Q1.
Can you guess how many electrons each of these isotopes have?
Answer
One electron each. Protium, deuterium and tritium all have exactly 1 electron.
| Isotope | Symbol | Protons | Neutrons | Mass number A | Electrons |
|---|---|---|---|---|---|
| Protium (~99.98%) | 11H | 1 | 0 | 1 | 1 |
| Deuterium (~0.015%) | 21H | 1 | 1 | 2 | 1 |
| Tritium (traces) | 31H | 1 | 2 | 3 | 1 |
Reasoning: all three are hydrogen, so Z = 1 for each. A neutral atom has as many electrons as protons, so each has 1 electron, sitting in the K-shell.
Why it happens: this is precisely why isotopes have the same chemical properties. Chemistry is decided by the valence electrons, and all three have the identical configuration “1”. They differ only in neutron count, which changes the mass — so their physical properties such as boiling and melting points do differ slightly.
Did you know? Water made with deuterium instead of protium is called heavy water, D₂O. It is chemically water, but it is about 10% denser and it freezes at 3.8 °C instead of 0 °C. Indian nuclear reactors such as those at Kakrapar and Kaiga use heavy water as a moderator.