Four elements P, Q, R and S have atomic numbers of 4, 10, 12 and 14 respectively. Which of these elements is noble gas?

Analyze the question: We are given four elements by atomic number and asked which one is a noble gas. The way to answer is to write out each electron configuration and see which one has a completely filled outer shell.

Concept: Noble gases (Group 18 / Group 0) have a full outermost shell:

• Helium has 2 electrons (duplet).
• All the others have 8 electrons (octet).

Because their outer shells are full, they are chemically unreactive (inert).

Solution: Write the electron configuration for each element.

| Element | Atomic number | Configuration | Outer electrons | Group | |---|---|---|---|---| | P | 4 | 2, 2 | 2 | Group 2 (Beryllium) | | Q | 10 | 2, 8 | 8 | Group 18 (Neon) | | R | 12 | 2, 8, 2 | 2 | Group 2 (Magnesium) | | S | 14 | 2, 8, 4 | 4 | Group 14 (Silicon) |

• P (Z = 4): Its outer shell has only 2 electrons in the second shell, which can hold 8. It is not full, so P is a reactive metal (Be).
• Q (Z = 10): Its configuration 2, 8 has a completely filled second shell. This is Neon, a noble gas.
• R (Z = 12): Its configuration 2, 8, 2 has 2 electrons in the third shell, so it is not full. This is Magnesium, a reactive metal.
• S (Z = 14): Its configuration 2, 8, 4 has 4 outer electrons, so it is not full. This is Silicon, a metalloid.

Answer: Only Q has a stable, completely filled outer shell, so Q is the noble gas (Neon). The correct option is B.

Tip: Memorise the atomic numbers of the noble gases: 2, 10, 18, 36, 54, 86 (He, Ne, Ar, Kr, Xe, Rn). If a question gives an atomic number from this list, it is a noble gas immediately, and you can answer in seconds.