system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is H²O harmless, but H²O²(hydrogen peroxide) very lethal? How does the addition of a single oxygen atom bring such a huge change?
gpt
Because a single oxygen atom is very dangerous in and of itself. Oxygen is very reactive and it *hates* being alone. Whenever it is by itself, it looks for the nearest thing it can attach to and attaches to it.

The oxygen in water is very cozy. It has two Hydrogen buddies that give it all the attention it wants and it has no desire to go anywhere else.

The oxygen in peroxide is different. This is a case of *three's company, four's a crowd.* The hydrogen-oxygen bonds here are quite weaker. Two Hydrogen can keep the attention of a single Oxygen just fine, but they can't keep the attention of two very well. The relationship is unstable and the slightest disturbance - shaking, light, looking at it wrong - causes one of those Oxygen to get bored and look for a better situation. If that situation happens to be inside your body then that can do bad things. The atoms of your body don't particularly like being ripped apart by oxygen atoms. Well, the atoms don't care, but the tissue, organs, and systems that are made of atoms don't like it.

EDIT:

As u/
breckenridgeback pointed out, it is more so the oxygen-oxygen bond that is the weak link here (the structure of H2O2 is, roughly: H-O-O-H). This would leave H-O and O-H when it broke apart but this itself isn't stable. If H2O2 is left to decompose by itself one of those H's will swap over to form H2O and the free O will combine with another free O to form O2.