Construct a part of the modern periodic table and show the position of the given element X.

From the Bohr model diagram:

• First shell has 2 electrons.
• Second shell has 8 electrons.
• Third (outermost) shell has 7 electrons. Total number of electrons = $2 + 8 + 7 = 17$. For a neutral atom, the number of electrons equals the atomic number. Therefore, the atomic number of Element X is 17. The electron configuration is $2, 8, 7$.

The element with atomic number 17 is Chlorine (Cl).

• The number of electron shells determines the period. Since Element X has 3 shells, it belongs to Period 3.
• The number of valence electrons determines the group for main group elements. Since Element X has 7 valence electrons, it belongs to Group 17 (also known as Group VIIA, the Halogens).

Here is a section of the modern periodic table with Element X (Chlorine) highlighted:

Period	Group 16 (Chalcogens)	Group 17 (Halogens)	Group 18 (Noble Gases)
2	Oxygen (O)	Fluorine (F)	Neon (Ne)
3	Sulfur (S)	Chlorine (Cl) (X)	Argon (Ar)
4	Selenium (Se)	Bromine (Br)	Krypton (Kr)

The position of Element X is in Period 3, Group 17.