Asked by: Alan Dunne, Ireland
The simple answer would be about one-twentieth of a nanometre. But this would only be reasonable if the electron were a solid particle.
Quantum mechanics tells us that we can only speak of an electron’s orbital position in a statistical sense. We talk about the electron’s ‘wave function’, which gives us a measure of that probability, which has a value arbitrarily close to the nucleus, and also on the other side of the galaxy. The maths work, even though we can’t picture what it is working on.