Explain how the period in which an element is located relates to the number of shells that contain electrons.

The period of an element in the periodic table directly corresponds to the number of electron shells that element has. Here's how:

1. The periodic table is organized into rows and columns, with each row of the table known as a 'period'.

2. The period number of an element signifies the highest energy level an electron in that element possesses when the atom is in its ground state (not excited).

3. This highest energy level is also the number of electron shells that the atom has.

4. For example, elements in the first period have one electron shell, elements in the second period have two electron shells, and so on.

5. Therefore, by looking at what period an element is in, you can determine how many electron shells that element has.

6. This is because electrons fill the lowest energy levels (closest to the nucleus) first, and then move to higher levels as more electrons are added.

7. So, the period number is a direct indication of the number of filled electron shells in that element's atoms.