A quantum particle (an atom, an electron, a photon…) has quantized properties, hence the term “quantum”. This means that its properties can only take certain specific values. For instance, if you tried to imagine a quantum object going up a staircase, it would suddenly go from one stair to another with no intermediate stage. Thus, the energy of the electron in an atom can only have certain, precise values.
This is why all atoms with the same number of electrons have exactly the same properties, whether it be in France or in Australia. If this quantization did not exist, the electron would adopt any energy and quickly collapse on the nucleus of the atom, leading to the rapid destruction of our universe!
Where does this quantization come from? We can try to understand the origin in the situation of the electron around the atom. The electron is in some way “stuck” around the atom as if it were trapped in a box. This electron is both a particle and a wave with peaks and troughs (physicist call them “periods”).
The situation is similar to that of waves in an aquarium full of water. The wave bounces off the edges. It is only if the waves juxtapose perfectly when they bounce that the wave can survive. The wave can have only certain distances between its peaks in order to persist; it is “quantized” (physicists call it modes of the waves). You will find a similar effect if you pluck the string of a guitar. The wave on a particular string can have only a certain shape and a certain number of peaks and troughs, because the length of the string, which makes a particular note, is definite, like the length of the aquarium.
Many properties are quantized on a nanometre scale: energy, momentum (mass times speed), magnetization (called “spin”), angular momentum, and magnetic flux as in a superconducting ring…
