Final answer:
The subscript in the nuclide notation 42/20 Ca represents the atomic number, which is the number of protons, not neutrons. The number of neutrons is found by subtracting the atomic number from the mass number.
Step-by-step explanation:
In the nuclide notation 42/20 Ca, the subscript represents the atomic number (Z) which signifies the number of protons in the atom, not the number of neutrons. The element symbol (Ca) stands for calcium. The notion that the subscript represents the number of neutrons is incorrect.
Instead, the number of neutrons (N) is calculated by subtracting the atomic number from the mass number (A), which in this case is 42 - 20 = 22 neutrons. The mass number (A) is the total number of protons and neutrons in the nucleus, and is typically presented as the superscript.
The correct interpretation of this notation is that the atom has 20 protons, because of its atomic number 20, and 22 neutrons, the difference between the mass number (42) and the atomic number (Z=20).
For isotopes of any element, including calcium, the structure of the nuclide is usually represented in the form of AZX, where A is the mass number, Z is the atomic number, and X is the chemical symbol of the element.