How many neutrons are in an atom of magnesium-25 ($Mg-25$)? (Atomic number of Mg is 12.)

• A. 12
• B. 37
• C. 13
• D. 25

Answer: (C)

Verified step by step guidance

1. Identify the atomic number (Z) of magnesium, which is given as 12. The atomic number represents the number of protons in the nucleus of the atom.
2. Recognize that the number 25 in magnesium-25 is the mass number (A), which is the total number of protons and neutrons in the nucleus.
3. Use the relationship between mass number, atomic number, and number of neutrons (N):
4. \[ N = A - Z \]
5. Substitute the known values into the equation:
6. \[ N = 25 - 12 \]
7. Calculate the difference to find the number of neutrons in the magnesium-25 atom.