The element phosphorus is located to the right of the element magnesium along the same period in the periodic table. Which best describes the relationship of the elements?

Across a period from left to right, atomic number increases. Since phosphorus is to the right of magnesium, phosphorus has the larger atomic number, and it also has a greater atomic mass: phosphorus is about 31 u, while magnesium is about 24 u. Therefore, the best choice is that phosphorus has a larger atomic mass.

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