Isotopes share the same number of protons but differ in neutrons

Steps:

• Identify the atomic number (number of protons) for each particle, as it determines the element.
• Check which particles have identical protons, indicating the same element.
• For those, confirm differing neutrons (protons + neutrons = mass number varies).
• Electrons may vary (ions) but do not affect the element identity.

Why C is correct:

• Particles 2 and 4 have the same protons (defining the element) but different neutrons, matching the isotope definition.

Why the others are wrong:

• A: Particle 1 (19 protons) and 2 differ in protons, so different elements.
• B: Particle 1 (19 protons) and 3 (20 protons) have different protons, so different elements.
• D: Includes 1 (19 protons) with 2 and 3, but mismatched protons across them.

Final answer: C