[philonossis]

PROBLEM: "What are the names of the elements that each of the following atoms represent?

a) Mass number = 45, 24 neutrons are present.

b) Atom with two more neutrons than (a)

MY ANSWER: (a) is Sc, scandium because we subtract number of neutrons from mass number: 45-24= 21. This is the atomic number. 21 is the atomic number of scandium.

My answer to (b) was Ti, titanium, because its atomic number is 22, and its amu is 47.868, which rounds to 48. Since the amu is a weighted average, some isotopes of Ti have to have at least 46 neutrons to get a weighted average of over 47.

amu 48-Atomic number 22 = 26. This is two more neutrons than 24.

BOOK ANSWER: (b) is also scandium.

It is possible that Ti ans Sc could be isobars, but in that case the book should have said the answer is both Sc and Ti.

If the only answer is scandium, it seems to me we need to know the isotopes of scandium, which was not provided or referenced in the book.

Am I missing something here?  Is there a way to know the answer is scandium and not titanium?

[sjb]

What is the definition of an isotope, of an isobar (specifically in this instance, I only know the weather related one)?

[Borek]

What differentiates elements?

[philonossis]

What differentiates elements?

The only thing differentiating elements is atomic number.

I considered that the question should have stated that the second atom had the same number of protons, i.e., the same atomic number. Then it would have been clear that the atom with two more neutrons was an isotope.

If the atom can have a different number of protons with the same number of neutrons, then both Ti and Sc would be correct, wouldn't they?

[philonossis]

What is the definition of an isotope, of an isobar (specifically in this instance, I only know the weather related one)?

an isobar is an element that has the same number of neutrons as another element. But it is a different element as defined by the different number of protons (atomic number) they have.

[sjb]

an isobar is an element that has the same number of neutrons as another element. But it is a different element as defined by the different number of protons (atomic number) they have.

That isn't quite right, an isobar has the same number of nucleons (so protons+neutrons), so for instance ¹⁸O and ¹⁸F are isobars

[typhoon2028]

I thought isobars were those lines on a weather map indicating areas of the same barometric pressure?

[JGK]

I thought isobars were those lines on a weather map indicating areas of the same barometric pressure?

They are, the chemical one is also correct but my Chambers dictionary has the term as "Isobare" (or isobar) - either of two atoms of different chemical elements but of identical atomic mass (e.g. an isotope of Titanium and an isotope of chromium both of atomic mass 50)

[typhoon2028]

I was joking a bit

[DrCMS]

I considered that the question should have stated that the second atom had the same number of protons, i.e., the same atomic number. Then it would have been clear that the atom with two more neutrons was an isotope.

No the question is quite clear to the rest of us it asked for the the name of an atom with 2 extra neutrons you just did not read it clearly, tried to be too clever and failed.  Do not blame the question for your own errors and misinterpretation.

[philonossis]

That isn't quite right, an isobar has the same number of nucleons (so protons+neutrons), so for instance ¹⁸O and ¹⁸F are isobars

yes, this is right.

so an atom that theoretically has one proton and 10 neutrons would have a mass number of 11.

another atom that had 2 protons and 12 neutrons would have 2 more neutrons, so this would satisfy the question, as far as I can see.

So far the comments made on this point have not clarified the matter for me, and it seems the emotional tone of the thread is worsening, so it may be best to drop the matter.

[tamim83]

I know that phillonossis wants to put this topic to bed but I want to weigh in and hopefully clarify things. 

The second part of the original question is a bit on the ambiguous side.  It simply asks for an atom with two more neutrons than (a).  Sc is an acceptable answer (it would be ⁴⁷Sc instead of ⁴⁵Sc), as the book stated but its not the only one.  If you want to go with isobars you could chose ⁴⁸Ti.  The question was clear, however it was not specific.  The book merely gave the "simplest" answer possible.  The book should have either been more specific (for example "Write the nuclide symbol for an isotope of (a) that has two more neutrons") or stated in the answer key that there is more than one acceptable answer and listed them.  Textbooks can make errors (or in this case overlook things). 

So, phillonossis, your answer to part b was correct.  The book was just not specific enough.  If this were an exam question, a "fair" professor or TA would have given you full credit. 

Hope this helps. 

[philonossis]

Yes, thanks for this explanation, because I was afraid I had missed some obvious and relatively simply explanation.

Whether my first answer was correct or I had gotten it wrong, I would have been glad to learn something.