Isotopes: the same element with different masses
Atoms of one element always have the same number of protons, so the same Z. But they can have different numbers of neutrons. Such atoms are called isotopes: same Z, different A. Hydrogen has three isotopes. Protium has 1 proton and no neutron, deuterium has 1 proton and 1 neutron, and tritium has 1 proton and 2 neutrons. All three are hydrogen, with one electron each. Isotopes are not different elements. The chemical behaviour of an atom depends on its electrons, and isotopes of an element have the same number of electrons, so they behave alike in chemical changes. They differ in mass, because they differ in neutrons.
| Isotope | Z | A | neutrons | electrons |
|---|---|---|---|---|
| protium ¹₁H | 1 | 1 | 0 | 1 |
| deuterium ²₁H | 1 | 2 | 1 | 1 |
| tritium ³₁H | 1 | 3 | 2 | 1 |
| Isotope | Z | A | n |
|---|---|---|---|
| ¹²₆C | 6 | 12 | 6 |
| ¹³₆C | 6 | 13 | 7 |
Isotope cards. Make cards for the three isotopes of hydrogen with their Z, A, protons and neutrons. Mix them with cards for carbon-12 and carbon-13. Sort the cards into two piles, hydrogen and carbon, and say what is the same and what is different inside each pile. Research task: find one use of isotopes and tell the class.