9.) the isotope chromium-51 has a half-life of 28 days. if the original sample had a 51cr mass of 32.0 grams, what mass of 51cr would remain after 84 days

Explanation:

The half-life of chromium-51 (28 da) is the time it takes for half of it to decay.

After one half-life, half of the original amount will remain.  

After a second half-life, half of that amount will remain, and so on.  

We can construct a table as follows:  

We see that, after 84 da (three half-lives), ⅛ of the original mass remains.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.

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explanation: