What is heavy water? (a) \(\mathrm{H}_{2} \mathrm{O}^{18}\) (b) \(\mathrm{D}_{2} \mathrm{O}\) (c) \(\mathrm{H}_{2} \mathrm{O}^{17}\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Deuterium, represented with the symbol 'D' or as \( {}^2H \), is one of the two stable isotopes of hydrogen. While the most common hydrogen isotope, protium (\( {}^1H \)), has just one proton in its nucleus, deuterium is unique because it has one proton and one neutron. This additional neutron makes deuterium twice as heavy as protium, hence the name 'heavy hydrogen'. Apart from heavy water, deuterium is used in various applications, such as in nuclear reactors for moderation of neutrons. It is also a potential fuel for nuclear fusion, which could provide a substantial source of energy in the future.

Due to its distinct atomic structure, deuterium forms stronger bonds with other atoms compared to protium. This is particularly significant in chemical reactions, where the deuterium isotope can serve as a tracer or in studies related to reaction mechanisms due to its heavier mass and slightly different chemical behavior.

Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons in their nuclei, giving them different atomic masses. Hydrogen has three naturally occurring isotopes: protium, deuterium, and tritium (\( {}^3H \)). Protium, with no neutron, is the lightest and most abundant isotope, making up over 99% of hydrogen found in nature. Deuterium, with one neutron, is less common but still naturally occurring. Tritium, which has two neutrons and is radioactive, is the least common and occurs in nature in trace amounts but can be produced artificially in nuclear reactors.

These isotopes exhibit nearly identical chemical behaviors but have different physical properties due to their mass difference. For instance, because of their varying masses, they will diffuse at different rates. This property is crucial in scientific techniques like isotope separation and in industries like medicine where isotopes can be used for diagnostic purposes.

A chemical formula represents the composition of a molecular substance using symbols and subscripts. It identifies the elements present in the compound and the relative number of atoms of each element. For water, \( H_2O \) is the chemical formula where 'H' represents hydrogen and 'O' is oxygen. The subscript '2' indicates there are two hydrogen atoms for every oxygen atom in a molecule of water.

In the context of heavy water, the chemical formula changes to \( D_2O \) to indicate that the hydrogen atoms are replaced with deuterium. The 'D' symbolizes the presence of this heavier isotope. While the structural layout of a water molecule is maintained in \( D_2O \), the physical and chemical properties of heavy water are distinct from those of ordinary water due to the presence of deuterium. The understanding of chemical formulas is fundamental in chemistry as it provides a quick reference to the makeup of compounds and helps in predicting potential chemical reactions.