Multiple choiceQuestion 1 · Recall
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By the end you can define an antiparticle, state the mass and charge of a positron compared with an electron, and use a positron in a decay equation.
Antiparticle
Every particle has an antiparticle with the same mass but the opposite charge. Other properties, such as the magnitude of the charge and the mass, are identical; only the sign of the charge is reversed.
The antiparticle of the electron is the positron, written or .
| particle | symbol | nuclide notation | charge | mass |
|---|---|---|---|---|
| electron, | about | |||
| positron, | about |
The two numbers in the nuclide notation say it all: both have nucleon number 0, because neither is a nucleon, and the proton numbers are and because the charges are equal and opposite.
A positron is not a proton. A proton has about 1836 times the mass of a positron and lives in the nucleus; a positron has the mass of an electron and is emitted from a nucleus when a proton changes into a neutron.
Because the masses are identical and the charges are exactly opposite, a particle and its antiparticle are mirror images: same inertia, opposite electrical behaviour. In a magnetic field they curve the same way in terms of radius, because the radius depends on mass and speed, but they curve in opposite directions, because the force depends on the sign of the charge.
That mirror property is what makes the pair useful in equations. A positron can take the place of an electron in a decay equation while conserving nucleon number (0 either way) as long as the charge balance is adjusted with it.
Worked example: Balancing a positron in a decay equation
A nucleus of decays by emitting a positron and becomes a nucleus of neon. Write the equation and show that charge and nucleon number are conserved.