3.5.1 Isotopes of Hydrogen

This topic explains the three isotopes of hydrogen — protium, deuterium, and tritium — and their differences in structure, abundance, and properties.

Definition

Isotopes are atoms of the same element that have the same atomic number but different mass numbers (different numbers of neutrons).

Isotopes of hydrogen are the three naturally occurring forms of hydrogen that differ in the number of neutrons in their nuclei.

Main Idea

Hydrogen has three isotopes:

• Protium (Hydrogen-1) — no neutron
• Deuterium (Hydrogen-2) — one neutron
• Tritium (Hydrogen-3) — two neutrons

All three have 1 proton (that's why they are all hydrogen), but they differ in the number of neutrons.

Simple explanation: Hydrogen is the simplest element — it has only 1 proton. Adding neutrons to this single-proton nucleus gives us three different isotopes.

Core Concept

Detailed Explanation

Protium (Hydrogen-1)

Symbol: ¹H

• 1 proton
• 0 neutrons

• Almost all hydrogen is Hydrogen-1 (protium).
• Because protium has only one proton, adding a neutron doubles its mass (to form deuterium).
• It is a colourless, odourless, and tasteless gas.
• It is insoluble in water.
• It is a highly inflammable gas.

Simple explanation: Protium is the most common form of hydrogen — it is the hydrogen we find in water (H₂O), in fuels, and in most chemical compounds.

Deuterium (Hydrogen-2)

Symbol: ²H (sometimes written as D)

• 1 proton
• 1 neutron

• Deuterium has one neutron.
• Its mass is about twice that of protium.
• Water that contains hydrogen-2 atoms in place of hydrogen-1 is called heavy water.
• Natural abundance: 0.0015%.

Simple explanation: Deuterium is a heavier form of hydrogen. When it replaces normal hydrogen in water, the water becomes "heavy water" — which has slightly different physical properties.

Heavy Water

Definition: Water that contains hydrogen-2 (deuterium) atoms in place of hydrogen-1 is called heavy water.

Comparison of ordinary water and heavy water:

Simple explanation: Heavy water is chemically similar to ordinary water, but its physical properties are slightly different because deuterium is heavier than protium.

Tritium (Hydrogen-3)

Symbol: ³H (sometimes written as T)

• 1 proton
• 2 neutrons

• Tritium has two neutrons.
• It is radioactive.
• It is rare.
• Tritium is not found in naturally occurring hydrogen because its nucleus is highly unstable.
• It is produced artificially in nuclear reactors.

Simple explanation: Tritium is the heaviest and only radioactive isotope of hydrogen. It doesn't occur naturally because it decays quickly.

Natural Abundance of Hydrogen Isotopes

Naturally occurring hydrogen contains:

Simple explanation: Almost all hydrogen in nature is protium. Deuterium is present in tiny amounts. Tritium is so rare and unstable that it is essentially absent from natural hydrogen.

Intermediate Level

Why Are They All Hydrogen?

All three isotopes have 1 proton in the nucleus. Since the atomic number (Z) is the number of protons, all three have Z = 1. That's why they are all hydrogen.

Why Do They Have Different Masses?

• Protium: 0 neutrons → mass number = 1
• Deuterium: 1 neutron → mass number = 2
• Tritium: 2 neutrons → mass number = 3

Chemical vs Physical Properties

Key point: Since isotopes have the same number of electrons, their chemical properties are similar. But their physical properties differ because of different masses.

Heavy Water — Why It Matters

• Nuclear reactors — as a moderator to slow down neutrons.
• Scientific research — as a tracer in studies.
• Production of deuterium compounds.

• Higher melting point (3.81 °C vs 0.00 °C)
• Higher boiling point (101.2 °C vs 100 °C)
• Higher density (1.1044 g/cm³ vs 0.99701 g/cm³)

Important Points

• - Hydrogen has three isotopes: protium, deuterium, tritium.
• - Protium (¹H): 1 proton, 0 neutrons; most abundant (99.99%).
• - Deuterium (²H): 1 proton, 1 neutron; abundance 0.0015%.
• - Tritium (³H): 1 proton, 2 neutrons; radioactive and rare.
• - Heavy water: water containing deuterium instead of protium.
• - Properties of heavy water: higher melting point, boiling point, and density than ordinary water.
• - Isotopes affect molecular mass and can change physical properties but do not change chemical properties.
• - Tritium is not found in naturally occurring hydrogen because its nucleus is highly unstable.

Remember

• - Protium (¹H): 1 proton, 0 neutrons — plain hydrogen.
• - Deuterium (²H): 1 proton, 1 neutron — heavy hydrogen.
• - Tritium (³H): 1 proton, 2 neutrons — radioactive hydrogen.
• - Heavy water: D₂O — water with deuterium.
• - Natural abundance: 99.99% protium, 0.0015% deuterium, negligible tritium.
• - Same chemical properties for all three; different physical properties.
• - Tritium is radioactive and not found naturally.

Common Mistakes

• 1. Thinking deuterium and tritium are different elements. They are all hydrogen — same element, different isotopes.
• 2. Thinking heavy water is just water with extra oxygen. Heavy water contains deuterium (²H), not extra oxygen.
• 3. Thinking tritium is common in nature. Tritium is radioactive and rare; it is not found in naturally occurring hydrogen.
• 4. Confusing protium with hydrogen gas. Protium is an isotope of hydrogen; hydrogen gas (H₂) is a molecule made of two hydrogen atoms.
• 5. Thinking isotopes have different chemical properties. They have the same chemical properties.
• 6. Forgetting that adding a neutron doubles protium's mass. Since protium has only 1 proton, adding 1 neutron doubles its mass (to form deuterium).

Quick Revision

• Hydrogen has three isotopes: protium, deuterium, tritium.
• Protium (¹H): 1 proton, 0 neutrons; almost all hydrogen is protium; colourless, odourless, tasteless, insoluble in water, highly inflammable.
• Deuterium (²H): 1 proton, 1 neutron; used to make heavy water.
• Tritium (³H): 1 proton, 2 neutrons; radioactive; rare; not found in naturally occurring hydrogen.
• Heavy water: water containing deuterium; melting point 3.81 °C; boiling point 101.2 °C; density 1.1044 g/cm³.
• Natural abundance: 99.99% protium, 0.0015% deuterium, negligible tritium.
• Isotopes affect molecular mass and physical properties, not chemical properties.

60-Second Revision

• Protium (¹H): 1 proton, 0 neutrons.
• Deuterium (²H): 1 proton, 1 neutron.
• Tritium (³H): 1 proton, 2 neutrons, radioactive.
• Heavy water: D₂O.
• Abundance: 99.99% protium, 0.0015% deuterium.
• Warning: all three are hydrogen — same element, different isotopes.