The element with the atomic number 12 belongs to the ____ group and _____ period.

The element with the atomic number 12 is **Magnesium (Mg)**. To determine its group and period:

### Group:
Magnesium belongs to **Group 2** of the periodic table, which is also known as the **alkaline earth metals**. Elements in this group have two valence electrons in their outermost shell.

### Period:
To determine the period, we need to look at the number of electron shells. The electron configuration of Magnesium (atomic number 12) is:

\[
1s^2\, 2s^2\, 2p^6\, 3s^2
\]

This shows that the outermost electrons are in the **3rd shell**. Therefore, Magnesium is located in the **3rd period**.

### Final Answer:
Magnesium (atomic number 12) belongs to the **2nd group** and **3rd period** of the periodic table.