The correct option is **B. The sum of the number of protons and neutrons present in the nucleus.** ### Detailed Explanation: 1. **Understanding Mass Number**: - The mass number (denoted as \( A \)) of an atom is defined as the total number of protons and neutrons in its nucleus. - Protons are positively charged particles, while neutrons are neutral particles. Together, they make up the bulk of an atom's mass. 2. **Formula**: - The mass number can be expressed mathematically as: \[ A = Z + N \] where: - \( A \) = mass number - \( Z \) = number of protons (also known as the atomic number) - \( N \) = number of neutrons 3. **Why Option B is Correct**: - Since the mass number is the sum of protons and neutrons, it directly reflects the total number of these nucleons in the nucleus. For example, if an atom has 6 protons and 6 neutrons (like carbon-12), its mass number is \( 6 + 6 = 12 \). ### Why the Other Options are Incorrect: - **Option A: Always less than its atomic number**: - This statement is incorrect. The mass number is not always less than the atomic number. In fact, for most elements, the mass number is greater than the atomic number because it includes both protons and neutrons. For example, in carbon (with an atomic number of 6), the mass number is 12, which is greater than 6. - **Option C: Always more than the atomic weight**: - This option is misleading. The atomic weight (or atomic mass) is a weighted average of the masses of all the isotopes of an element, taking into account their relative abundances. It is usually expressed in atomic mass units (amu) and can be less than the mass number due to the presence of isotopes and the binding energy of the nucleus. For example, the atomic weight of carbon is approximately 12.01 amu, which is less than its mass number of 12. - **Option D: A fraction**: - This statement is incorrect because the mass number is always a whole number. It represents a count of particles (protons and neutrons), and you cannot have a fraction of a particle in this context. For example, the mass number of oxygen is 16, not 16.5 or any other fraction. ### Common Pitfalls: - Confusing mass number with atomic weight: Remember that mass number is a count of nucleons, while atomic weight is an average based on isotopes. - Misunderstanding the relationship between protons and neutrons: It’s important to remember that the mass number includes both, and the number of neutrons can vary in isotopes of the same element. ### Revision Summary: - The mass number is the total number of protons and neutrons in an atom's nucleus. - It is calculated using the formula \( A = Z + N \). - The mass number is always a whole number and is not always less than or more than the atomic weight. - Understanding the distinction between mass number and atomic weight is crucial for solving related problems.