Caption: An ABSTRACT diagram of a hydrogen atom changing energy levels and emitting a photon. In general, a change in energy levels is called a atomic transition.
Features:
Atoms can be imaged directly in various ways. But they do NOT have sharp edges, and so they look like fuzzy balls. In general, a real image of an atom does NOT give a clear picture of its structure in any direct way.
But it is right in that there are NOT a continuum of energy states allowed for the electrons that surround the atomic nucleus which for hydrogen (the simplest of all atomic nuclei) is a single proton represented by the central dot in the diagram. A hydrogen atom also has only one electron represented by a small dot in the diagram.
Instead of a continuum of energy states, there is only a discrete set with discrete energies. This is main reason why we call quantum mechanics quantum mechanics: the energy states are QUANTIZED.
The quantized energy levels in the ABSTRACT diagram are represented by a quantized set of circular orbits.
The larger the circular orbit, the higher the energy of the energy level.
The smallest circle represents the ground state, the lowest energy energy level allowed by quantum mechanics.
The Bohr atom did posit actual circular orbits, but that turned out to be WRONG.
In fact, each energy level is a spread out density distribution for an electron. The electron exists in a continuum superposition of positions with the amount of it at any point being determined by the density distribution.
The electron is usually only one energy level at at time---but it can be in a superposition of energy levels.
The energy of the emitted or absorbed photon is determined by the conservation of energy.
An absorption process requires an incident photon.
In the ground state, the atom simply has no REMOVABLE energy.
It does have an IRREMOVABLE zero-point energy dictated by quantum mechanics.
This means only certain frequencies and wavelengths are allowed for the emitted or absorbed photons.
Wavelength is given by de Broglie relation ΔE=hc/λ, where λ is wavelength.