Which of the following is NOT a halogen gas?

Correct — B. Helium (He) is a noble gas, not a halogen. It sits in Group 18, its outer shell is already full at two electrons, and it forms no compounds in ordinary chemistry.

The halogens are the Group 17 elements: fluorine, chlorine, bromine, iodine and astatine. Cl, Br and F are all on that list, so helium is the element that does not belong — option (b).

Why the others are wrong

• (a)
  Cl — Chlorine is a Group 17 halogen and a greenish-yellow gas at room temperature, so it fits both halves of the question's description.
• (c)
  Br — Bromine is a halogen. It is a liquid rather than a gas at room temperature, which makes it tempting here, but the question asks which element is not a halogen.
• (d)
  F — Fluorine is the first halogen and the most electronegative element known — squarely inside Group 17.

Group 17, the halogens, have seven electrons in the outermost shell. One electron short of a full shell, they are strongly reactive and typically pick up an electron to form negative ions, which is why they combine with metals to make salts.

Group 18, the noble gases, are the opposite case: the shell is already complete, so there is nothing to gain or lose and the elements stay unreactive. Helium is the first of them, with just two electrons.

Key facts

• The halogens are the Group 17 elements: fluorine, chlorine, bromine, iodine and astatine.
• Helium belongs to Group 18, the noble gases, and has a filled outer shell of two electrons.
• At room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.
• The word halogen means salt-former, from the salts these elements form with metals.

Common traps

• Choosing bromine because it is a liquid, when the question asks about the group and not the state.
• Reading the symbol He as an unfamiliar halogen instead of helium.

No directly related past PYQ was found.