3.5.3 Isotopes of Chlorine

This topic explains the two natural isotopes of chlorine — chlorine-35 and chlorine-37 — including their structure, natural abundance, and properties.

Definition

Isotopes of chlorine are atoms of chlorine that have the same atomic number (17 protons) but different mass numbers due to different numbers of neutrons.

Simple explanation: All chlorine atoms have 17 protons. But they can have 18 or 20 neutrons — giving two natural isotopes.

Main Idea

There are two natural isotopes of chlorine:

• Chlorine-35 (³⁵Cl) — 17 protons, 18 neutrons
• Chlorine-37 (³⁷Cl) — 17 protons, 20 neutrons

Both have 17 protons (that's why they are both chlorine), but they differ in the number of neutrons.

Core Concept

Natural abundance:

Simple explanation: About three-quarters of all chlorine atoms are chlorine-35; about one-quarter are chlorine-37.

Detailed Explanation

Chlorine-35 (³⁵Cl)

Symbol: ³⁵Cl

• 17 protons
• 18 neutrons

• It is the more abundant isotope of chlorine.
• Natural abundance: about 75%.
• It is stable (non-radioactive).

Simple explanation: Chlorine-35 is the lighter and more common form of chlorine.

Chlorine-37 (³⁷Cl)

Symbol: ³⁷Cl

• 17 protons
• 20 neutrons

• It is the less abundant isotope of chlorine.
• Natural abundance: about 25%.
• It is stable (non-radioactive).

Simple explanation: Chlorine-37 is the heavier and less common form of chlorine.

Properties of Chlorine

The book states:

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

Note: These properties apply to chlorine as an element, regardless of isotope. Isotopes have the same chemical properties; their physical properties differ slightly.

Determining Relative Atomic Mass of Chlorine

The relative atomic mass of chlorine can be calculated from its isotopes and their abundances:

Relative atomic mass = (75 × 35) + (25 × 37)100

= 2625 + 925100

= 3550100

= 35.5 amu

Simple explanation: The relative atomic mass of chlorine is 35.5 amu — a weighted average of the two isotopes (75% × 35 + 25% × 37).

Comparison of Chlorine Isotopes

Intermediate Level

Why Chlorine Has Two Isotopes

Chlorine's nucleus contains 17 protons. To be stable, the nucleus needs a certain number of neutrons. Chlorine can be stable with either 18 neutrons (³⁵Cl) or 20 neutrons (³⁷Cl). Both are naturally occurring and stable.

Why the Relative Atomic Mass Is 35.5

Most elements have relative atomic masses that are not whole numbers because they are weighted averages of their isotopes.

For chlorine:

Relative atomic mass = (75 × 35) + (25 × 37)100 = 35.5 amu

This is why the periodic table shows chlorine's atomic mass as 35.5 (or 35.45 in more precise tables).

Chemical vs Physical Properties

Key point: Since both isotopes have 17 electrons, their chemical properties are the same. Their physical properties differ slightly because of different masses.

Real-World Importance

• Chlorine-35 and chlorine-37 are both stable and non-radioactive.
• Their relative abundance is used to calculate chlorine's atomic mass (35.5 amu).
• Chlorine is widely used in water treatment, bleach production, and PVC manufacturing.

Important Points

• - Chlorine has two natural isotopes: ³⁵Cl and ³⁷Cl.
• - Chlorine-35: 17 protons, 18 neutrons; 75% abundant.
• - Chlorine-37: 17 protons, 20 neutrons; 25% abundant.
• - Both are stable (non-radioactive).
• - Same chemical properties (same number of electrons).
• - Different physical properties (different masses).
• - Relative atomic mass of chlorine: 35.5 amu (weighted average).
• - Chlorine is a greyish-yellow gas with a sharp, pungent, irritating smell; fairly soluble in water.

Remember

• - Chlorine isotopes: ³⁵Cl and ³⁷Cl.
• - Both have 17 protons (same element).
• - ³⁵Cl: 18 neutrons; 75% abundant.
• - ³⁷Cl: 20 neutrons; 25% abundant.
• - Both stable (non-radioactive).
• - Relative atomic mass of chlorine = 35.5 amu.
• - Chlorine: greyish-yellow gas; sharp, pungent, irritating smell; fairly soluble in water.

Common Mistakes

• 1. Thinking chlorine-35 and chlorine-37 are different elements. They are both chlorine — same element, different isotopes.
• 2. Thinking the atomic mass of chlorine is 35 or 37. It is 35.5 — a weighted average.
• 3. Thinking chlorine isotopes have different chemical properties. They have the same chemical properties.
• 4. Forgetting that both isotopes are stable. Neither ³⁵Cl nor ³⁷Cl is radioactive.
• 5. Confusing isotope mass with relative atomic mass. Isotope mass is a whole number (35 or 37); relative atomic mass is a weighted average (35.5).
• 6. Thinking the abundance is 50-50. It is about 75% ³⁵Cl and 25% ³⁷Cl.

Quick Revision

• Chlorine has two natural isotopes: chlorine-35 and chlorine-37.
• Chlorine-35: 17 protons, 18 neutrons; about 75% abundant; stable.
• Chlorine-37: 17 protons, 20 neutrons; about 25% abundant; stable.
• Both have the same chemical properties (same number of electrons).
• Different physical properties (different masses).
• Relative atomic mass of chlorine: 35.5 amu — a weighted average.
• Chlorine: greyish-yellow gas; sharp, pungent, irritating smell; fairly soluble in water.

60-Second Revision

• Chlorine isotopes: ³⁵Cl and ³⁷Cl.
• ³⁵Cl: 17p, 18n; 75%.
• ³⁷Cl: 17p, 20n; 25%.
• Both stable; same chemical properties.
• Relative atomic mass of chlorine = 35.5 amu.
• Warning: chlorine's atomic mass is 35.5, not 35 or 37.