Which of the following representations is correct for an atom X with 28 electrons and 30 neutrons?

Analyze the question: We need the correct nuclide notation for an atom with 28 electrons and 30 neutrons.

Concept: Nuclide notation is written as ᴬ_Z X, where:

• Z = atomic number = number of protons
• A = mass number = protons + neutrons

For a neutral atom, the number of electrons equals the number of protons.

Solution:

1. The atom is neutral, so protons = electrons = 28. This gives Z = 28.
2. Mass number A = protons + neutrons = 28 + 30 = 58.
3. The notation is therefore ⁵⁸₂₈X.

Checking the options:

• A. ⁵⁸₂₈X: A = 58, Z = 28. Correct.
• B. ³⁰₂₈X: the mass number is wrongly given as 30, the neutron count.
• C. ²⁸₃₀X: the mass number and atomic number are swapped, and the values are wrong.
• D. ⁵⁸₃₀X: the atomic number is wrongly given as 30, the neutron count.

(The element is nickel-58.)

Tip: The top number (mass number) is always the larger one, because it counts protons plus neutrons. The bottom number is the atomic number, which counts protons only. Remember: "Atomic number = bottom = protons = electrons (in a neutral atom)."