# Homework Help: Specific charge. Don't understand how to convert to Coulombs

1. Apr 3, 2009

### orgla

1. The problem statement, all variables and given/known data
An ion of a magnesium isotope has A=24 and Z=12. Find the specific charge

2. Relevant equations

3. The attempt at a solution

I know that the charge of a proton in Coulombs is $$1.60\times10^{-19}$$. From the information given, I know that I have 12 protons and $$24-12=12$$ neutrons. Neutrons have no charge so I can ignore them. Therefore my charge is $$12 \times 1.60\times10^{-19} = 1.92\times10^{-18}$$

This is very wrong. My answer should be $$+3.2\times10^{-19}$$

I know that the mass of a proton is $$1.67\times10^{-27}$$ which is equal to a neutron. From the information given I have 24 nucleons and so a mass of $$24\times1.67\times10^{-27} = 4.008\times10^{-26}$$.

This is very wrong. My answer should $$3.98\times10^{-26}$$

I realise that electrons have charge and mass too, and in a stable atom, equal the number of protons. Persumably, if this is an ion, it's lost/gained some electrons and so is not stable. So I don't know how to tell how many electrons there are in this atom.

Thank you very much for reading. I hope someone can sort out this confusion!

Charlie.

Last edited: Apr 3, 2009
2. Apr 3, 2009

### Vagrant

This list of common ions should help you to find the number of electrons in Magnesium ion.
http://www.rose.edu/faculty/Aslagle/ion%20list.pdf" [Broken]

Last edited by a moderator: May 4, 2017
3. Apr 3, 2009

### orgla

I'm sorry but that hasn't helped me. I don't understand what means what in that document. This is a very basic A level physics question. Ions haven't been introduced. I only knew what I mentioned in the previous post from GCSE studies.

Last edited by a moderator: May 4, 2017
4. Apr 3, 2009

### Vagrant

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge.
http://en.wikipedia.org/wiki/Ion" [Broken]

Magnesium ion = Mg2+, which means that it has lost two electrons or has +2 (positive) charge.

Last edited by a moderator: May 4, 2017
5. Apr 3, 2009

### orgla

Would I be right in thinking that without being given how many electrons have been gained/lost, this question can not be answered without a further understanding of ions? If so I might have wasted some time here as the question was an example in my book and the charge and mass were given. I just didn't understand where the numbers came from.

If this is the case I'm terribly sorry for wasting your time.

Charlie.

6. Apr 3, 2009

### orgla

Oh I see, so with that knowledge you arrive at the answer by 2 x 1.60-19[\sup] = 3.2 x10-19[\sup].

Is this how it's done?

Last edited by a moderator: May 4, 2017
7. Apr 3, 2009

### Vagrant

No matter

Yes, this is right.