Here's how it works:

* Beta decay is a type of radioactive decay where a neutron in the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino.

* Iodine-131 has 53 protons and 78 neutrons. When it undergoes beta decay, one neutron changes into a proton, increasing the atomic number to 54, which is Xenon.

* The decay equation is: ¹³¹₅₃I → ¹³¹₅₄Xe + β− + ν̅ₑ

This decay mode is important to know because it explains how iodine-131 is used in medical applications, such as treating thyroid cancer.