Question

The fourth period ends with the element ________

(a) Helium

(b) Argon

(c) Neon

(d) Krypton

I had been asked this question in homework.

The origin of the question is Electronic Configurations of Elements and the Periodic Table topic in section Classification of Elements and Periodicity in Properties of Chemistry – Class 11

Answer 1

The correct choice is (d) Krypton

To elaborate: Every period ends with a noble gas, as the element at the end of the period is with the fully filled orbitals. The 4^th period has an outer shell as O i.e. n = 4. So the configuration is [Ar}4s^23d^104p^6, that is Krypton.