Lithium has an atomic number of 3 and a mass number of 7. Which of the following is the number of protons in a lithium atom?
A. 12
B. 3
C. 4
D. 7
The number of protons in an atom is equal to its atomic number. In this case, lithium has an atomic number of 3, so it contains 3 protons.
The other choices (A, C, and D) do not represent the correct number of protons for a lithium atom.
Therefore, the Correct Answer is B.