An atom that has an atomic number of 20 and a mass number of 42 is an isotope of an atom that has:
A)an atomic number of 21 and a mass number of 42
B)an atomic number of 20 and a mass number of 40
C)22 neutrons and 20 protons
D)22 protons and 20 neutrons

Question

Quizplus · Accepted Answer

The atomic number of an element is the same as the number of protons in its nucleus. Therefore, the isotope with atomic number 20 has 20 protons in its nucleus. The mass number of an atom is the sum of its protons and neutrons. Therefore, the isotope with mass number 42 has 42 - 20 = 22 neutrons in its nucleus. The only plausible option is option B, since it represents the same element with the same number of protons (atomic number of 20) and a slightly different number of neutrons (mass number of 40 instead of 42). Therefore, the isotope with mass number 42 is an isotope of the atom with atomic number 20 and mass number 40.

An Atom That Has an Atomic Number of 20 and a Mass