Q1.
Helium contains two protons in its nucleus and two electrons. In which way will the two electrons be arranged in its atomic shell?
Answer
Both electrons go into the K-shell: the configuration of helium is simply 2.
Rule: a shell can hold at most 2n² electrons, and shells are filled from the innermost outward.
For the K-shell, n = 1
Maximum electrons = 2n²
= 2 × 1²
= 2
Helium has 2 electrons, and the K-shell can take exactly 2
⇒ configuration of He = 2, with the L-shell empty
Maximum electrons = 2n²
= 2 × 1²
= 2
Helium has 2 electrons, and the K-shell can take exactly 2
⇒ configuration of He = 2, with the L-shell empty
So helium's only shell is completely full. That is the whole reason helium is chemically inert — it has nothing to gain, lose or share. For the first shell, 2 electrons (a duplet) plays the same role that an octet plays for later shells.
Why it happens: electrons fill the lowest available energy level first, and the K-shell is the closest to the nucleus and lowest in energy. The second electron has no reason to go to the L-shell while there is still room in K.
Check it yourself: lithium has 3 electrons. The K-shell takes 2 and is then full, so the third must start the L-shell — giving 2, 1. That single outer electron makes lithium a reactive metal, while helium, one place before it, is a gas that reacts with nothing.