| Date | May 2014 | Marks available | 2 | Reference code | 14M.2.sl.TZ2.2 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate | Question number | 2 | Adapted from | N/A |
Question
Boron is most often encountered as a component in borosilicate glass (heat resistant glass).
The naturally occurring element contains two stable isotopes, \(_{\;{\text{5}}}^{{\text{10}}}{\text{B}}\) and \(_{\;{\text{5}}}^{{\text{11}}}{\text{B}}\).
State the number of protons, neutrons and electrons in an atom of \(_{\;{\text{5}}}^{{\text{11}}}{\text{B}}\).
The relative atomic mass of boron is 10.8, to three significant figures. Calculate the percentage of \(_{\;{\text{5}}}^{{\text{10}}}{\text{B}}\) in the naturally occurring element.
Isotopes of boron containing 7 and 8 neutrons also exist. Suggest why releasing isotopes containing more neutrons than the stable isotope into the environment can be dangerous.
(i) State the formula of the compound that boron forms with fluorine.
(ii) Explain why this compound acts as a Lewis acid.
Markscheme
;
\(10x + 11(100 - x) = 10.8 \times 100\);
\((x = )20\% \);
Award [2] for correct final answer.
Do not allow ECF.
radioactive/radioisotope(s)/give out radiation;
Accept answers that outline the effects of radioactive pollution of the environment.
Do not accept “unstable”.
(i) \({\text{B}}{{\text{F}}_3}\);
(ii) incomplete valence shell / electron deficient / OWTTE;
capable of accepting an electron pair;
Examiners report
This question in general was well answered. Most candidates were able to identify the elementary particles of atomic boron with an encouraging number of students calculating the proportions of the two isotopes. A significant number did leave the question blank however although it should be a familiar example. Most candidates were able to state the formula of boron trifluoride and describe the action of Lewis acids although only a minority could explain its behaviour in terms of boron’s incomplete octet.
This question in general was well answered. Most candidates were able to identify the elementary particles of atomic boron with an encouraging number of students calculating the proportions of the two isotopes. A significant number did leave the question blank however although it should be a familiar example. Most candidates were able to state the formula of boron trifluoride and describe the action of Lewis acids although only a minority could explain its behaviour in terms of boron’s incomplete octet.
This question in general was well answered. Most candidates were able to identify the elementary particles of atomic boron with an encouraging number of students calculating the proportions of the two isotopes. A significant number did leave the question blank however although it should be a familiar example. Most candidates were able to state the formula of boron trifluoride and describe the action of Lewis acids although only a minority could explain its behaviour in terms of boron’s incomplete octet.
This question in general was well answered. Most candidates were able to identify the elementary particles of atomic boron with an encouraging number of students calculating the proportions of the two isotopes. A significant number did leave the question blank however although it should be a familiar example. Most candidates were able to state the formula of boron trifluoride and describe the action of Lewis acids although only a minority could explain its behaviour in terms of boron’s incomplete octet.
