Important Elements & Their Compounds – NDA Chemistry Notes

This chapter covers some of the most important non-metallic elements and their compounds. The elements covered are hydrogen, oxygen, nitrogen, and carbon in the form of its gaseous compounds. Questions from this chapter appear regularly in NDA, mostly as direct property identification, match lists, and assertion-reason pairs.

The chapter rewards students who know specific facts. Memorise the key properties, know the important compounds, and know the exact facts that examiners test year after year.

1. Hydrogen

Hydrogen is the lightest of all known elements. It is the first element in the periodic table and the most abundant element in the universe. Hydrogen gas is colourless, odourless, and tasteless. [NDA 2022-I] It is highly flammable. A mixture of hydrogen and oxygen is explosive.

Isotopes of Hydrogen

Hydrogen has three isotopes. This is the only element whose three isotopes each have a different name.

Name	Symbol	Protons	Neutrons	Mass Number
Protium (ordinary hydrogen)	H	1	0	1
Deuterium (heavy hydrogen)	D	1	1	2
Tritium	T	1	2	3

Protium is by far the most abundant, more than 99.98% of all hydrogen. Deuterium is stable. Tritium is radioactive and used in nuclear research. All three isotopes have identical chemical properties because they have the same number of protons and electrons.

The statement “Hydrogen has no isotopes” is FALSE.

Oxidation States of Hydrogen

Hydrogen can show more than one oxidation state.

Oxidation State	When it occurs	Example
+1	Bonded to non-metals (most common)	HCl, H₂O, H₂SO₄
−1	Bonded to metals, in metal hydrides	NaH, CaH₂, LiH
0	In its elemental form	H₂ gas

The statement “oxidation number of hydrogen is always +1” is FALSE. It can also be −1 in metal hydrides, and 0 in H₂.

Hydrogen as a Reducing Agent

Hydrogen has a strong affinity for oxygen. This is what makes hydrogen a reducing agent.

At high temperature, hydrogen reduces metal oxides to give the pure metal. PbO + H₂ → Pb + H₂O: hydrogen reduces lead oxide to elemental lead. [NDA 2012-I] CuO + H₂ → Cu + H₂O: hydrogen reduces copper oxide to copper.

Hydrogen reduces PbO because it has great affinity for oxygen. The oxygen leaves the PbO and joins with hydrogen to form water. [NDA 2012-I]

Storing Large Volumes of Hydrogen

Storing hydrogen as a compressed gas requires large heavy tanks and is risky. A better method is to convert hydrogen into alkali metal hydrides. [NDA 2019-II]

Alkali metals (lithium, sodium, potassium) react with hydrogen to form solid hydrides. Example: 2Na + H₂ → 2NaH. These solid hydrides are compact and store a very large amount of hydrogen in a small volume. When hydrogen is needed, the hydride reacts with water: NaH + H₂O → NaOH + H₂.

Why Helium is Preferred Over Hydrogen for Airships

Both helium and hydrogen are light gases. Yet airships use helium, not hydrogen.

Property	Hydrogen	Helium
Density	Very low	Low (slightly higher than H₂)
Flammability	Highly flammable, explosive	Non-flammable
Reactivity	Reactive	Completely inert (noble gas)
Safety	Dangerous	Safe

Helium is preferred because it is chemically less reactive, NOT because of its density. [NDA 2006-I] Hydrogen is flammable and forms an explosive mixture with oxygen. Helium does not react with anything under normal conditions. Safety is the deciding factor.

Hydrogen + Sulphuric Acid + Calcium

When dilute sulphuric acid is added to calcium, very little hydrogen gas is produced. [NDA 2013-II]

The reaction produces calcium sulphate (CaSO₄), which is insoluble in water. This insoluble CaSO₄ coats the surface of the calcium metal. The coating acts as a barrier. The acid can no longer reach the calcium. So the reaction stops very quickly and very little H₂ is produced.

Ca + H₂SO₄ → CaSO₄ + H₂ (very little, then stops)

Heavy Water (D₂O)

Water made from deuterium instead of ordinary hydrogen is called heavy water.

