An original sample of k-40 has a mass of 25.00 grams. after 3.9 m 109 years, 3.125 grams of the original sample remains unchanged. what is the half-life of k-40?

An original sample of K-40 has a mass of 25.00 grams. After 3.9 m 109 years, 3.125 grams of the original sample remains unchanged. What is the half-life of K-40?

First step is to determine the remaining decimal amount.

3.125 grams /25.00 grams = 0.125

Second step is to determine the number of half lives.

(1/2)^n = 0.125

N log (1/2) = log 0.125

N = 3 years

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.