Chemistry · Periodic Classification Of Properties · NEET
Their outermost shell is completely filled. Helium has 1s2, and Ne, Ar, Kr, Xe, Rn all have ns2np6 (8 electrons in the outer shell). A full shell is a very low-energy, stable state. To react, an atom must gain, lose, or share electrons, but a noble gas already has a full shell, so it has no drive to do this. That is why they stay as single atoms and hardly form compounds.
A closed shell means the s and p orbitals of the outermost energy level are completely filled with electrons. For example, argon is [Ne]3s2 3p6 — its 3rd shell has the full ns2np6 set. This complete arrangement holds the electrons tightly and leaves no easy way to add or remove one, so the atom is chemically settled.
Helium's first shell (the K shell) can only hold 2 electrons, and 1s2 fills it completely. So helium also has a completely filled valence shell, just like the others. NCERT notes helium is placed in Group 18 (p-block) with the other noble gases because this full 1s2 shell gives it the same unreactive, noble-gas behaviour. The '8 electrons' rule only applies from neon onward.
Because a filled outer shell (the noble gas configuration) is extra stable. Atoms react by gaining, losing, or sharing electrons so that their outer shell becomes like the nearest noble gas. For example, sodium loses 1 electron to become Na+ (like neon), and chlorine gains 1 electron to become Cl- (like argon). Noble gases are the 'goal' configuration that other atoms aim for.
They are extremely unreactive, but not 100% inert. The heavier ones like xenon and krypton can form a few compounds (for example XeF2, XeF4) under special conditions, because their outer electrons are farther from the nucleus and held less tightly. This is why the older name 'inert gases' was dropped in favour of 'noble gases'. For NEET, remember: very low reactivity, but not truly zero for heavy members.
Ionization enthalpy is the energy needed to remove an electron. A completely filled shell is very stable, so pulling an electron out of it needs a lot of energy. That is why noble gases have the highest ionization enthalpies in their periods. This same stability is the reason they resist forming positive ions and stay unreactive.
The element expected to form the largest ion to achieve the nearest noble gas configuration is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
ns2np6 for the outer shell (for example Ne = 2s2 2p6, Ar = 3s2 3p6), except helium which is 1s2. In every case the outermost shell is completely filled.
Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Oganesson (Og, Z=118) is the newest, man-made member of the family.
Because for a long time no chemist could make them react at all. The name was changed to 'noble gases' after xenon and krypton compounds were made, showing they are not completely inert.
Yes. Because their shells are full, they have no need to bond, so they exist as single (monatomic) atoms, unlike O2 or N2 which form molecules.
Radon is chemically unreactive like the others, but it is radioactive. Its danger comes from radiation, not from chemical reactions.
