system
Given the following post, respond as if you're commenting on Reddit:
human
Why heavy water is toxic, while ordinary water isn't?

If chemical properties of substances depend only on the outermost "layer" of electrons around the molecule, how come ordinary water isn't toxic, while heavy water is?
gpt
There are a few reasons. Heavy water is called so because there are a lot of deuterium atoms in the water molecules instead of normal hydrogen. Hydrogen is just a proton with an electron. Deuterium is a proton, a neutron, and an electron. This alters the atom in some subtle ways.

First, the mass of the atom is basically doubled. A neutron has a slightly higher mass than a proton, and an electron's mass is negligible. That added mass can affect the chemical properties of the water.

Second, and most importantly, the hydrogen bonds formed by heavy water are slightly stronger than those of normal water. There are thousands of processes in the body that rely on the very specific strength of hydrogen bonds. The stronger bonds found in heavy water can inhibit cell division, make protein action inefficient, or disrupt the proper functioning of cell membranes.

Heavy water isn't radioactive like many other isotopes can be, but its particular chemical properties can cause symptoms similar to radiation poisoning. It's non-toxic in small doses, and is actually used for some diagnostic tests in medicine, but once the levels increase to 25% or more of the water in your body, severe symptoms start to appear.