What is the atomic mass number of an atom?
A The number of protons, neutrons, and electrons in the atom
B The number of protons and neutrons in its nucleus
C The average weight of all isotopes of an element
D The number of protons in its nucleus
E The number of neutrons in its nucleus
The correct answer and explanation is:
The correct answer and explanation is:
The correct answer is B. The number of protons and neutrons in its nucleus.
The atomic mass number, often referred to simply as the mass number, is a whole number that represents the total number of protons and neutrons in an atom’s nucleus. This number is used to describe the mass of an atom, as protons and neutrons contribute almost all of the atom’s mass, while electrons have a negligible mass.
Each element is defined by its number of protons, known as the atomic number. For example, all carbon atoms have 6 protons. However, the number of neutrons can vary, resulting in different isotopes of the same element. The atomic mass number, therefore, is the sum of these protons and neutrons, which can vary between isotopes of the same element.
For example, the most common isotope of carbon is carbon-12, which has 6 protons and 6 neutrons, so its atomic mass number is 12. Another isotope, carbon-14, has 6 protons and 8 neutrons, making its atomic mass number 14.
This number is important in various scientific fields, such as chemistry and nuclear physics, as it helps in determining the behavior of the atom, its stability, and its interactions with other atoms or particles.
Options A, C, D, and E are incorrect because:
- A refers to the total count of protons, neutrons, and electrons, which is not the definition of atomic mass number.
- C refers to the average atomic mass (which considers isotopic distribution), not the mass number.
- D is the definition of atomic number, not the mass number.
- E only accounts for neutrons, which is incomplete because the mass number includes both protons and neutrons.