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Isotopes

222
Ra is radium
88

A
Ra is a neutral atom of a different isotope of radium
Z

State the possible values for A and Z?

Could you explain what I should be thinking please?

Reply 1

alkyone

first of all z is the same, the proton number is the same. the number of neutrons can vary though, for example a different isotope could have mass number of 220 or something close to 222, as the neutron number varies in isotopes

Reply 2

Charries

Yes - got Z is the same, but what about the A number

Actually made a typo on the question - should read

228
Ra is radium
88

In the mark schems they say accept 228 + or - 10?

Is there a reason for the 10?

Reply 3

alkyone

they just accept something close to that value, if i were you i would just answer to + or -1 number, it's as close as possible and still an isotope. it is just a suggest question, and there is no reason to risk giving a number with a great difference

Reply 4

Charries

Just looked at another question paper :

133
Cs is the most abundant and stable isotope of Caesium
55

Question says write down an Isotope of that which is likely to be a Beta minus emitter?

Mark scheme says

133->154
Cs
55

Suggesting I caould say anything between 133 and 154 - but WHY?

Reply 5

whitemagesonia

z=88
An isotope is a different atomic form of the same element, with the same number of protons but a different number of neutrons.

Reply 6

alkyone

it says that it is a STABLE isotope, so to emit beta particles it must have a greater mass number, so it becomes unstable and radioactive

Reply 7

Charries

OK - and the reason for the range is just the same as before?

Reply 8

alkyone

i suppose that the range is set in a way so to acept anything logical, close values are usually the logical ones

Reply 9

Charries

Ok thanks very much ...

Reply 10

Cora Lindsay

Original post by Charries

Advice given so far is good.

Why 10? Because, for a given element, the stable isotope(s) lie in the middle of a series so if you go a few up, or a few down from them, you'll have plausible radioactive ones. Ten is probably slightly generous for most elements.

The Cs question. 133 is the only stable Cs isotope so isotopes with A>133 or <133 will be radioactive. Beta minus decay (which turns a neutron into a proton) is characteristic of neutron-rich nuclei (ie those with A>stable) and moves the proton-neutron ratio towards a more stable value. So for a beta minus emitting Cs you need A>133. They happen to allow up to 154 in the answer, but that's just arbitrary. For interest, all Cs isotopes from 113 to 149 are known, as well as 151.

If you had a neutron-poor Cs isotope (A<133) it wouldn't don't decay by beta minus emission, because that moves the proton-neutron ratio further in the wrong direction. Such isotopes decay by positron emission or electron capture, both of which turn a proton into a neutron.