Write the isotope symbol for calcium-40 with 20 protons and 20 neutrons.

• A. 40/20 Ca
• B. 40Ca
• C. Ca-40
• D. 20/40 Ca

Answer: (A)

In isotope notation, the mass number (total protons plus neutrons) is placed as the superscript or upper-left number, and the atomic number (number of protons) is the subscript or lower-left number, followed by the element symbol. Calcium has 20 protons, so its atomic number is 20. If calcium-40 has 20 protons, it must have 40 − 20 = 20 neutrons, giving a mass number of 40. Therefore the symbol with both numbers is 40/20 Ca. This format makes the nuclear composition explicit. Other notations either omit one of the numbers or swap them: 40Ca lacks the atomic number, Ca-40 uses a different naming style without the explicit nuclear notation, and 20/40 Ca would imply 40 protons, which is incorrect.