What are Isotopes?
CSEC Physics: The Family Members of Atoms
Essential Understanding: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number (Z) but different mass numbers (A). Despite these differences, they share the same chemical properties!
Understanding Isotopes
The word “isotope” comes from Greek words meaning “same place.” This refers to the fact that isotopes of the same element occupy the same place in the periodic table because they have the same number of protons!
The Definition of Isotopes
Z (Atomic Number) = Number of protons (defines the element)
N (Neutron Number) = Number of neutrons (varies in isotopes)
A (Mass Number) = Z + N (changes with neutron count)
Every element has multiple isotopes. For example, carbon has 15 known isotopes, but only 3 occur naturally in significant amounts. Let’s explore what makes isotopes special!
What Stays the Same vs What Changes
Properties That Are THE SAME
- Atomic Number (Z) – Same number of protons
- Electron Configuration – Same number of electrons
- Chemical Properties – Same reactivity and bonding
- Position in Periodic Table – Same location
- Element Identity – Still the same element!
Isotopes of carbon all behave chemically like carbon – they form the same types of compounds.
Properties That Are DIFFERENT
- Mass Number (A) – Different total nucleons
- Atomic Mass – Different actual mass
- Density – Slightly different packing
- Physical Properties – Melting point, boiling point
- Nuclear Stability – Some are radioactive
These differences are due to the different number of neutrons in the nucleus.
Interactive Isotope Explorer
Compare Hydrogen Isotopes
Objective: Click the buttons below to explore the three naturally occurring isotopes of Hydrogen. Notice how the number of protons stays the same while neutrons change!
Hydrogen-1 (Protium)
The most common isotope of hydrogen. It has 1 proton and 0 neutrons.
Notation: ¹₁H
Carbon Isotopes: A Detailed Comparison
| Property | Carbon-12 (¹²₆C) | Carbon-13 (¹³₆C) | Carbon-14 (¹⁴₆C) |
|---|---|---|---|
| Protons (Z) | 6 | 6 | 6 |
| Neutrons (N) | 6 | 7 | 8 |
| Mass Number (A) | 12 | 13 | 14 |
| Natural Abundance | 98.9% | 1.1% | 0.0001% (trace) |
| Stability | Stable | Stable | Radioactive (unstable) |
| Uses | Standard for atomic mass | NMR spectroscopy | Carbon dating |
Natural Abundance of Carbon Isotopes
Most carbon in nature is Carbon-12. Carbon-13 is much rarer, and Carbon-14 exists only in trace amounts due to its radioactivity.
Why This Matters
Carbon-14 is continuously formed in the upper atmosphere when cosmic rays collide with nitrogen atoms. All living organisms absorb carbon (including Carbon-14) while they are alive. After death, they stop absorbing carbon, and the Carbon-14 slowly decays. By measuring the remaining Carbon-14, scientists can determine when an organism died – this is called radiocarbon dating!
Real-World Applications of Isotopes
Archaeology & Dating
Carbon-14 Dating: Used to determine the age of ancient artifacts, fossils, and archaeological specimens up to 50,000 years old.
Uranium-Lead Dating: Used to determine the age of rocks and geological formations (billions of years).
Medical Applications
Cobalt-60: Used in radiation therapy to treat cancer and in medical equipment sterilization.
Iodine-131: Used to diagnose and treat thyroid disorders.
Technetium-99m: The most widely used radioactive isotope in medical imaging.
Industry & Research
Tracer Isotopes: Used to track the movement of materials in industrial processes and biological systems.
Nuclear Power: Uranium-235 and Uranium-238 are used as fuel in nuclear reactors.
Irradiation: Gamma rays from Cobalt-60 are used to preserve food and kill bacteria.
CSEC Practice Arena
Test Your Understanding
Both isotopes are chlorine, so they both have Z = 17 protons.
For Cl-35: A = 35, so N = A – Z = 35 – 17 = 18 neutrons ✓
For Cl-37: A = 37, so N = A – Z = 37 – 17 = 20 neutrons
Answer: Cl-37 has 20 neutrons.
Explanation: Isotopes must be atoms of the same element, meaning they must have the same number of protons (same Z).
• Atom 1: Z = 6, A = 14 → This is Carbon-14
• Atom 2: Z = 7, A = 14 → This is Nitrogen-14
These are different elements! They just happen to have the same mass number by coincidence.
Chapter Summary
Key Definitions
- Isotope: Atoms of the same element with different numbers of neutrons
- Same Z: Same number of protons, same element
- Different N: Different number of neutrons
- Different A: Different mass number (A = Z + N)
Important Facts
- Chemical properties are identical for isotopes
- Physical properties may differ slightly
- Some isotopes are stable, others are radioactive
- Carbon-14 is used for radiocarbon dating
Remember!
Isotopes of the same element behave identically in chemical reactions but have different masses.