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D, it is changing the identity of itself, it is reacting to the air
1/2 of the original parent has decayed, thus one half- life has passed, and the rock is 1.3 billion = 1300 million years old
(4.0 moles of Na) x (6.02 x 10^23) / (1 mole) = 2.4 x 10^24 atoms of Na
There are 2.4 x 10^24 atoms of Na in 4.0 moles.