How do we know that most of an atom is empty space? And since that is so, why can’t we just walk through solid objects?

Question

How do we know that most of an atom is empty space? And since that is so, why can’t we just walk through solid objects?

In: 13

Answer 1

tl;dr: prison bars are mostly empty, but you still can’t walk through them.

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Rayleigh’s experiments involved firing alpha particles (what we now know are helium nuclei) at a thin sheet of gold. He expected the particles to bounce off in a broad spread of reflection angles, but to his surprise, almost none bounced off the gold atoms, except for a few, which reflected nearly 180° – this implies the nucleus isn’t a smooth blob (the ‘plum pudding’ module), but rather has almost all its mass concentrated into a tiny volume/dot – which we now know is a very accurate description.

So, atoms are mostly empty in the classical ‘electrons orbiting a nucleus’ sense. We can’t walk through a material because those electrons exert a strong electrostatic repulsion against the electrons in your body/clothes when you get close enough.

So _atoms_ are mostly empty_, but the force fields exerted by electrons, protons, and neutrons, fill all space per an inverse square law. Quantum mechanics adds an extra degree of blur, but isn’t necessary to explain the broad phenomenon.