Electrons are negatively charged particles that have almost no mass. They constantly whiz around the nucleus of an atom. Sometimes, the electrons in an atom behave like particles orbiting the nucleus, just like the planets around the Sun. In others, the electrons behave like waves frozen in position around the nucleus. Because of this, scientists have put these electrons into shells, with each shell containing a different amount of electrons.
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It is impossible to tell where an electron is at any given time. For this reason, the location of an electron is described as a probability function around the nucleus. The picture on the right shows the "cloud model". The purple dot is the nucleus, while the haze around it is where the electrons might be. The darker the area, the more likely the electrons are there. The 'rings of darkness' are organized into shells. Each shell has subshells. Subshells can only hold a certain amount of electrons. When a subshell is full, the remaining electrons will have to move to the next shell.