Explain what is meant by an isotope and state that an element may have more than one isotope
5.1.2 The Nucleus – Isotopes
In the nucleus of an atom, the number of protons (the atomic number $Z$) defines the element. However, the number of neutrons can vary. When atoms of the same element have different numbers of neutrons, they are called isotopes.
What is an Isotope?
An isotope is an atom that has the same number of protons but a different number of neutrons. The mass number $A$ (protons + neutrons) changes, but the chemical identity stays the same.
Think of isotopes like different models of the same car brand. They all belong to the same brand (element), but each model has a different engine size (neutron count). The cars look similar and perform similar functions, but their weight and performance differ.
Examples of Isotopes
Carbon: $^{12}\text{C}$ (6 protons, 6 neutrons) • $^{13}\text{C}$ (6 protons, 7 neutrons)
Hydrogen: $^{1}\text{H}$ (1 proton, 0 neutrons) • $^{2}\text{H}$ (1 proton, 1 neutron) – also known as deuterium
Uranium: $^{235}\text{U}$ (92 protons, 143 neutrons) • $^{238}\text{U}$ (92 protons, 146 neutrons)
Why Is It Important?
- Isotopes have the same chemical behaviour because they have the same electron configuration.
- They differ in physical properties such as mass and stability.
- Some isotopes are radioactive and decay over time, which is useful in dating fossils and medical imaging.
Isotopes in a Table
| Element | Isotope | Mass Number ($A$) | Neutrons |
|---|---|---|---|
| Carbon | $^{12}\text{C}$ | 12 | 6 |
| Carbon | $^{13}\text{C}$ | 13 | 7 |
| Hydrogen | $^{1}\text{H}$ | 1 | 0 |
| Hydrogen | $^{2}\text{H}$ | 2 | 1 |
Exam Tip Box
Remember: Isotopes have the same atomic number $Z$ but different mass numbers $A$.
When you see a question about “different forms of the same element”, think “isotopes”. Use the formula $A = Z + N$ to find the number of neutrons.
🔍 Tip: If a problem mentions a specific mass number, you can immediately identify the isotope.
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