The atomic number of magnesium is 12. This means that its nucleus must contain:

• A. 12 neutrons
• B. 12 protons
• C. 6 protons and 6 neutrons
• D. 24 protons

Answer: (B)

Verified step by step guidance

1. Recall that the atomic number of an element is defined as the number of protons in the nucleus of an atom of that element.
2. Since magnesium has an atomic number of 12, it means that every magnesium atom has 12 protons in its nucleus.
3. The number of neutrons is not given by the atomic number; instead, it is related to the mass number, which is the sum of protons and neutrons.
4. Therefore, the statement that the nucleus must contain 12 neutrons is incorrect because the atomic number only tells us about protons.
5. Conclude that the correct interpretation is that the nucleus of magnesium contains 12 protons.