The mass number of a nucleus is?

Analysis

The question asks for the definition of the "mass number" of an atom. To answer this, we need to recall the fundamental structure of an atom and how its subatomic particles contribute to its total mass.

Concept Explanation

An atom consists of a central nucleus containing protons and neutrons, surrounded by electrons orbiting in shells.

• Atomic Number ($Z$): This represents the number of protons in the nucleus of an atom.
• Mass Number ($A$): This represents the total number of nucleons (the particles found inside the nucleus). Since protons and neutrons are the heavy particles that contribute significantly to the mass of an atom, their sum defines the mass number.

The formula is:

Step-by-Step Solution

1. Evaluate Option A: The mass number is always greater than or equal to the atomic number (because it includes neutrons), so this is incorrect.
2. Evaluate Option B: As defined above, the mass number is the total count of protons and neutrons in the nucleus. This matches the scientific definition perfectly.
3. Evaluate Option C: The mass number is an integer (a whole number), while atomic weight is often a decimal value representing the weighted average of isotopes. They are not directly related in this way.
4. Evaluate Option D: The mass number is always a whole number because you cannot have a fraction of a proton or a neutron.

Conclusion

The mass number is defined as the sum of protons and neutrons in the nucleus. Therefore, option B is the correct definition.

Exam Tip: Remember the acronym "MAN": Mass number = Atomic number + Neutrons. This will help you quickly calculate the number of neutrons if you are given the mass number and atomic number in a JAMB question!