Hydrogen

Hydrogen is unique because it resembles both group 1 and group 17 elements. It has a single valence electron, yet it can also gain one electron to complete a duplet. NEET often asks this dual-position logic directly.

Its three isotopes are protium ($^1$H), deuterium ($^2$H or D), and tritium ($^3$H or T). Tritium is radioactive, while deuterium is widely used in isotopic labeling and heavy-water chemistry.

Hydrides are compounds of hydrogen with other elements. They are broadly classified into ionic (saline) hydrides, covalent hydrides, and metallic/interstitial hydrides.

Ionic hydrides like NaH contain H$^-$ and are strongly basic. Covalent hydrides dominate the p-block, while metallic hydrides are often non-stoichiometric and seen with transition metals.

Water owes its unusual properties to extensive hydrogen bonding. Ice is less dense than liquid water because its open tetrahedral arrangement occupies more volume.

Hardness of water is due to Ca$^{2+}$ and Mg$^{2+}$ salts. Temporary hardness comes from bicarbonates and can be removed by boiling. Permanent hardness comes from chlorides and sulfates and is removed by methods such as washing soda, zeolite, ion exchange, or Calgon treatment.

Hydrogen peroxide, H$_2$O$_2$, contains a peroxide linkage with oxygen in the -1 oxidation state. It can behave as both an oxidizing agent and a reducing agent depending on the reacting partner.

Its bleaching action is due to oxidation. This dual redox behavior makes H$_2$O$_2$ a favorite NEET topic, especially in oxidation-state and reaction-product questions.

A strong revision flow for this chapter is: isotopes, hydride classes, hardness of water, heavy water, then hydrogen peroxide. Most direct questions come from these five areas, and many are concept-based rather than numerical.