radioactive decay...what the?!

[aye]

i never really paid attention to this until it was on the 4r. i don't understand it conceptually....

basically, when the atom loses or gains protons/neutrons, wouldn't that result in the atom havin a net charge?? for example, in beta -, you convert a neutron into a proton and eject an electron... so that would give the atom a positive charge?? and vice versa for beta +. i would think it was reverse: beta - would lose a positron and beta + would lose an electron... so it would be electrically neutral...

can anyone please help me out... thanks!

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[Psycho Doctor]

Many nuclei are radioactive which is ultimately unstable, and will eventually decay by emitting a particle, transforming the nucleus into another nucleus, or into a lower energy state. During radioactive decay, principles of conservation apply: conservation of energy, conservation of momentum, conservation of charge and conservation of nucleon number. Conservation of nucleon number means that the total number of nucleons must be the same before and after a decay.

In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable. A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. In gamma decay, the nucleus changes from a higher-level energy state to a lower level. It's jus tlike the energy levels for electrons in the atom, the nucleus also has energy levels. The concepts of shells, and more stable nuclei having filled shells, apply to the nucleus as well.

 

[chopper]

Do not think of 'total charge' on an atom with decay problems - electrons have nothing to do with it. You are ONLY worried about what is happening in the nucleus. So don't worry about the 'net charge' and all that.

Psycho doctor may be right - but I think he's over-complicating it for MOST everything you will need on the MCAT. Just think of this: All the mass numbers (total neutrons and protons) and all the charge (protons and electrons) must be conserved across the 'equals' sign. So, for example - the Beta decay of a C-14 nucleus. On the left, you have 6 '+' charges from the 6 protons, and 14 total mass units from the 6 protons and 8 neutrons. On the other side, you have 7 '+' charges from 7 protons and 1 '-' charge from the beta particle. Total is +6 again. You still also have 14 total mass units (7 protons now with 7 neutrons).

Just make sure things balance, and it will work out. there was that one TPR passage that talks about nuclear energy levels - but I've NEVER seen anything like that on diags or the real thing.

-chop

 

[PRamos]

I did a passage where the talked about alpha particles being accelerated by an electric field. The point of the passage was that the alpha particle carried a +2 charge and therefore could be accelerated, so an alpha particle beam was generated. It emphasized field lines and deflection. I can't remember where I saw it (AAMC and my study books are all blurring together), but it wasn't that hard. I remember being a bit freaked out that alpha particles were charged, but it made sense when I thought of it as a "helium nucleus" and not a helium atom.

And as far as nuclear chemistry goes, it's all algebra. The mass is conserved and the charge is conserved. As long as the total mass of the reactants equals the total mass of the products and the total charge of the reactants equals the total charge of the products, you're fine. Once you do a couple passages, it becomes easy.

 

[Psycho Doctor]

i love those radioactivity problems because it's just like math.