I am a bit confused about the idea of isotope notation. I know that the top number is the mass number, equivalent to the number of neutrons + protons, while the bottom is the atomic number, equivalent to the number of protons.
What I don't understand is why during beta decay, the electron emitted is represented as having an atomic number of -1. What exactly does it mean to have a negative atomic number? Can the atomic number be considered as just the charge?
Also, my understanding is that the -1 charge balances the gain in the charge due to the proton being formed from a neutron. However, where does this electron originate from if the nucleus originally only contains protons and neutrons?
Thanks for any help, I appreciate it.