When an atom of radium-226 undergoes alpha decay, it becomes radon-222.

Here's why:

* Alpha decay involves the emission of an alpha particle, which is essentially a helium nucleus (2 protons and 2 neutrons).

* Radium-226 has 88 protons and 138 neutrons (226 total).

* When it emits an alpha particle, it loses 2 protons and 2 neutrons.

* This results in an atom with 86 protons and 136 neutrons, which is the composition of radon-222.

So, the decay equation is:

Radium-226 → Radon-222 + Alpha particle