Electrons

So far, we have talked mainly about what is inside the nucleus of an atom: protons and neutrons. Circling around outside the nucleus are electrons, the negatively charged subatomic particle. Electrons spin billions of times every second as they circle the nucleus. They are moving very fast and the path that they travel is not the same each time. This means that if we could see these electrons they might appear to look like a cloud around the nucleus.

According to current theory electrons are arranged in electron shells around the nucleus, where each shell has a different discrete energy level. This implies that the energy of an electron is restricted to a few particular energy levels. In other words, the energy of an electron and the energy level of electron shells are said to be quantized, meaning that it cannot vary continuously over a range, but instead are limited to certain values. When electrons gain or lose energy, they jump between shells as they are rotating around the nucleus. For example, as electrons gain energy from photons (small bundles of energy), they might move from the second to the third energy level shell. Then, as they lose energy by emitting photons, they might move back to the second energy level shell or even to the first energy level shell. This process is depicted in the applet below. 

The photon shown in the graphic can be considered a small bundle of energy.

Another thing to note is that only a certain number of electrons can be in an electron shell at the same time. The number of electrons in electron shells follow a very predictable pattern. The closest shell to the nucleus can have up to 2 electrons. The second shell from the nucleus can have up to 8 electrons. The third shell can have up to 18 electrons. The fourth shell can have up to 32 electrons, and so on. Atoms canmany electrons spread out through their electron shells according to this pattern. The greater the distance between the electrons in the outer shells and the protons in the nucleus the less of a force of attraction the outer shell electrons experience to the nucleus. The smaller the distance between electrons and the protons in the nucleus, the more strongly they're attracted to each other.