 This video's topic will be nuclear chemistry. We will focus on units. Units is a small lesson to learn, but I'd like to separate it from the other lessons to make sure that you're very clear about the numbers involved. First of all, we have activity, which is simply the ratio of decays per time. It's a very factual way of going about nuclear chemistry. It simply is how many times are there decays? How many times do particles come off the nucleus? And in this case, we have, first of all, the curie, which historically, of course, comes from Marie Curie. And this is if you have one gram and you wait one second of radium, you're going to get 3.7 times 10 to the 10th decays. That's about 37 billion decays. That's just the number. That's just what she measured. Also, we have the becquerel after Henry Becquerel, who thought, you know what, one decay, I'll take that unit for me. Simple. Now let's get into something that's more practical, that has to do with safety. And we will have a lecture later on about dealing with safety with nuclear materials. Radiation-absorbed dose, or rad, a term that you may have heard often, deals with the amount of radiation absorbed by one gram of body tissue. This is an important variable because it does involve health and safety. There is one unit and that is the gray. The gray in this case doesn't have to do with the number of decays over time. It has rather to do with the amount of energy. In chemistry, we often study the energy unit of calorie. In this case, we will deal with the unit of the jewel. A gray is one jewel over a kilogram, but it's the amount of jewels that are in fact absorbed by the tissue. So if a body has radiation emitted upon it, say 100 joules of radiation and only three are absorbed, 97 will go through, three then will be absorbed. That would then be an absorption of three gray of rads. The radiation equivalent in humans is the REM, which is probably the most common way of expressing nuclear effect on humans. And we will get to this later on, especially as it relates to health and working with a Geiger counter, for example. The radiation equivalent in humans is that it measures the biological damage or effect on different types of radiation. And the biological effect in this case may have to do with breaking down cell walls. It may have to do with having effects on cell reproduction. And in many cases, it will have effect on the effectiveness of DNA to maintain its integrity. DNA strands may be breaking up because of this rain of different types of particles. And here we come into the fact that all radiation is not created equal. The REMs, or the radiation equivalent in humans, is equal to the rads that we've seen from over here, multiplied by a type factor. You see, the REM does its best to put apples, oranges, and bananas on an equal footing. In this case, we have alpha radiation, which is rather soft and doesn't get absorbed much by the body. We have beta radiation, which does get absorbed somewhat by the body and is dangerous. And then finally, gamma radiation, which does a significant amount of damage. REMs then are equal to the rads, which have to do simply with the amount of energy multiplied by a type factor. That is to say, one jewel of alpha particles will not have the damage or effect that one jewel of beta particles will. And finally, the unit of the REM is the sievert. Thank you for your attention.