neutral atom. We need the number of electrons and neutrons.

Concept:

• Mass number (A) = protons + neutrons
• Atomic number (Z) = number of protons
• In a neutral atom, electrons = protons

The question doesn't give the atomic number directly, so we have to identify the element. Mass number 31 corresponds to the stable isotope of phosphorus (P-31), which has atomic number 15. This is the only stable isotope of phosphorus, and it is the well-known one with mass number 31.

Solution:

• Protons = 15 (atomic number of phosphorus)
• Electrons = protons = 15 (the atom is neutral)
• Neutrons = A − Z = 31 − 15 = 16

So the electrons and neutrons are 15 and 16 respectively.

Checking the other options:

• B (16 and 31): neutrons can't equal the whole mass number, because protons are also part of it.
• C (15 and 31): same problem; the neutron count is wrong.
• D (16 and 15): this reverses the numbers.

Tip: For "electrons and neutrons" questions, remember: electrons = Z for a neutral atom, and neutrons = A − Z. Also memorise the first 20 elements and their atomic numbers (P = 15, S = 16, Cl = 17), since JAMB often gives only the mass number and expects you to recognise the element.