"The Isotope potassium K (atomic number = 40 mass number = 19)"
No – it’s the other way around. Atomic number (Z) = 19, mass number (A) = 40.
Z (atomic number) is the number of protons = 19.
A (mass number) is the number of (protons + neutrons) = (19 protons + 21neutrons) = 40 nucleons
What happens in β+ decay is that a proton (p) changes into:
- a neutron (n) and
- a positron (the β+ particle),
- an electron-neutrino (often just called a neutrino, symbol Greek letter 'nu', 'ν'):
p -> n + (β+) + ν
You will need to learn this.
The new nucleus therefore now has 18 protons and 22 neutrons. It is a different element: the new atomic number is 18 (the mass number is the same (40) as the proton has simply turned into a neutron, hardly affecting the mass).
This new element (Z=18) is argon. So K-40 has turned in Ar-40. We could write the equation as:
K(40,19) -> Ar(40,18) + (β+) + ν
If you couldn’t find out the symbol of the new element, you would put 'X(40,18)' and write a footnote saying 'X is the symbol for the new element with Z=18’, or words to that effect; you'll still get full marks,