The correct option is **B. Atomic mass is the weighted average of all isotopes of an element based on their natural abundance.** ### Detailed Explanation: 1. **Understanding Atomic Mass**: - Atomic mass is a measure of the mass of an atom, typically expressed in atomic mass units (amu). It reflects the total number of protons and neutrons in the nucleus of an atom, as electrons contribute negligibly to the overall mass. 2. **Isotopes**: - Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. For example, carbon has isotopes like Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons). While they are the same element, their atomic masses differ due to the varying number of neutrons. 3. **Weighted Average**: - The atomic mass of an element is not simply the mass of one isotope but rather a weighted average of all its naturally occurring isotopes. This means that the atomic mass takes into account both the mass of each isotope and its relative abundance in nature. - For example, if Carbon-12 makes up 98.9% of carbon found in nature and Carbon-14 makes up 1.1%, the atomic mass of carbon is calculated as: \[ \text{Atomic Mass} = (0.989 \times 12 \, \text{amu}) + (0.011 \times 14 \, \text{amu}) = 12.011 \, \text{amu} \] ### Why Other Options Are Incorrect: - **Option A: Atomic mass is determined solely by the number of protons in an atom.** - This statement is incorrect because atomic mass is influenced by both protons and neutrons. While the number of protons defines the element (its atomic number), the number of neutrons affects the mass. Therefore, atomic mass cannot be determined solely by protons. - **Option C: Atomic mass is the same for all isotopes of an element.** - This statement is false because different isotopes of the same element have different numbers of neutrons, leading to different atomic masses. For instance, Carbon-12 and Carbon-14 have different atomic masses due to their differing neutron counts. - **Option D: Atomic mass only considers the neutrons present in the nucleus.** - This option is misleading because atomic mass considers both protons and neutrons. While neutrons do contribute to the mass, protons are equally important since they define the element and contribute to the overall mass. ### Common Pitfalls: - Students often confuse atomic mass with atomic number. Remember, atomic number refers to the number of protons, while atomic mass is the total mass of protons and neutrons. - It’s also important to remember that atomic mass is an average value, not a whole number, due to the presence of isotopes and their varying abundances. ### Revision Summary: - Atomic mass is the weighted average of all isotopes of an element based on their natural abundance. - Isotopes have the same number of protons but different numbers of neutrons, leading to different atomic masses. - Atomic mass is influenced by both protons and neutrons, not just one or the other. - Understanding the concept of weighted averages is crucial for calculating atomic mass accurately.