in today's video we're going to do a quick review of everything that you need to know about the atom from chemistry like its structure and what isotopes are we'll also look at how electrons can move between different energy levels in the center of every atom is a nucleus which contains individual protons and neutrons the protons are positively charged so one plus while the neutrons are neutral and we say that both of them have a relative mass of one then opening around the nucleus in shells we have electrons which are about two thousand times smaller than protons or neutrons and have a negative charge of one minus if you look at a periodic table each box is known as a nucleus symbol and represents a particular element like this one for lithium inside the box is all the important information that you need to know about that element on the right we have the elemental symbol which represents the element so li for lithium in the bottom left we have the atomic number which is how many protons the atoms of that element have and this one's really important because it's a number of protons that determines which element it is so if our atom over here had four protons instead of three then it wouldn't be lithium anymore in the top left we have the mass number which tells us the total number of protons and neutrons together so lithiums is seven because it has three protons and four neutrons which together makes seven now the nuclear symbol doesn't directly tell you the number of electrons you just have to remember that the number of protons and electrons in an atom is the same which is why lithium has three electrons the next thing we need to look at are isotopes the isotopes of an element are atoms with the same number of protons so the same atomic number but different numbers of neutrons so different mass numbers for example as well as lithium-7 which we've been looking at so far we can also get lithium-6 which has three neutrons and lithium-8 which has five so these three would all be classed as isotopes of lithium because they all have the same number of protons but different numbers of neutrons now generally only one or two of an element's isotopes are actually stable the other isotopes which are unstable tend to decay into other elements by emitting radiation like alpha beta or gamma radiation or sometimes they just emit neutrons we call this process radioactive decay and we'll take a closer look at it in the next video for now though we need to take a closer look at the electrons the electrons in an atom are arranged in a series of energy levels which we call shells with each shell getting progressively further away from the nucleus and increasing in energy level although electrons normally stick to a single shell as they orbit the nucleus they can occasionally jump up to a higher energy level if they gain enough energy and at that point we'd say that the electron has become excited the energy for this comes from electromagnetic radiation and it will only work if the radiation carries just the right amount of energy to get the electron from one energy level to the next pretty soon afterwards though the electron will fall back down to the lower energy level and will re-emit the energy as electromagnetic radiation the very last thing we need to look at is ionization very occasionally one of the outermost electrons should absorb so much energy that is able to completely leave the atom this leaves the atom with more protons than electrons and so it's left with a positive charge which means that we now call it a positive ion rather than an atom also if you hear the term ionizing radiation all it means is that it's able to knock electrons off atoms and so ionize them like we just did for lithium anyway that's everything for this video so if you enjoyed it then do give us a like and subscribe and we'll see you next time