let's suppose it does not emit an alpha particle. it can emit a beta particle. if it emits a beta particle. beta particle, beta particle, beta--what's a beta particle? - electron. - electron. now we have to make the symbol for electron. for electron, get it? mm-hmm. what's the atomic mber of an electron? - negative o. - atomic number? negativene. what is atomic number, anyway? charge, right? what's the charge of an electron? - negative one. - negave one. what's the mass of an electron? zero. well, it's closer to zero than it is to one if one is the mass of a proton neutron. and this has got, like, about 2,0th the mass. that's closer to zero than to one. so we round off atomic mass number, anyway. this is zero. now, what does the thorium become? well, what do we got over here for an atomic number? 91 do you see it's 91? yeah. 91. and what's the mass gonna be? - 89. - now, what has 91? protactinium. - is it gonna be 91, gang? - yeah. let's check. - 91 minus 1 equals 90. - correct. it's 91. that's interesting. some people think that radioactive decay, you start wit

u238 emits alpha particles, but 239 emits beta particles. and guess what the 239 does, gang? it turns into an element beyond uranium. we talked last time about decaying up the periodic table. what's gonna be the atomic number of the element that this will decay into? what number plus a negative one give me 92? 93. lee, okay. anyone else besides lee? how many of you say it's 93? all right. okay. you guys are just being a little reticent, right? okay. it's gonna be a 93. and that 93 is a brand-new element. it's beyond uranium called transuranic and that's neptunium. you know why they call it neptunium? you know what the first planet was they discovered in terms of newton's laws? neptune, gang. neptune. so they call this neptunium. first element beyond uranium and that's neptunium 239. it's got a very, very short half-life. and you know what it does? it decays. and guess what it emits? begins with a b. no, not alpha. try it again. it emits a beta particle. okay. it emits a beta particle. and what does it turn into? how many people can figure out what the number is here that coul