The part of an atom that causes radioactivity is its nucleus. Specifically, it's the unstable combination of protons and neutrons within the nucleus that leads to radioactivity.

Here's why:

* Protons: These particles have a positive charge and repel each other.

* Neutrons: These particles are neutral and help to hold the protons together.

When the ratio of protons to neutrons in the nucleus is off-balance, the nucleus becomes unstable. This instability manifests in various ways:

* Alpha Decay: The nucleus emits an alpha particle (2 protons and 2 neutrons), essentially a helium nucleus.

* Beta Decay: A neutron transforms into a proton, emitting an electron (beta particle) and an antineutrino.

* Gamma Decay: The nucleus releases energy in the form of gamma rays (high-energy photons).

These decay processes are what we call radioactivity, and they are the result of the nucleus trying to achieve a more stable configuration.

It's important to note that not all atoms are radioactive. Only those with an unstable nucleus will undergo these decay processes.