# As Physics help: The Isotope potassium K (atomic number = 40 mass number = 19) can decay by positron emission?

The Isotope potassium K (atomic number = 40 mass number = 19) can decay by positron emission to form an isotope of aragon. Complete the following equation which represents this decay:

K (40,19) - ....... + ...... + .........

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"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,

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• Mass Number Potassium

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