* Beta decay: Beta decay involves the emission of a beta particle, which is an electron. This happens when a neutron within the nucleus decays into a proton, releasing an electron (beta particle) and an antineutrino.

* Atomic number changes: Since a neutron turns into a proton, the atomic number of the atom increases by one.

* Mass number stays the same: The total number of protons and neutrons (mass number) remains the same because a neutron is simply converted into a proton.

Applying this to Sr-90:

* Sr-90: Strontium (Sr) has an atomic number of 38 and a mass number of 90.

* Beta decay: When Sr-90 emits a beta particle, the atomic number increases by one, but the mass number stays the same.

* Result: The new isotope has an atomic number of 39 and a mass number of 90.

Looking at the periodic table, the element with atomic number 39 is Yttrium (Y).

Therefore, the isotope formed when Sr-90 emits beta particles is Yttrium-90 (Y-90).