There is no element in period 3 with an atomic mass of 12. Here's why:

* Atomic mass: The atomic mass of an element is the average mass of all its isotopes. It's usually a decimal number, not a whole number.

* Period 3: Period 3 of the periodic table includes elements like sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar). None of these elements have an atomic mass of 12.

The element with an atomic mass close to 12 is carbon (C), which is located in period 2.

Let me know if you have any other questions!