Ok – What exactly IS radioactivity? Can someone explain what it is, and how it hurts people and things? The wikipedia entry makes my head go spinny.
How exactly does a rock, like uranium, cause someone to get cancer just by exposure?
IANAPhysicist
Imagine a rock. Over time basic wear and tear will cause the rock to fall apart into smaller granules.
Atoms appear to be like this, and the heavier they are, the faster they fall apart. But when atoms fall apart, their energy radiates out similar to how a split atom releases a ton of energy and destroys bits of Japan, except slower. The energy of these is high enough that it affects your body at an atomic level, throwing bits and pieces of it out of wack. You can think of this like there being a pool full of balls and then someone starts shooting bullets into it. It might visually look the same after since the bullets are so small compared to an entire pool, but really you’re going to have quite a bit of damage in various unknown places. And since this is damage at the atomic level, it’s not like a regular wound that can be healed via a scab. It’s like chopping a guy’s leg off–it ain’t going to grow back–except the guy is your cells.
And remember that cancer is, essentially, just cells that have gone out of wack. Cancer isn’t a disease in the common sense, rather it’s cells that keep multiplying beyond control. Eventually some of those cells will float about in your body and start multiplying themselves in other places, borking up the whole system.
radiation = mutilated, mutant cells = cancer and other badness
Large atoms like uranium are unstable. Sometimes they will spontaneously split apart into smaller atoms. The split throws off debris – high-energy electromagnetic waves (gamma rays) or fast moving particles (alpha and beta rays). That’s radiation.
When radiation hits other atoms or molecules it can do all sorts of destructive things. It can cause those atoms to split and turn into something else, or break the bonds holding atoms together triggering a chemical reaction.
If the radiation hits the molecules that control metabolism inside a cell it can trigger bad effects. It can kill off the cell, but it can also switch on the molecular machinery that controls cell division. When a cell starts dividing uncontrollably and can’t stop, that’s cancer.
Why are radioactive things unstable? What makes a ball of iron not break down and shoot off energy compared to a uranium rock?
Of the top of my head…
There are four different types of radioactivity.
Gamma rays are a very energetic type of electromagnetic (EM) radiation. EM radiation includes light, radio, and x-rays. Gamma rays are energetic enough that they can destroy or alter the DNA of individual cells in your body, which either results in the death of the cell, or can cause the cell to go nuts and reproduce uncontrollably (i.e. cancer).
You’ve also got beta particles (which are energetic electrons) and alpha particles (energetic helium nuclei). Being charged, both of these are easily blocked (even by your skin), but they can cause problems if something emitting these is inhaled or ingested.
Finally, you’ve got neutrons, which are emitted by nuclear reactions (including reactors and nuclear weapons). These do damage due to their speed. If they hit an atom in your body, they cause damage.
Since you asked, uranium decays by emitting alpha particles. This chart shows the decay chain for uranium-238 (the most common isotope). The daughter product of uranium decay is thorium-234, which itself decays by emitting beta particles and gammas.
It is, just slower.
The coolest analogy I’ve come across for a chain reaction is to imagine each molecule in a radioactive substance to be a mouse trap with a ping pong ball on it. If there are none of these molecules in a room, and I throw a ping pong ball (neutron) in it, nothing’s going to happen. If there are, say, ten contraptions, one or two of them might go off. However, if there are 1,000 of them in the room, the original ping pong ball will set one off, which will likely set two off, which will likely set four off, etc.
While radioactive elements can spontaneously decay, they can also be induced to decay by being bombarded with neutrons. This is why critical mass is also dependent on density and purity. In this model, radioactive “poisons” could be modeled as sticky mouse traps.
As a professional layman, I’ll see if I can remember the answers I got from some of the dopers on this board previously.
The nucleus of an atom has neutrons and protons. they’re held together by the “strong force.” But protons are also electrically charged. Since they all have the same positive charge, they are trying to fly apart. The neutrons have no charge to make them fly apart but still have the strong force holding them together. So if you get enough of them in there, they can hold the protons too. When the atoms start getting too big, even the neutrons can’t that many protons together and that’s when pieces start flying off. Those pieces are the radioactivity. Neutrons apparently are unstable too. So too many of them will create an unstable atom too.
