TBR GChem question

[helix]

Which of the following is an isotope of element 35 containing 44 neutrons?

1. 
   A. 81Br-
   B. 79Br
   C. 81Br
   D. 79Kr
   
   
   They're asking for an isotope so I picked C. But its wrong.
   The answer is B.
   Please explain.
   Thank you!

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[Pisiform]

79 - 35 = 44 neutrons ... whereas in choice C 81-35=46 neutrons ... So B is corerect

Neutrons = Mass Number - Atomic Number

If question wasnt clear, they are asking which of the following is an isotope of Br, that has 44 neutrons

 

[theseeker4]

Which of the following is an isotope of element 35 containing 44 neutrons?

1. 
   A. 81Br-
   B. 79Br
   C. 81Br
   D. 79Kr
   
   
   They're asking for an isotope so I picked C. But its wrong.
   The answer is B.
   Please explain.
   Thank you!

1. 
   Element 35 means 35 protons. 44 neutrons obviously means there are 44 neutrons. Every atom with 35 protons is a Bromine atom, so it is Br. The number in front designates the total number of protons and neutrons (aka the atomic mass) so 35 + 44 = 79.
   
   Why did you pick C? Both 79Br and 81Br would be isotopes of Bromine (element 35).

 

[chiddler]

Which of the following is an isotope of element 35 containing 44 neutrons?

1. 
   A. 81Br-
   B. 79Br
   C. 81Br
   D. 79Kr
   
   
   They're asking for an isotope so I picked C. But its wrong.
   The answer is B.
   Please explain.
   Thank you!

1. 
   
   Adding or removing protons from an atom changes the element. Like adding a proton to carbon makes it nitrogen. So if it's element 35, it MUST be bromine. Next we just add the neutrons. 35 protons +44 neutrons = 79. Bromine with 79 atomic weight. B!

 

[helix]

Thanks you guys!
The question was not clear to me. I thought I have to pick an isotope of an element that has 35 protons and 44 neutrons.

 

[milski]

Do you get a periodic table at the exam? Or do they seriously expect me to remember that Br is #35?

 

[helix]

They do provide a periodic table.

 

[MedPR]

Thanks you guys!
The question was not clear to me. I thought I have to pick an isotope of an element that has 35 protons and 44 neutrons.

You do.. Isotopes vary only in the number of neutrons. So picking an isotope of an element that has 35 protons and 44 neutrons = element #35 (presumably Br, judging from the previous posts. I'm too lazy to look at a periodic table) that also has 44 neutrons. You know it can't be C because 35+44 =/= 81.

 

[milski]

You do.. Isotopes vary only in the number of neutrons. So picking an isotope of an element that has 35 protons and 44 neutrons = element #35 (presumably Br, judging from the previous posts. I'm too lazy to look at a periodic table) that also has 44 neutrons. You know it can't be C because 35+44 =/= 81.

After a second read, it can be handled without periodic table. Krypton is a noble gas, which means full outer shell of electrons, which means it has to have an even number in the periodic table. So #35 has to be Br.

 

[MT Headed]

I think the OP was confused on terminology, and thought 'isotope' meant 'an atom with a different weight than you'd expect'.

Actually, isotope just means 'a specific instance of an atom with a specific weight'.

Carbon is an atom. Carbon-14 is a heavy radioactive isotope of carbon. But carbon-12 is also an isotope of carbon.