The mass of the radioactive isotope americium- \( 241 \) remaining at the end of every period of \( 432.2 \) years is half of the mass at the beginning of the period. If the initial mass of americium- \( 241 \) is \( 5 \) grams, which equation represents the mass \( y \) , in grams, of americium-241 remaining after \( x \) years?
A) \( y = 5 ( 432.2 ) ^ { x } \)
B) \( y = 5 ( 432.2 ) ^ { \frac { x } { 2 } } \)
C) \( y = 5 ( \frac { 1 } { 2 } ) ^ { 432.2 x } \)
D) \( y = 5 ( \frac { 1 } { 2 } ) ^ { \frac { x } { 432.2 } }\)