Hydrogen (Class 11): Position, Isotopes, Hydrides and Water

Why its position is anomalous

It also differs from both: unlike alkali metals it is a non-metal gas with high ionisation energy, and unlike halogens it is far less electronegative. The honest answer to "where does hydrogen belong" is that it belongs to neither group entirely, and the reasons on both sides should be given.

Isotopes

Protium has no neutron, deuterium one, tritium two. Tritium is radioactive; the other two are stable. Because the mass ratio between them is so large — deuterium is twice protium — their physical properties differ noticeably, unlike isotopes of heavier elements where the relative difference is small.

This is why heavy water has measurably different melting and boiling points from ordinary water, and why kinetic isotope effects involving hydrogen are so pronounced.

Hydrides

A gap in hydride formation occurs across part of the transition series, where the elements form no stable simple hydride. Naming that gap is a standard recall question.

Water

Why water is anomalous

Its high boiling point relative to comparable molecules, its high surface tension and its high heat capacity all follow from extensive hydrogen bonding. Each water molecule can form up to four hydrogen bonds, which is unusually many for such a small molecule.

Ice is less dense than liquid water because hydrogen bonding forces an open tetrahedral arrangement in the solid, which collapses partly on melting. That is why ice floats, and it is one of the most frequently asked explanations in the chapter.

Hard and soft water

Hardness is caused by dissolved calcium and magnesium salts.

• Temporary hardness comes from hydrogencarbonates and is removed by boiling, which decomposes them to insoluble carbonates.
• Permanent hardness comes from chlorides and sulphates and is not removed by boiling. It requires chemical treatment or ion exchange.

The distinction and its removal methods are asked together almost every year, and the answer should name the anion responsible in each case.

Heavy water

Water in which the hydrogen is deuterium. It is obtained by prolonged electrolysis, since ordinary water is electrolysed faster, leaving heavy water behind. It is used as a moderator in certain nuclear reactors and as a solvent in spectroscopy.

Hydrogen peroxide

Its structure is non-planar, with the two O–H bonds lying in different planes — an open-book shape. This structural point is asked directly and is easy to state incorrectly.

Its chemistry is defined by the oxygen being in an intermediate oxidation state, so it can act as either an oxidising or a reducing agent depending on the partner. That dual behaviour, with an example of each, is the expected answer.

It decomposes readily, which is why it is stored in dark bottles — light accelerates decomposition — and often with a stabiliser.

Frequently asked questions

Why does ice float on water?

Because hydrogen bonding holds the molecules in an open tetrahedral lattice with more empty space than the liquid, making the solid less dense.

Why is temporary hardness removed by boiling but permanent hardness not?

Because hydrogencarbonates decompose on heating to give insoluble carbonates, while chlorides and sulphates are unaffected by heat.

Why can hydrogen peroxide act as both oxidising and reducing agent?

Because its oxygen is in an intermediate oxidation state, so it can be either reduced or oxidised depending on what it reacts with.

Why do the isotopes of hydrogen differ so much in properties?

Because the relative mass difference is very large — deuterium is twice the mass of protium. For heavier elements the corresponding relative difference is small, so isotope effects are negligible there.