Electrons don’t move identical around nucleus. There is a big difference between an atom that has one electron and an atom that has two or more electrons. Electrons have the same electric charge, negative. They can not come closer to each other as they want, just as they are allowed by electrostatic force of rejection of electric field of each electron they meet during their movement rotation around the nucleus.
Electrons can have circular or elliptic trajectory. When the electron’s trajectory is elliptical, the proton must be in one of the two focuses. During the rotation movement of electron, circular of elliptical, the plan of movement tilts more or less against the proton, against its own orbital, against its own rotation axis, and against many other things. Ugly, isn’t it? Because inside of the atom are more than one referential, at the same time an electron is in centipede or centrifuge accelerated movement. Truly difficult stuff!
During its movement around nucleus, the electron is not always at the same distance to the nucleus. Maximum distance to the nucleus an electron can be is called electronic layer. The electron releases or absorbs energy while it comes closer or further from the nucleus; multiple quantum of energy. The smallest amount of energy that can not be divided is called quantum. In order for an electron to escape the last electric layer, it needs additional energy from outside the atom. This energy is called ionization energy because the atom becomes a positive ion.