Chemical formula: D₂O (deuterium oxide). Molecular weight: 20 u (ordinary water = 18 u).

Property	Ordinary Water (H₂O)	Heavy Water (D₂O)
Formula	H₂O	D₂O
Molecular weight	18 u	20 u
Boiling point	100°C	101.4°C
Freezing point	0°C	3.8°C
Density	1.0 g/cm³	1.1 g/cm³

Heavy water is used as a moderator in nuclear reactors. [NDA 2009-I] A moderator slows down fast neutrons to slow (thermal) neutrons. Slow neutrons are needed to sustain the nuclear chain reaction. Heavy water is preferred because it slows neutrons effectively without absorbing too many of them.

Heavy water is NOT the same as hard water. Hard water contains dissolved Ca²⁺ and Mg²⁺ salts. Heavy water is a pure compound, D₂O.

Hydrogen Peroxide (H₂O₂)

Hydrogen peroxide is a colourless liquid with strong oxidising properties. Formula: H₂O₂. Structure: H—O—O—H.

It is a strong oxidising agent. It decomposes slowly into water and oxygen: 2H₂O₂ → 2H₂O + O₂. Decomposition is speeded up by light, heat, and MnO₂.

Use	Detail
Bleaching agent	Bleaches hair, paper, textiles
Antiseptic	3% solution used to clean wounds
Disinfectant	Kills bacteria by oxidation
Rocket propellant	Concentrated form decomposes rapidly releasing O₂ and energy

H₂O₂ is NOT the same as heavy water (D₂O). They have completely different formulas, properties, and uses.

Standard Hydrogen Electrode

The standard hydrogen electrode is the reference electrode in electrochemistry.

Standard electrode potential of hydrogen = 0 (by definition). This is measured at 25°C, with H₂ at 1 atm pressure, and H⁺ ion concentration of 1 mol/L. All other electrode potentials are measured relative to this zero value. The electrode potential is zero only at 25°C.

2. Oxygen

Oxygen is the most abundant element in the Earth’s crust. It makes up 21% of the atmosphere by volume. Oxygen gas is colourless and odourless. It supports combustion. It helps other things burn. Oxygen itself does not burn.

When oxygen reacts with non-metals, it forms acidic oxides. [NDA 2013-II] When oxygen reacts with metals, it forms basic oxides.

Allotropes of Oxygen

Oxygen has two allotropes. Both are forms of the same element but with different structures.

Allotrope	Formula	Found in
Oxygen	O₂	Atmosphere, for breathing
Ozone	O₃	Stratosphere (ozone layer)

Oxygen and ozone are allotropes. [NDA 2009-I]

Ozone (O₃)

Ozone is an allotrope of oxygen with three oxygen atoms per molecule. It has a pungent, distinctive smell and is pale blue in colour.

Ozone is found in the stratosphere, 15 to 35 km above the Earth’s surface. The ozone layer absorbs harmful ultraviolet (UV) radiation from the Sun. [NDA 2010-I] This protects all life on Earth from damage caused by UV rays.

Chlorofluorocarbons (CFCs) release chlorine radicals when they reach the stratosphere. These chlorine radicals destroy ozone in a chain reaction. Cl + O₃ → ClO + O₂

This destruction of the ozone layer allows more UV radiation to reach Earth, causing skin cancer, eye damage, and harm to ecosystems.

3. Nitrogen

Nitrogen is the most abundant gas in the atmosphere, 78% by volume. It is colourless and odourless. It does not support combustion and does not burn.

Nitrogen is chemically very inert under normal conditions. The reason is the strong triple bond between the two nitrogen atoms (N≡N). Breaking this bond requires a very large amount of energy. This is why nitrogen does not react easily with other substances at room temperature.

Nitrogen is essential for life. It is a key component of proteins, DNA, and amino acids. But plants cannot use nitrogen gas directly from the air. Nitrogen must first be converted into a usable form by nitrogen-fixing bacteria or by industrial processes.

