[dipz04]

I've been looking through AQA past exam papers and there is always a question about deducing atoms. I just don't understand it.

Example: An atom has twice as many protons as, and four more neutrons than, an atom of 9Be.
Deduce the symbol, including the mass number, of this atom.

Please help.

Thanks in advance  

[sjb]

The question asks about protons and neutrons of an unknown, relative to a known. So, how many protons and neutrons are there in ⁹Be?

[dipz04]

yeah so there are 4 protons and 5 neutrons. the answer was ¹⁷O
but i don't know how to get to that...

[sjb]

OK, so if there are 4 protons in Be, what is twice that? What element has that number of protons? Similarly, what is 5 + 4? So how many nucleons in total?

[dipz04]

It would be oxygen. But why ¹⁷O and not ¹⁸O.

[dipz04]

Ohhh...I think i get it.
Please check if the method is right please:

If there are twice as many protons it would be 8.
8+5=13 ----->mass number
4 more neutrons + 13 = 17

[DrCMS]

If there are twice as many protons it would be 8.
8+5=13 ----->mass number
4 more neutrons + 13 = 17

This does not really show clear thinking.

⁹Be as you said correctly before is made up of 4 protons and 5 neutrons.
 
If you have an atom with twice as many protons and 4 more neutrons it would have:
4x2 = 8 protons
5+4 = 9 neutrons.

At atom with 8 protons has an atomic number of 8 and therefore is oxygen.
At atom with 8 protons and 9 neutron has a mass number of 8+9 = 17

so the answer is ¹⁷O

It can not be ¹⁸O because that has 8 protons and 10 neutrons i.e 1 too many neutrons
It can not be ¹⁶O because that has 8 protons and 8 neutrons i.e 1 too few neutrons

[dipz04]

This does not really show clear thinking.

⁹Be as you said correctly before is made up of 4 protons and 5 neutrons.
 
If you have an atom with twice as many protons and 4 more neutrons it would have:
4x2 = 8 protons
5+4 = 9 neutrons.

At atom with 8 protons has an atomic number of 8 and therefore is oxygen.
At atom with 8 protons and 9 neutron has a mass number of 8+9 = 17

so the answer is ¹⁷O

It can not be ¹⁸O because that has 8 protons and 10 neutrons i.e 1 too many neutrons
It can not be ¹⁶O because that has 8 protons and 8 neutrons i.e 1 too few neutrons

THANK YOU SO MUCH!!!

That just cleared it all up for me and made it easier to understand.
I also tried it on questions similar to the one I asked about and they all worked.

Thanks once again