Here's why:

* Deuterium is an isotope of hydrogen, meaning it has the same number of protons (1) but a different number of neutrons (1 for deuterium, 0 for protium). This makes deuterium heavier than protium.

* Heavy water is chemically similar to regular water (H₂O) but the hydrogen atoms are replaced with deuterium atoms (²H or D), forming D₂O.

Key points to remember about heavy water:

* It's heavier than regular water due to the extra neutron in deuterium.

* It has different physical properties compared to regular water (e.g., higher boiling point, lower freezing point).

* It can be toxic in large quantities, as it disrupts biological processes.

* It's used in nuclear reactors and scientific research.