Important Nitrogen Compounds. Nitrogen forms several important gaseous and industrial compounds, each with distinct properties and uses.

Nitrous Oxide (N₂O): Laughing Gas

Nitrous oxide is a colourless gas with a sweet smell. Its common name is laughing gas. Inhaling small amounts causes euphoria. [NDA 2010-I] It is used as an anaesthetic in dentistry and minor surgery. It is also used as a propellant in whipped cream dispensers.

Nitric Oxide (NO)

Nitric oxide is a colourless, toxic gas. It is produced when nitrogen and oxygen from air react at very high temperatures, for example in diesel engines. [NDA 2010-II] NO converts to NO₂ in the atmosphere, which then causes acid rain.

Nitrogen Dioxide (NO₂)

Nitrogen dioxide is a brown/reddish-brown gas. It is produced from vehicle exhaust and industrial emissions. It contributes to acid rain by reacting with water and oxygen to form nitric acid (HNO₃).

Ammonia (NH₃)

Ammonia is a colourless gas with a sharp pungent smell. It is lighter than air and highly soluble in water. It is produced industrially by the Haber process: N₂ + 3H₂ → 2NH₃. [NDA 2009-I]

Ammonia is the starting material for manufacturing fertilisers: urea, ammonium nitrate, ammonium sulphate. Among common nitrogen fertilisers, urea has the highest percentage of nitrogen by mass, 46.7%. [NDA 2009-I]

4. Carbon Monoxide (CO)

Carbon monoxide is a colourless, odourless, highly toxic gas. It is produced by incomplete combustion of carbon-containing fuels.

CO combines with haemoglobin in the blood. CO binds to haemoglobin about 250 times more strongly than oxygen does. The compound formed (carboxyhaemoglobin) is stable and cannot carry oxygen. The person is effectively suffocated even when air is present.

CO does NOT turn lime water milky. That is CO₂. CO₂ is exhaled normally and does not bind to haemoglobin in a harmful way.

Sources of CO: vehicle exhaust, burning coal, wood, or petrol in insufficient air.

5. Carbon Dioxide (CO₂)

Carbon dioxide is a colourless, odourless gas. It makes up about 0.04% of the atmosphere. CO₂ does not support combustion. It extinguishes fire. This is why CO₂ is used in fire extinguishers.

CO₂ dissolves in water to form carbonic acid (H₂CO₃), slightly acidic. CO₂ turns lime water milky. This is the standard laboratory test for CO₂.

CO₂ + Ca(OH)₂ → CaCO₃↓ (white precipitate) + H₂O

If CO₂ is passed through lime water for a long time, the white precipitate dissolves.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble, clear solution returns)

CO₂ absorbs infrared radiation from the Earth’s surface. This traps heat in the atmosphere, the greenhouse effect. Burning fossil fuels increases CO₂ in the atmosphere, leading to global warming.

6. Water Gas

Water gas is a fuel gas made by passing steam over red-hot coke (carbon). [NDA 2016-I]

Preparation: C + H₂O → CO + H₂ (at very high temperature)

Water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂). It is also called syngas (synthesis gas). It is used as an industrial fuel and as a raw material for making methanol and ammonia.

Water gas is NOT the same as producer gas.

Gas	Composition	How made
Water gas	CO + H₂	Steam + hot coke
Producer gas	CO + N₂	Air + hot coke
Biogas	Mainly CH₄	Anaerobic decomposition of organic waste

7. Aqua Regia

Aqua regia is a special mixture of acids that can dissolve gold and platinum, metals that no single acid can dissolve.

Composition: 3 parts concentrated HCl + 1 part concentrated HNO₃ (ratio 3:1).

It is a yellow or reddish-brown fuming liquid. It is extremely corrosive. The name means “royal water” in Latin. It dissolves the “royal metals”. HNO₃ oxidises the metal surface while HCl provides Cl⁻ ions that form stable soluble complexes, preventing the reverse reaction.

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