Topic 2 · page 2 of 15
Isotopes
Atoms of one element can have different numbers of neutrons. They behave the same way chemically, but they have different masses.
Read first: Inside the atom
What an isotope is 2.5
Look at the Periodic Table and chlorine has a relative atomic mass of 35.5. Yet every atom contains a whole number of protons and neutrons, each with a relative mass of 1. No atom can have a mass of 35.5.
The answer is that chlorine atoms are not all the same.
The number of protons fixes the element, so isotopes are all chlorine. The number of neutrons changes, so their mass numbers differ.
| ³⁵Cl | ³⁷Cl | |
|---|---|---|
| Protons | 17 | 17 |
| Neutrons | 18 | 20 |
| Electrons (neutral atom) | 17 | 17 |
| Mass number | 35 | 37 |
| Electronic configuration | the same | the same |
Isotopes are named by their mass number: chlorine-35 and chlorine-37. Hydrogen has three isotopes with 0, 1 and 2 neutrons: ¹H, ²H (deuterium) and ³H (tritium). Each has one proton and one electron.
Most elements in nature are a mixture of isotopes, and the mixture is nearly always the same wherever the sample comes from. About three chlorine atoms in every four are ³⁵Cl and one in four is ³⁷Cl. The relative atomic mass of 35.5 is an average over that mixture, closer to 35 because there is more of it. Relative atomic mass shows how to work the average out.
Same chemistry, different mass
Chemical reactions are about electrons. Both chlorine isotopes have 17 electrons in the same arrangement, so both form Cl⁻ ions and both react in exactly the same way.
The neutrons only change the mass. So properties that depend on mass differ slightly:
- density, because the same volume of heavier atoms has more mass
- rate of diffusion, because lighter particles at the same temperature move faster
Check yourself
- Define the term isotope.
- ²⁴Mg and ²⁶Mg are isotopes of magnesium. State one similarity and one difference between their atoms.
- Atom A has 17 protons and 18 neutrons. Atom B has 17 protons and 20 neutrons. Atom C has 18 protons and 22 neutrons. Which two are isotopes of the same element?
- Why do ³⁵Cl and ³⁷Cl react in the same way?
- Hydrogen gas made of ¹H and hydrogen gas made of ²H diffuse at different rates. Which diffuses faster, and why?
- Give the number of protons, neutrons and electrons in each of ¹H, ²H and ³H.
Answers cover these until you have tried the questions
- Atoms of the same element with the same number of protons but different numbers of neutrons.
- Similarity: the same number of protons and electrons (12 each), so the same chemical properties. Difference: ²⁶Mg has two more neutrons (14, against 12), so it has a greater mass.
- A and B. Both have 17 protons, so both are chlorine, with mass numbers 35 and 37. Atom C has 18 protons, so it is argon (a different element).
- They have the same number of electrons in the same arrangement, and chemical reactions depend on electrons.
- ¹H diffuses faster. The lighter particles move faster at the same temperature, so they spread out more quickly.
- ¹H: 1 proton, 0 neutrons, 1 electron. ²H: 1 proton, 1 neutron, 1 electron. ³H: 1 proton, 2 neutrons, 1 electron.