Structure of Matter
5. Isotopes
Learning outcomes
- I can define isotopes as atoms of the same element with different numbers of neutrons.
- I can explain why isotopes have the same chemical properties but different masses.
- I can identify isotopes using atomic notation.
- I can calculate the numbers of protons, neutrons, and electrons in isotopes.
- I can describe common applications of isotopes in science, medicine, and industry.
Introduction
Not all atoms of an element are exactly the same. While every atom of an element has the same number of protons, some atoms contain different numbers of neutrons. These different forms of the same element are called isotopes.
Most elements found in nature exist as mixtures of isotopes. Although isotopes have different masses, they behave almost identically in chemical reactions because they have the same number of electrons. Understanding isotopes is important in chemistry, medicine, archaeology, agriculture, and nuclear science.
What Are Isotopes?
Isotopes are atoms of the same element that have:
- the same number of protons
- the same number of electrons (if neutral)
- different numbers of neutrons
Because they have the same number of protons, isotopes are the same element.
Definition:
Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons.
Why Do Isotopes Exist?
The number of protons determines an element.
The number of neutrons can vary without changing the element.
For example, every carbon atom contains:
- 6 protons
However, carbon atoms may contain:
- 6 neutrons
- 7 neutrons
- 8 neutrons
These are three different isotopes of carbon.
| Isotope | Protons | Neutrons | Mass Number |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 |
| Carbon-13 | 6 | 7 | 13 |
| Carbon-14 | 6 | 8 | 14 |
Why Do Isotopes Have Different Masses?
The mass number is:
Protons + Neutrons
Since isotopes have different numbers of neutrons, they have different masses.
Example:
Carbon-12
- 6 protons
- 6 neutrons
Mass number:
12
Carbon-14
- 6 protons
- 8 neutrons
Mass number:
14
The extra neutrons increase the mass.
Why Do Isotopes Have the Same Chemical Properties?
Chemical reactions involve electrons, not neutrons.
Since isotopes of the same element have:
- the same number of protons,
- the same number of electrons,
they have:
- the same electron arrangement,
- the same chemical behaviour.
This is why carbon-12 and carbon-14 react almost identically in chemical reactions.
Identifying Isotopes Using Atomic Notation
Isotopes are commonly written using nuclear notation.
Example:
\( {14 \brack 6} C \)
This tells us:
- Atomic number = 6
- Mass number = 14
Therefore:
Protons = 6
Neutrons = 14 − 6 = 8
Electrons = 6 (for a neutral atom)
Worked Example 1
Interpret:
\( {23 \brack 11} Na \)
Solution
Protons:
11
Neutrons:
23 − 11 = 12
Electrons:
11
Calculating Subatomic Particles
Remember:
| Quantity | Formula |
|---|---|
| Protons | Atomic number |
| Electrons (neutral atom) | Atomic number |
| Neutrons | Mass number − Atomic number |
Worked Example 2
An isotope has:
- Atomic number = 17
- Mass number = 37
Determine:
- protons
- neutrons
- electrons
Solution
Protons:
17
Neutrons:
37 − 17 = 20
Electrons:
17
Examples of Common Isotopes
Hydrogen
Hydrogen has three naturally occurring isotopes.
| Isotope | Protons | Neutrons |
|---|---|---|
| Hydrogen-1 (Protium) | 1 | 0 |
| Hydrogen-2 (Deuterium) | 1 | 1 |
| Hydrogen-3 (Tritium) | 1 | 2 |
Although they all behave chemically like hydrogen, they have different masses.
Carbon
Carbon commonly exists as:
- Carbon-12
- Carbon-13
- Carbon-14
Carbon-14 is radioactive and is widely used for dating ancient organic materials.
Applications of Isotopes
Isotopes have many important uses.
Medicine
Radioactive isotopes are used to:
- diagnose diseases,
- produce medical images,
- treat certain cancers.
Examples include:
- Technetium-99m for medical imaging.
- Iodine-131 for treating thyroid disorders.
Archaeology
Carbon-14 is used in radiocarbon dating.
Scientists can estimate the age of:
- fossils,
- bones,
- ancient wood,
- cloth,
- archaeological artefacts.
Industry
Radioactive isotopes are used for:
- measuring material thickness,
- detecting leaks in pipelines,
- sterilising medical equipment,
- checking weld quality.
Agriculture
Isotopes help scientists:
- trace nutrient movement in plants,
- improve crop production,
- study water use in agriculture.
Stable and Radioactive Isotopes
Most isotopes are stable.
They remain unchanged over time.
Some isotopes are radioactive.
These isotopes:
- have unstable nuclei,
- emit radiation,
- gradually change into different atoms.
Examples:
Stable:
- Carbon-12
- Oxygen-16
Radioactive:
- Carbon-14
- Uranium-238
Radioactive isotopes will be studied in greater detail in later chemistry and physics courses.
Worked Example 3
A carbon atom has:
- 6 protons
- 8 neutrons
Question
Which isotope is it?
Solution
Mass number:
6 + 8 = 14
Answer:
Carbon-14
Worked Example 4
Two atoms each contain 8 protons.
One contains 8 neutrons.
The other contains 10 neutrons.
Question
Are they different elements?
Solution
No.
Both contain 8 protons.
Therefore, both are oxygen atoms.
They are different isotopes of oxygen because they have different numbers of neutrons.
Real-World Connection
Isotopes improve lives in many ways. Doctors use radioactive isotopes to detect diseases and monitor organ function without surgery. Engineers use isotopes to inspect pipelines and machinery for hidden defects, while environmental scientists trace the movement of pollutants through rivers and groundwater. Archaeologists rely on carbon-14 dating to determine the ages of ancient artefacts and fossils, helping us understand human history and the evolution of life.
Did You Know?
Every second, tiny amounts of carbon-14 are produced naturally in Earth's atmosphere when cosmic rays from space interact with nitrogen atoms. Living organisms continuously absorb carbon-14 while they are alive. After death, the carbon-14 slowly decays, allowing scientists to estimate the age of organic remains that are up to about 50,000 years old.
Key Terms
- Isotope — atoms of the same element with different numbers of neutrons.
- Atomic number — the number of protons in an atom.
- Mass number — the total number of protons and neutrons.
- Stable isotope — an isotope with a stable nucleus that does not undergo radioactive decay.
- Radioactive isotope (radioisotope) — an isotope with an unstable nucleus that emits radiation.
- Radiocarbon dating — a method of estimating the age of ancient organic materials using carbon-14.
- Nuclear notation — a standard way of writing an atom using its mass number, atomic number, and chemical symbol.
Key Takeaways
- Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons.
- Because isotopes have different numbers of neutrons, they have different mass numbers.
- Isotopes have nearly identical chemical properties because they have the same number and arrangement of electrons.
- Atomic notation can be used to determine the numbers of protons, neutrons, and electrons in an isotope.
- Isotopes have important applications in medicine, archaeology, industry, agriculture, and scientific research.
- Understanding isotopes provides the foundation for studying atomic mass, radioactivity, and nuclear chemistry.