3.5 Isotopes

This topic explains isotopes — atoms of the same element that have different numbers of neutrons. It covers their definition, examples, properties, and how relative atomic mass is calculated from isotope data.

Definition

Isotopes are atoms of an element whose nuclei have the same atomic number but different mass number.

This is because atoms of an element can differ in the number of neutrons.

Simple explanation: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

• "isos" means same
• "tope" means place

So isotopes occupy the same place in the periodic table.

Main Idea

• Same atomic number → same element
• Different mass number → different number of neutrons
• Isotopes are chemically alike but differ in their physical properties

Simple explanation: Isotopes behave the same way in chemical reactions because chemistry depends on electrons, not neutrons. But they have different masses, so their physical properties differ.

Core Concept

Key Features of Isotopes

Why Isotopes Have Similar Chemical Properties

Chemical properties of an element depend upon the number of protons and electrons. Neutrons do not take part in ordinary chemical reactions. Therefore, isotopes of an element have similar chemical properties.

Simple explanation: Chemical reactions involve electrons. Since isotopes have the same number of electrons, they react in the same way.

Why Isotopes Have Different Physical Properties

Physical properties (like mass, density, melting point, boiling point) depend on the mass of the atom. Since isotopes have different numbers of neutrons, they have different masses — so their physical properties differ.

Detailed Explanation

Isotopes of Hydrogen

Hydrogen has three isotopes:

• Protium has no neutron. Almost all hydrogen is protium.
• Deuterium has one neutron.
• Tritium has two neutrons.
• Because protium (hydrogen-1) has only one proton, adding a neutron doubles its mass.

• Colourless, odourless, tasteless gas
• Insoluble in water
• Highly inflammable gas

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

Comparison of ordinary water and heavy water:

• Protium: 99.99%
• Deuterium: 0.0015%
• Tritium: radioactive and rare; not found in naturally occurring hydrogen because its nucleus is highly unstable.

Isotopes of Carbon

Carbon has three isotopes:

• ¹²C = **98.8%**
• ¹³C = **1.1%**
• ¹⁴C = **0.009%**

• Different forms of carbon are black or greyish-black solids except diamond.
• They are odourless and tasteless.
• They have high melting and boiling points.
• They are insoluble in water.

Isotopes of Chlorine

There are two natural isotopes of chlorine:

• ³⁵Cl = about **75%**
• ³⁷Cl = about **25%**

• Chlorine is a greyish-yellow gas with a sharp, pungent, irritating smell.
• It is fairly soluble in water.

Isotopes of Uranium

Uranium has three isotopes with mass numbers 234, 235, and 238.

• ²³⁴U = **0.006%**
• ²³⁵U = **0.72%**
• ²³⁸U = **99.27%**

• ²³⁵U is used in nuclear reactors and atomic bombs.
• ²³⁸U lacks the properties necessary for these applications.
• ²³⁴U is rare.

Nuclear reactions: - When uranium-238 decays into thorium-234, it emits an alpha particle (a doubly positively charged helium nucleus):

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

• The fission of uranium-235 yields smaller nuclei, neutrons, and energy.
• The nuclear energy released by the fission of one kilogram of uranium-235 is equivalent to the chemical energy produced by burning more than 17,000 kg of coal.

Determination of Relative Atomic Mass

The relative atomic mass of an element can be calculated from the relative masses of its isotopes and their relative abundance.

Formula:

Relative atomic mass = (abundance of isotope 1 × mass of isotope 1) + (abundance of isotope 2 × mass of isotope 2) + …100

Example from the book (Carbon):

• ¹²C = 98.8%
• ¹³C = 1.1%
• ¹⁴C = 0.009%

Solution:

Relative atomic mass of C = (98.8 × 12) + (1.1 × 13) + (0.009 × 14)100

= 1185.6 + 14.3 + 0.126100

= 1200.026100

= 12.00 amu

Uses of Isotopes

Stable and radioactive isotopes have many applications in science and medicine:

• Radioactive iodine-131 — used as a tracer in diagnosing thyroid problems.
• Na-24 — used to trace the flow of blood and detect possible constrictions or obstructions in the circulatory system.
• Iodine-123 — used to image the brain.
• Cobalt-60 — commonly used to irradiate cancer cells in the hope of killing or shrinking the tumors.
• Carbon-14 — used to trace the path of carbon in photosynthesis.

Carbon Dating

Carbon-14 is used to estimate the age of carbon-containing substances.

• Carbon atoms circulate between the oceans and living organisms at a rate very much faster than they decay.
• As a result, the concentration of C-14 in all living things keeps on increasing.
• After death, organisms no longer pick up C-14.
• By comparing the activity of a sample of skull or jaw bones with the activity of living tissues, we can estimate how long it has been since the organism died.

This process is called dating.

Intermediate Level

Isotopes and the Periodic Table

Isotopes occupy the same place in the periodic table because they have the same atomic number.

