An instructor displays a plot of the first-ionization energy, IE1, versus atomic number (Z) for the elements with 1≤Z≤20. The plot shows sharp maxima at Z=2, Z=10, and Z=18. Immediately after each maximum, the curve plunges to a local minimum. Which atomic number in this range is most likely associated with an alkali-metal element?
a) 17
b) 11
c) 18
d) 12