师：同学们，今天我们一起来学习高中粤教版物理选择性必修三第五章第二节放射性元素的衰变。先来看学习目标：一、了解放射性现象，明确放射性现象是由原子核变化引起；二、知道三种射线的特性；三、知道半衰期的概念。
伦琴发现X射线后，不少科学家认为荧光来自X射线。于是，法国物理学家贝可勒尔把一种晶体铀盐放在阳光下照射，因为要产生荧光，就需将荧光物质放在阳光下照射。他发现晶体铀盐使底片感光了，便认为荧光就是X射线。但科学是严谨的，为防止偶然性，他准备重复几次实验。然而天公不作美，一连几天都是阴雨天。他懊丧地从抽屉里拿出样品，冲洗底片，检查纸包是否漏光，结果大吃一惊，底片居然被感光了，正好是铀盐的像。
经过反复实验，贝可勒尔发现，只要把铀盐和照相底片放在一起，不管在多么黑暗的地方，底片都会感光。贝可勒尔断定，含铀的物质能够自发地放出一种射线，这种射线不同于X射线，是从原子核内部发射出来的，同样可以使底片感光。这是科学界最早发现的射线现象，铀也是人们发现的第一个放射性元素，从此对原子核的研究掀开了新篇章。
贝克勒尔发现放射性的消息公布后，立刻引起了居里夫妇的注意，居里夫人当时是贝可勒尔的学生。下面我们通过《放射性物质》这个电影的片段来了解一下。
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Pitch blend, which is a special ore that comes all the way from a mine in Bohemia. And then we use a scientific process to remove the elements we know we can remove and look at what's left.
And what scientific processes might we use? These are very good questions. We crush it.
Which is extremely hard work, and we boil it, which results in the removal of substances. We add certain chemical solutions to it until only that which is pure remains. And there's a lot of substances to remove, but it will be worth it. It's very exciting. And why are you looking at uranium? Thursday at thrill, I'm on. A competent scientist, Becquerel discovered accidentally that uranium salts wrapped in a black cloth left in a drawer left an impression on a photographic plate. And he thought that the uranium was having an unusual chemical reaction with something in the atmosphere.
I say I feel myself a very magical thought. Imagine a grape. How do you turn a grape into wine? By crushing and fermenting. Yes. So imagine a grape crushes itself, ferments itself, changes its very being. And what if I tell you that as the grape turns itself into wine, there's a powerful surge of energy, power that can make things happen? You'd be excited. Am I right? I can see someone. So science is changing. And the very people who are running science, people who believe the world is flat, and I'm gonna prove them wrong, just as Galileo did. We are. Thank you.
Yes.
That's radium, a pinprick of radium gathered from four tons of pitchblende. Is it the most beautiful thing you've ever seen? And you're sure of it. I said yes, it's the most beautiful thing I've ever seen. Well, I guess everything changes now, doesn't it? Yes.
居里夫妇发现一种沥青铀矿的放射性强度比预计的大得多，说明这些沥青矿物中含有一种量少但放射性很强的新元素。在非常简陋且缺乏经费支持的实验室里，居里夫妇合力攻关，发现了这种新元素，它比纯铀的放射性要强400倍。为纪念居里夫人的祖国波兰，新元素被命名为Po。后来他们又发现了第二种放射性元素，其放射性比Po还强，便将这种元素命名为镭。居里夫妇从30吨的铀沥青残渣中提炼出了0.1克的镭盐，接着初步测定了镭的原子量，因此在1911年，居里夫人获得了诺贝尔化学奖。
物质放射出射线的性质叫做放射性，具有放射性的元素叫做放射性元素。研究发现，原子序数大于82的所有元素都能自发地发出射线，原子序数小于83的元素中也有部分具有放射性。我们把能够自发地放出射线的这些元素都叫做天然放射性元素。
同学们，元素的放射性是原子的性质还是原子核的性质呢？我们提供一个实验结果：如果一种元素具有放射性，那么不论它是以单质的形式存在，还是以某种化合物的形式存在，放射性都不受影响，即放射性与元素所处的化学状态无关。同时还表明，放射性的强度也不受温度、外界压强的影响，即放射性与元素存在的状态也无关。
因为元素的化学性质取决于原子核外电子，所以可以断定射线来源于原子核，原子核具有内部结构。