The half-life of a specific element was calculated to be 5,200 years. what percentage of a sample of the pure element will remain after two half-lives?

25% will be remaining.

Explanation: Half live is the time in which the a radioactive substance remains half of its initial amount.

Given half life is 5200 years. let's say we have 100 grams(assume any amount of the element). In one half life(5200 years), half of it's initial amount that is 50 grams will be remaining. Now, in second half life, 25 grams of the element will be remaining.

Percentage of the remaining element =

percentage of the remaining element =  = 25%

So, 25% of the element will be remaining in two half lives.

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons