Question

after an isotope decays by the loss of a beta particle, it has one more proton. where did the extra proton come from?

Explanation:

In beta - decay, a neutron in the nucleus of the isotope transforms into a proton. This occurs because a neutron is made up of an up - quark and two down - quarks. During beta - decay, one of the down - quarks changes into an up - quark through the weak nuclear force. As a result, the neutron becomes a proton, and an electron (beta particle) and an antineutrino are emitted to conserve charge and other quantum numbers.

Answer:

The extra proton comes from the transformation of a neutron in the nucleus of the isotope through the weak nuclear force, where a down - quark in the neutron changes to an up - quark.