Example: Chlorine-35 and chlorine-37 are both placed at atomic number 17 in the periodic table.

Why Chemical Properties Are the Same

Chemical reactions involve electrons (especially valence electrons). Isotopes have the same number of electrons because they have the same number of protons. Therefore, they react chemically in the same way.

Why Physical Properties Differ

• Density
• Melting point
• Boiling point
• Rate of diffusion
• Spectroscopic properties

Calculating Relative Atomic Mass

The relative atomic mass is a weighted average of the masses of all naturally occurring isotopes.

General formula:

Relative atomic mass = ∑ (abundance × isotopic mass)100

Worked Example (from the book):

An element has two isotopes A and B. The relative atomic mass of the element is 35.5 amu. Relative abundance of isotope A is 75.77% and its isotopic mass is 35. Find the isotopic mass of B if its relative abundance is 24.23%.

Solution:

Let the isotopic mass of B be x.

35.5 = (75.77 × 35) + (24.23 × x)100

3550 = 2651.95 + 24.23x

24.23x = 3550 - 2651.95 = 898.05

x = 898.0524.23 = 37.06 amu

So the isotopic mass of B is approximately 37 amu.

Important Points

• - Isotopes: atoms of the same element with same atomic number but different mass number.
• - Same protons, same electrons, different neutrons.
• - Chemically alike, but physically different.
• - Word origin: "isos" (same) + "tope" (place) → same place in periodic table.
• - Hydrogen isotopes: protium (¹H), deuterium (²H), tritium (³H).
• - Heavy water: water containing deuterium instead of protium.
• - Carbon isotopes: ¹²C, ¹³C, ¹⁴C.
• - Chlorine isotopes: ³⁵Cl, ³⁷Cl.
• - Uranium isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• - Relative atomic mass is calculated as a weighted average of isotope masses.
• - Uses: iodine-131 (thyroid), Na-24 (blood flow), iodine-123 (brain imaging), cobalt-60 (cancer treatment), carbon-14 (photosynthesis tracer).
• - Carbon dating: uses C-14 to estimate age of once-living things.

Remember

• - Isotopes = same element, different neutrons.
• - Same atomic number, different mass number.
• - Chemically alike, physically different.
• - Hydrogen isotopes: protium, deuterium, tritium.
• - Heavy water = water with deuterium.
• - Carbon isotopes: ¹²C, ¹³C, ¹⁴C.
• - Chlorine isotopes: ³⁵Cl (75%), ³⁷Cl (25%).
• - Uranium isotopes: ²³⁴U, ²³⁵U, ²³⁸U.
• - Relative atomic mass = weighted average of isotopes.
• - Uses: iodine-131, Na-24, iodine-123, cobalt-60, carbon-14.
• - Carbon dating uses C-14.

Common Mistakes

• 1. Thinking isotopes have different chemical properties. They have the same chemical properties.
• 2. Confusing isotopes with ions. Isotopes differ in neutrons; ions differ in electrons.
• 3. Thinking isotopes have different atomic numbers. They have the same atomic number.
• 4. Forgetting that relative atomic mass is a weighted average. It is not simply the mass of the most common isotope.
• 5. Thinking heavy water is just water with extra oxygen. It is water with deuterium instead of protium.
• 6. Confusing ²³⁵U and ²³⁸U uses. ²³⁵U is used in nuclear reactors and bombs; ²³⁸U lacks these properties.

Quick Revision

• Isotopes: atoms of the same element with same atomic number but different mass number.
• Same protons, same electrons, different neutrons.
• Chemically alike; physically different.
• Word origin: "isos" (same) + "tope" (place).
• Hydrogen isotopes: protium (¹H), deuterium (²H), tritium (³H).
• Heavy water: water with deuterium instead of protium.
• Carbon isotopes: ¹²C, ¹³C, ¹⁴C.
• Chlorine isotopes: ³⁵Cl (75%), ³⁷Cl (25%).
• Uranium isotopes: ²³⁴U (0.006%), ²³⁵U (0.72%), ²³⁸U (99.27%); ²³⁵U used in reactors/bombs.
• Relative atomic mass: weighted average of isotope masses.
• Uses: iodine-131 (thyroid), Na-24 (blood flow), iodine-123 (brain), cobalt-60 (cancer), carbon-14 (photosynthesis tracer).
• Carbon dating: uses C-14 to estimate age of once-living things.

60-Second Revision

• Isotopes = same element, different neutrons.
• Same Z, different A.
• Chemically alike, physically different.
• Hydrogen: protium, deuterium, tritium.
• Carbon: ¹²C, ¹³C, ¹⁴C.
• Chlorine: ³⁵Cl, ³⁷Cl.
• Uranium: ²³⁴U, ²³⁵U, ²³⁸U.
• Relative atomic mass = weighted average.
• Warning: isotopes have the same chemical properties but different physical properties.