In atomic structure, the number of neutrons in an atom is equal to which of the following expressions?

Introduction / Context:
Understanding the basic structure of the atom is fundamental in chemistry and physics. Atoms are made up of protons, neutrons, and electrons, and simple relationships between mass number and atomic number allow us to calculate the number of each type of particle. This question specifically tests whether you know how to find the number of neutrons using the mass number and atomic number of an atom.

Given Data / Assumptions:

• Mass number is usually denoted by A and is the total number of protons and neutrons in the nucleus.
• Atomic number is usually denoted by Z and is the number of protons in the nucleus.
• For a neutral atom, the number of electrons equals the number of protons, but that is not directly used to compute neutrons.
• We assume we are dealing with neutral atoms and standard definitions from basic chemistry.

Concept / Approach:
The nucleus of an atom contains protons and neutrons. The mass number A equals the sum of protons (Z) and neutrons (N): A = Z + N. Rearranging this relationship, the number of neutrons is N = A - Z. This simple formula is the key to answering the question. Atomic number alone gives only the number of protons. The mass number alone gives the combined count of protons and neutrons, but not neutrons separately. Therefore, the only correct expression for the number of neutrons is mass number minus atomic number.

Step-by-Step Solution:
1. Start with the definition that mass number A is the total of protons plus neutrons: A = Z + N.2. Atomic number Z is defined as the number of protons in the nucleus.3. To isolate N, subtract Z from both sides of the equation A = Z + N.4. This gives N = A - Z, which means neutrons are found by subtracting atomic number from mass number.5. Compare this with the options and note that the expression mass number - atomic number matches N = A - Z.6. Therefore, the number of neutrons equals mass number minus atomic number.

Verification / Alternative check:
Consider an example such as carbon 12, which has mass number 12 and atomic number 6. Using the formula N = A - Z, we get N = 12 - 6 = 6 neutrons. This matches the known composition of a carbon 12 nucleus with 6 protons and 6 neutrons. Another example is oxygen 16, with A = 16 and Z = 8, giving N = 16 - 8 = 8 neutrons. These checks confirm that the expression is correct for typical elements studied in school level chemistry.

Why Other Options Are Wrong:
Option A, mass number, is incorrect because it counts both protons and neutrons together. Option B, atomic number, counts only protons, not neutrons. Option D, number of electrons, equals the number of protons only in a neutral atom and does not directly give the neutron count. None of these alone can provide the neutron number without additional information, while mass number minus atomic number directly gives the number of neutrons.

Common Pitfalls:
Students sometimes confuse mass number with atomic mass or think that mass number directly equals neutrons, which is wrong. Another pitfall is to treat atomic number as the total number of particles in the nucleus. Remembering the simple relationship A = Z + N and writing it down before answering can prevent these mistakes. Always check whether the question is asking for protons, neutrons, or electrons and use the appropriate formula.

Final Answer:
The number of neutrons in an atom is equal to mass number - atomic number.