Chemistry · Periodic Classification Of Properties · NEET
Both answers are partly right, and that is why it confuses students. By its electron configuration (1s2), the last electron enters an s-orbital, so helium STRICTLY belongs to the s-block. But in the actual periodic table we PLACE it in the p-block, in Group 18, with the noble gases. So: s-block by configuration, p-block by position. NEET questions usually reward the position answer (Group 18) unless they ask specifically about block from configuration.
Group 2 (beryllium, magnesium) elements also have ns2 outer electrons, so on paper helium seems to fit above beryllium. But Group 2 metals have an EMPTY p-subshell above their s2, so they easily lose 2 electrons and are very reactive. Helium's 1s2 is a COMPLETE shell (the K-shell holds only 2 electrons, there is no 1p). So helium is stable and unreactive, the opposite of beryllium. We match elements by chemical behaviour, not just by outer-electron count, so helium goes with the unreactive noble gases, not with the reactive Group 2 metals.
All Group 18 elements (neon, argon, etc.) have a completely filled valence shell, which makes them stable and almost non-reactive. Helium also has a completely filled valence shell (1s2, the full first shell). So helium shows the same key property: it is inert. The periodic table groups elements by similar properties, so helium sits with the family it behaves like, the noble gases.
No. Helium has zero p-electrons; its configuration is just 1s2. This is the exact reason it is called an EXCEPTION. Normally p-block means the last electron entered a p-orbital, but helium is placed in the p-block only because of its noble-gas behaviour, not because it has p-electrons. Remember: helium is in the p-block by exception, not by rule.
NCERT (Class 11, Classification of Elements and Periodicity) says: 'Strictly, helium belongs to the s-block but its positioning in the p-block along with other group 18 elements is justified because it has a completely filled valence shell (1s2) and as a result, exhibits properties characteristic of other noble gases.' So NCERT itself calls it an exception and gives the full-shell reason.
No. NCERT lists two special cases in this topic: helium (s-block by config, placed in p-block Group 18) and hydrogen (1s1, placed separately at the top because it can act like Group 1 OR Group 17). If a question asks for the exception among the noble gases, the answer is helium; if it asks about elements placed away from where their configuration suggests, both helium and hydrogen count.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
By electronic configuration (1s2) it is s-block, but its position in the periodic table is p-block (Group 18). If the question asks about its placement/family, answer p-block / noble gases; if it asks about block from configuration, answer s-block.
Because helium's 1s2 shell is completely full and stable, so it is inert. Group 2 metals have an empty p-subshell and lose electrons easily, so they are reactive. Helium behaves like a noble gas, not a Group 2 metal.
A noble gas needs a completely filled valence shell. For helium the first shell (only 1s, holding 2 electrons) is full at 1s2, so its shell is complete without any p-electrons. That full shell gives it the same stability and low reactivity as neon, argon, etc.
Helium and hydrogen. Helium is s-block by configuration but placed in the p-block (Group 18). Hydrogen (1s1) is placed separately at the top because it can resemble both Group 1 (alkali metals) and Group 17 (halogens).
