Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because they are the same element, they have the same proton number but different nucleon numbers.
Why isotopes behave alike chemically
Chemical properties depend on the number and arrangement of electrons, not neutrons. Isotopes of an element have the same electron arrangement, so they react in the same way. They differ only in mass, which affects physical properties such as density and rate of diffusion.
| Isotope | Protons | Neutrons |
|---|---|---|
| Carbon-12 (126C) | 6 | 6 |
| Carbon-14 (146C) | 6 | 8 |
Key idea / 要点
Isotopes have the same proton number but different nucleon numbers (different neutrons). Same electrons → same chemical properties; different mass → different physical properties.
Worked example / 例题
Chlorine has two isotopes: 3517Cl (both have 17 protons; one has 18 neutrons) and 3717Cl (20 neutrons). Both react with sodium to form the salt NaCl in exactly the same way, because both have the electron arrangement 2, 8, 7. Their average gives chlorine's relative atomic mass of about 35.5.
Uses of isotopes
- Medicine: cobalt-60 in radiotherapy to treat cancer; iodine-131 to check the thyroid gland.
- Archaeology: carbon-14 dating to estimate the age of ancient remains.
- Industry: tracers to detect leaks in underground pipes; checking the thickness of metal sheets.
- Agriculture: labelled fertilisers to study how plants absorb nutrients.
These uses are possible because a radioactive isotope gives out radiation that can be detected, while still behaving chemically like the ordinary element. This lets scientists follow where an atom goes inside a pipe, a plant or the human body. Such applications must always be handled with care, since too much radiation can harm living cells.
Remember / 记住
- Isotopes: same protons, different neutrons.
- Same chemical properties, different physical properties.
- Carbon-14 is used for dating; cobalt-60 in cancer treatment.