Isotopes - Same Element, Different Number of Neutrons
In 1946, Willard Libby invented carbon-14 dating, a technique that can date bones and wood up to 50,000 years old using just a few grams of material.
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Q1 · Can two atoms of the same element be slightly different from each other? What do you think could vary?
Q2 · Why do you think it matters whether atoms of the same element have different numbers of neutrons?
● Know
- isotopes have the same proton number
- isotopes differ in neutron number
- different isotopes can have different mass numbers
● Understand
- same element means same atomic number
- isotopes show why proton number matters more than mass number for identity
- some isotopes have useful practical applications
● Can do
- identify whether two atoms are isotopes
- explain isotope examples clearly
- link isotope uses to the concept without doing advanced maths
Carbon atoms in a diamond look identical, but roughly 1 in every trillion is slightly heavier, that tiny mass difference is what separates carbon-14 from carbon-12, and it's the basis of isotopes: atoms of the same element with different numbers of neutrons. The key rule is: same proton number means same element. Carbon-12 and carbon-14 are both carbon because both have 6 protons. The difference is in the neutrons: carbon-12 has 6, carbon-14 has 8.
Isotopes have the same chemical properties because they have the same electron arrangement, but some isotopes are radioactive while others are stable. At this level, you do not need to calculate isotope abundance, you only need to understand that proton number fixes identity while neutron number can vary.
Hydrogen has three isotopes: hydrogen-1 (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron) and tritium (1 proton, 2 neutrons). All are hydrogen because all have 1 proton. Deuterium is stable and used in nuclear research. Tritium is radioactive and used in glow-in-the-dark emergency signs. Same element, different neutrons, different properties.
Australian archaeology: Carbon-14 dating, used by archaeologists at universities across Australia, relies on the predictable decay of carbon-14 to determine the age of organic samples up to about 50,000 years old. Indigenous Australian artefacts have been dated using this technique, connecting modern science with ancient history.
'Isotopes are different elements.' They are not. Isotopes are different versions of the same element. The element is defined by proton number, and isotopes share that proton number. What changes is the neutron count, which affects mass number but not chemical identity. Carbon-12 and carbon-14 are both carbon, just as deuterium and tritium are both hydrogen.
Use the Isotope Builder interactive below. What is one thing you learned from using it?
Complete this isotope calculation for carbon-14.
Number of neutrons = A − Z = − = .
Neutrons change the mass number but not the element name. Carbon-12 and carbon-14 are both carbon because both still have six protons. The changing neutron count explains the different mass numbers. At this level, the lesson stays conceptual and does not move into isotope abundance calculations.
The important skill is recognising that proton number fixes identity, while neutron number can vary. When you see two atoms with the same Z but different A, you should immediately think 'isotopes.' When you see different Z values, you should think 'different elements,' regardless of neutron count.
Oxygen has three stable isotopes: oxygen-16 (8 protons, 8 neutrons), oxygen-17 (8 protons, 9 neutrons) and oxygen-18 (8 protons, 10 neutrons). All are oxygen. All react the same way chemically. But oxygen-18 is slightly heavier and is used in medical tracer studies to track how the body processes water.
Australian healthcare: ANSTO produces medical isotopes like molybdenum-99, which decays to technetium-99m for diagnostic imaging. Understanding isotopes is essential for Australia's healthcare system, these isotopes help doctors detect cancer, assess heart function and diagnose bone disorders.
'All isotopes are radioactive.' This is false. Many isotopes are completely stable. Carbon-12 is stable and makes up 99% of all carbon. Oxygen-16 is stable and makes up 99.8% of all oxygen. Whether an isotope is radioactive depends on the balance between protons and neutrons in the nucleus, not on being an isotope.
Isotope questions are useful because they force you to apply the rules carefully. Ask first: are the proton numbers the same? If yes, the atoms are the same element. Then ask: are the neutron numbers different? If yes, they are isotopes. This two-step check is more reliable than guessing from names or pictures.
If the proton numbers differ, the atoms are different elements regardless of neutron count. Never use mass number alone to identify an element, always look at the proton number first.
Atom A has Z = 6, N = 6. Atom B has Z = 6, N = 8. Same protons → same element (carbon). Different neutrons → isotopes. Atom C has Z = 8, N = 8. Different protons from A and B → different element (oxygen). The neutron count does not matter for element identity. Only proton number matters.
Australian geology: Geologists at the University of Queensland use this exact logic when analysing rock samples. By comparing proton and neutron counts in mineral isotopes, they can trace the geological history of Australian landscapes and date ancient rock formations in the Pilbara and Yilgarn cratons.
'If two atoms have the same mass number, they must be the same element.' Not true. Carbon-14 (6 protons, 8 neutrons) and nitrogen-14 (7 protons, 7 neutrons) both have mass number 14, but they are different elements because their proton counts differ. Always check proton number first, mass number can be the same by coincidence.
Sort each pair of atoms into the correct bin.
Write a short explanation of why carbon-12 and carbon-14 are both carbon even though the numbers are different.
Isotopes are not just abstract atom examples. They can be useful in the real world.
At this level level, you should know that some isotopes are used in medicine, tracing and imaging because scientific understanding of substances can change how people use them.
The emphasis is on the broad idea of use, not detailed nuclear science.
Compare three atom descriptions and decide which pair are isotopes. Explain your reasoning using proton number and neutron number.
At the start of this lesson, you were asked about carbon-14 being used in archaeological dating while carbon-12 is in every living thing, and how two carbon atoms can share the same number of protons but have different masses.
Now that you have worked through everything, write your answer below. How has your thinking changed, and what surprised you most?
Q1. Define an isotope using proton number and neutron number.
Q2. Explain why two atoms with the same atomic number but different mass numbers can still be the same element.
Q3. Describe one broad real-world use of isotopes and explain why this matters for science learning.
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: D. Isotopes have the same proton number but different neutron numbers.
2: A. The proton number must stay the same.
3: B. Same atomic number but different mass number suggests isotopes.
4: C. Isotopes differ in neutron number, which changes mass number.
5: A. Some isotopes have practical uses such as tracing and medicine.
Short Answer 1
An isotope is an atom of the same element that has the same number of protons but a different number of neutrons.
Short Answer 2
They are still the same element because element identity depends on proton number. Different neutron numbers change the mass number, but not the atomic number.
Short Answer 3
One broad use is medical tracing or imaging. This matters because it shows that scientific understanding of atoms and substances can influence how materials are used in society.
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: D. Isotopes have the same proton number but different neutron numbers.
2: A. The proton number must stay the same.
3: B. Same atomic number but different mass number suggests isotopes.
4: C. Isotopes differ in neutron number, which changes mass number.
5: A. Some isotopes have practical uses such as tracing and medicine.
Short Answer 1
An isotope is an atom of the same element that has the same number of protons but a different number of neutrons.
Short Answer 2
They are still the same element because element identity depends on proton number. Different neutron numbers change the mass number, but not the atomic number.
Short Answer 3
One broad use is medical tracing or imaging. This matters because it shows that scientific understanding of atoms and substances can influence how materials are used in society.
● Isotope Rule
Same protons, different neutrons.
● Identity
Proton number decides the element.
● Mass Number
Different neutron numbers can change mass number.
● Use
Some isotopes have practical roles in medicine and tracing.