How does the electronic configuration of an atom of an element relate to its position in the modern periodic table? Explain with one example.

The electronic configuration of an atom determines its position (period and group) in the Modern Periodic Table as follows:

1. Period Number: The total number of occupied electron shells in an atom is equal to its period number.
2. Group Number: The number of valence electrons (electrons in the outermost shell) determines the group number:

• If the number of valence electrons is $1$ or $2$, the group number is equal to the number of valence electrons.
• If the number of valence electrons is $\ge 3$, the group number is equal to $\text{number of valence electrons} + 10$.

Example: Sodium ($Na$)

• Atomic number = $11$
• Electronic configuration = $2, 8, 1$
• Since it has $3$ occupied shells (K, L, M), it belongs to Period 3.
• Since it has $1$ valence electron, it belongs to Group 1.