Chemistry · Periodic Classification Of Properties · NEET
Hydrogen has the electron configuration 1s1. This one detail makes it fit two groups at the same time. It has one valence electron, exactly like Group 1 alkali metals (ns1). But it is also just one electron short of the stable helium configuration, exactly like Group 17 halogens (ns2 np5, which need one electron). Because it genuinely shares properties of both families and does not match either one completely, no single spot fully fits it. This is why we call its position anomalous (not normal).
Three ways. (1) Same valence configuration: hydrogen is 1s1, and alkali metals like Li, Na are ns1 — one outer electron each. (2) Same common ion: hydrogen loses its electron to form H+, just as Na forms Na+. (3) Similar valence and compounds: hydrogen shows valence 1 and forms oxides (H2O like Na2O), halides (HCl like NaCl), and it is a reducing agent. Because of this, hydrogen is usually written at the top of Group 1 in most periodic tables.
Also three ways. (1) One electron short of stability: hydrogen needs 1 electron to reach the helium duplet, just as F needs 1 to reach the neon octet. (2) Forms a negative ion: hydrogen can gain an electron to form the hydride ion H- (like Cl forms Cl-). (3) It is a diatomic gas (H2) and a nonmetal, just like F2, Cl2. Its ionisation enthalpy is also high, closer to halogens than to soft, low-ionisation alkali metals. So some tables place hydrogen above Group 17 instead.
Under normal conditions hydrogen is a NONMETAL — it is a light diatomic gas (H2), a poor conductor, and it forms covalent bonds (like in H2O, CH4, HCl). It is not shiny, malleable, or a good conductor like real metals. Note: at extremely high pressure hydrogen can turn metallic, but that is not relevant for NEET. For your exam, treat hydrogen as a nonmetal that has a few metal-LIKE traits (H+ and reducing power), which is exactly why its position is confusing.
Because hydrogen has an incomplete shell (only 1 electron, not the full 2 of helium), it is highly reactive and readily forms bonds — the opposite of noble gases, which have complete shells and are almost unreactive. Helium (1s2) is the truly stable one and belongs with noble gases; hydrogen (1s1) is one electron short and reactive, so it does not fit Group 18. Do not confuse hydrogen with helium.
Hydrogen has a much higher ionisation enthalpy (about 1312 kJ/mol) than alkali metals (Na is about 496 kJ/mol), so it does not lose its electron as easily. It usually forms covalent bonds, not the ionic bonds typical of alkali metals. And H2 is a gas, while Na, K are soft solids. These sharp differences are the reason its Group-1 placement is only partial — hence anomalous.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It means hydrogen cannot be given one fixed group in the periodic table because its 1s1 configuration makes it resemble both Group 1 alkali metals (one valence electron, forms H+) and Group 17 halogens (one electron short of a full shell, forms H-). It shares properties of both but matches neither completely.
Most modern periodic tables place hydrogen at the top of Group 1 (above lithium) because it has one valence electron like the alkali metals. But this placement is only partial; some tables also show it above Group 17 because of its halogen-like traits.
Yes, a little. Both hydrogen and carbon have half-filled valence shells (H needs 1 more to fill its duplet, C needs 4 to fill its octet), similar electronegativity, and both mainly form covalent bonds. But this similarity is weak and is not the main reason for hydrogen's anomalous position; the Group 1 and Group 17 resemblances are the key points for NEET.
Hydrogen holds its single electron tightly, giving it a high ionisation enthalpy (about 1312 kJ/mol), which is close to the halogens and far above the alkali metals (which lose electrons easily). This high value is one reason hydrogen behaves like a nonmetal rather than a soft, reactive metal.
