Question

do atoms in a period fill a different last main energy level?

Answer 1

Question

Do atoms in a period fill a different last main energy level?

Answer:

Since the one energy level is determined as the set of orbitals with the same principal quantum numbers (main quantum number (n) and orbital quantum number (l)), the answer may be given as follows.

In the first period elements (H and He) fill the same last energy level ( $n = 1$ , $l = 0$ (s-orbitals)). So the first period is the only period wherein elements fill the same energy level (answer no to the question)

In other periods, elements fill different energy levels (answer yes to the question), although some of elements of a period fill the same energy level. Namely, the elements of $1^{\text{st}}$ and $2^{\text{nd}}$ groups in each period fill the same energy level ( $l = 0$ (s-orbitals)) and the elements of $13^{\text{th}} - 18^{\text{th}}$ groups in each period fill the same energy level ( $l = 1$ (p-orbitals)). In $4^{\text{th}}$ and $5^{\text{th}}$ periods the elements of $3^{\text{rd}} - 12^{\text{th}}$ groups and in $6^{\text{th}}$ and $7^{\text{th}}$ periods the elements of $4^{\text{th}} - 12^{\text{th}}$ groups fill the same energy level ( $l = 2$ (d-orbitals)). In $6^{\text{th}}$ and $7^{\text{th}}$ periods the elements of $3^{\text{rd}}$ group fill the same energy level ( $l = 3$ (f-orbitals)).