For atoms, it’s really complicated. While people generally think in terms of really heavy elements, it also applies to even the lightest ones too. E.g., an isotope of hydrogen with 1 proton and 2 neutrons that has a half life of a little over 12 years.
One key matter in atomic stability is if the number of protons and neutrons are both even or not. The number of long term stable isotopes with even-even counts is something like 164. The number of long term stable odd-odd isotopes is only 4 (IIRC). 3 of those are only occur as traces compared to the more common version of the elements. The 4th is Nitrogen-14 (7 and 7). It’s the only common odd-odd stable isotope in the universe.
Physicists in the 40’s and 50’s worked out a model of nuclei that explains this. Not exactly simple message board-level material. But things like to come in pairs.
When it comes to individual particles, e.g., mesons that decay in an incredibly tiny fraction of a second, there is little explanation other than “that’s the nature of the beast”. There are some models that have predicted decay times for some particles but I don’t know offhand of any that have been confirmed. And if they are confirmed, again, you’d need to know some serious Physics as well as Math to understand it.
At at the deepest level, a particle (even a nuclei) is viewed as a smeared out cloud of probabilities. At some point the probabilities just happen to shift into a position such that decay will happen. Under most models of Quantum Mechanics, there are “no hidden variables”. I.e., no underlying state of the particle that could be measured to determine whether it is “about to break up.”
A simple example of a probabilistic effect is bouncing photons off a thin piece of glass at a sharp angle. If the glass is thick enough, all will reflect off. Make it thin enough, then a big fat fuzzy photon (in terms of a probability cloud) might luck out and find itself on the other side of the glass and continue on. So you end up with two beams off the glass, one reflected and one refracted. It just comes down to chance. You can calculate the odds, but you can’t mess with them. Ditto with decay. (When it comes to just measuring.)
You may as well ask why a certain stack of cannonballs is unstable. As you stack them higher and higher, the potential energy increases due to gravity and distance from the ground. There are only certain ways you can add more cannonballs, otherwise some of them will release the energy by falling to the ground (call it “decay”). Note that even very small stacks can decay if they are not stacked in a stable way.
Nuclear physics is not the same as classical physics, but the principles are similar. An atomic nucleus is a “stack” of protons and neutrons. Nuclei are stable only in certain configurations. In unstable configurations, the nucleus decays as the unstable particles “fall off”. Radiation is produced by decay.
To answer your question, uranium is less stable than iron because it has a less stable configuration than iron. The atomic weight of iron is 55, uranium is 238. Many stable configurations exist for lighter elements. For elements as heavy as uranium, very few stable configurations exist. That’s why there aren’t many natural elements heavier than uranium.
Here is a graph of islands of stability showing which combinations of protons and neutrons are likely to be stable for heavier elements.
Something that hasn’t been explicitly mentioned is that when we talk about radioactive elements, we are talking about nuclear radiation. Electrons in an atom (at least the outermost ones) get rubbed off and recaptured all the time. You can observe this by rubbing a balloon on your head. When you do that, you have rubbed off some of the electrons. Electrons will often get “bumped up” to higher energy levels analogous to moving a cannon ball up to a higher stair on a flight of stairs. When the electron moves back down, it gives off a photon. Depending on how far it fell, that photon will be heat or light. This kind of activity is called ordinary, non-nuclear or chemical to distinguish it from nuclear reactions.
In many nuclear reactions, new elements are created and old ones are destroyed. What makes an element a particular type of element is the number of protons in its nucleus. Change that and you have a new element with different properties.
FWIW,
Rob
Generally, we speak of radioactivity when something gives off ionizing radiation – i.e. radiation energetic enough to rip electrons from atoms to ionize them. This can especially destroy chemical bonds, damaging molecules; if either the DNA is damaged directly or is damaged by free radicals created by this process, we have a mutation, and this mutation might cause a cell to become cancerous (to paint with a broad brush).
As for why certain nuclei decay while others are stable (btw, iron, contrary to what was said upthread, is stable in its most common isotopes), this threaddiscusses the matter in some detail.