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✅ Correct Answer: 45 ✔️

Rationale:

The atomic number of an element is defined as the number of protons in its nucleus. In this case, the unknown element contains 45 protons, which directly corresponds to an atomic number of 45. The atomic mass (which is 64) refers to the total number of protons and neutrons, but it is not the same as the atomic number. Thus, the atomic number is 45.

Why the Other Choices Are Incorrect:

❌ Option A (It cannot be determined):
This is incorrect because the atomic number is always equal to the number of protons, which is clearly given as 45 in the question.

❌ Option B (19):
This is incorrect because the atomic number of 19 corresponds to the element potassium (K), not the unknown element in question.

❌ Option D (64):
This is incorrect because 64 is the atomic mass, not the atomic number. The atomic mass refers to the sum of protons and neutrons, whereas the atomic number only counts protons.

3a:T676,

About Atomic Structure

Atomic Structure HESI A2 Practice Questions

Rationale