The electron is a standing wave — ψ(r,θ,φ) = Rnℓ(r) · Yℓm(θ,φ).
Only states where the wavefunction closes on itself
(integer wavelengths around the orbit) are allowed — this is why
energy is quantized.
The bottom panel shows the radial probability density r²|R(r)|²
— the likelihood of finding the electron at distance r.