20. Lithium has an atomic number of 3 and a mass number of 7. What is the number of protons in a lithium atom?
Answer: D
The number of protons in a lithium atom is 3.
A lithium atom, with an atomic number of 3, contains 3 protons in its nucleus.
A) 7
Option A is incorrect because while the mass number of lithium is 7, this number represents the total number of protons and neutrons in the nucleus, not just the protons.
B) 4
Option B is also incorrect. The atomic number indicates the number of protons in an atom, and for lithium, that number is 3, not 4.
C) 12
Option C is incorrect as well. A mass number of 12 does not correspond to lithium; instead, lithium's mass number is 7. Hence, 12 cannot represent the number of protons.
D) 3
Option D is correct because the atomic number of lithium is 3, which directly indicates that there are 3 protons in a lithium atom.
Conclusion
The correct answer is 3 protons, as indicated by lithium's atomic number. Options A, B, and C misinterpret the atomic characteristics of lithium, failing to recognize that the atomic number defines the number of protons. Thus, Option D is definitively right in identifying the number of protons in a lithium atom.