A represents atomic mass and 2 represents an atomic number, which of the following describes an atom after a proton is emitted?
A. A-1,2-1
B. A-1,2+1
C. A,2-1
D. A+1,2-1
This is because when a proton is emitted from an atom, the atomic mass decreases by one unit, and the atomic number decreases by one unit. The atomic mass is the sum of protons and neutrons in the nucleus, and the atomic number is the number of protons in the nucleus. Therefore, if a proton is lost, both numbers decrease by one.
Therefore, the Correct Answer is A.