The nuclear atom: notes and practice questions
- This topic focuses on interpreting mass spectra to determine the relative atomic masses of elements.
- Interpret mass spectra in terms of isotopic identity and relative abundance.
- Understand that the peaks in a mass spectrum correspond to different isotopes.
- The operational details of a mass spectrometer are not assessed.
How it is examined
Paper 1A carries the subatomic-particle counting. Weighted-average Ar calculations from isotopic abundance are standard 2-mark Paper 2 parts. At HL the mass spectrum version gives a bar chart of m/z against relative abundance and asks for Ar, still 2 marks. A question asking a student to recall the mass of a proton in kg is a bad question: the booklet supplies it.
Actual masses and charges of the proton, neutron and electron. Relative atomic masses to two decimal places in the periodic table. The relative values (proton and neutron 1, electron negligible; charges +1, 0, -1) are recall.
- 1.2.1 Atoms contain a positively charged, dense nucleus of protons and neutrons (nucleons). Negatively charged electrons occupy the space outside the nucleus. Students use the nuclear symbol (mass number A upper left, atomic number Z lower left, element symbol X) to deduce the number of protons, neutrons and electrons in atoms and ions.
- 1.2.2 Isotopes are atoms of the same element with different numbers of neutrons. Students perform calculations involving non-integer relative atomic masses and isotopic abundance from given data.
The operational details of the mass spectrometer will not be assessed (HL).
Guiding questions
- How do the nuclei of atoms differ?
Linking questions
- Structure 1.3 What determines the different chemical properties of atoms?
- Structure 3.1 How does the atomic number relate to the position of an element in the periodic table?
- Nature of science, Reactivity 3.4 How can isotope tracers provide evidence for a reaction mechanism?
- Structure 3.2 (HL) How does the fragmentation pattern of a compound in the mass spectrometer help in the determination of its structure?
Practice questions
3 questions · 2 medium · 1 hardQuestion 1
MediumPaper 1A · calculator1 markThe line emission spectrum of hydrogen in the visible region shows a series of lines that become closer together at shorter wavelengths. What can be deduced from this observation?
A. The energy of an emitted photon is directly proportional to its wavelength.
B. The electron transitions responsible for this series all end at the level.
C. The energy levels of the hydrogen atom converge at higher energies.
D. The energy levels in the hydrogen atom are separated by equal amounts of energy.
Recall the relationship between the energy of emitted light and the difference between electron energy levels. How does the pattern of spectral lines (getting closer) reflect the pattern of the energy levels?
Question 2
HardPaper 2 · calculator23 marksA sample of chlorine consists of two isotopes, and .
(a) Contrast the sub-atomic structure of these two isotopes.
(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the ion. The spectrum shows three peaks at m/z values of 70, 72 and 74. Explain the origin and relative heights of these three peaks, given that the abundance of is approximately three times that of .
(ii) A more precise measurement finds the composition by mass to be: : 75.76%, : 24.24%. Calculate the relative atomic mass of chlorine from this sample, giving your answer to two decimal places. (Use isotopic masses of 35.0 and 37.0 for this calculation).
Magnesium chloride, , and manganese(II) chloride, , are two ionic compounds.
(c) (i) Deduce the type of bonding in magnesium chloride, , using electronegativity values from section 9 of the data booklet.
(ii) Determine the lattice enthalpy of magnesium chloride, assuming the bonding is purely ionic. Use sections 9, 10 and 12 of the data booklet and the following data:
Enthalpy of formation of magnesium chloride =
(iii) Explain, with reference to electron configurations, why the ionic radii of , and are different. Use section 10 of the data booklet.
(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, , or magnesium chloride.
Magnesium chloride is white, but manganese(II) chloride is pale pink.
(d) (i) State the condensed electron configuration of a manganese atom.
(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.
(iii) Manganese(II) chloride absorbs light with a wavelength of approximately 530 nm. Describe why this is consistent with the observed colour of the compound. Use sections 2 and 15 of the data booklet.
A copper key is to be electroplated with manganese using an aqueous solution of manganese(II) chloride as the electrolyte.
(e) (i) Deduce the half-equations for the reactions occurring at the anode (made of pure manganese) and the cathode (the copper key).
(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.
Isotopes of an element have the same number of protons but a different number of another sub-atomic particle. What is this particle and how does its number differ between and ?
The peaks correspond to different combinations of the two chlorine isotopes in a diatomic molecule. The height of each peak is related to the probability of that specific combination occurring. Consider the relative abundances of the isotopes.
The relative atomic mass is the weighted average of the isotopic masses. Multiply each isotopic mass by its fractional abundance and sum the results.
Find the electronegativity values for magnesium and chlorine. The difference in their electronegativity values will indicate the type of bonding.
Construct a Born-Haber cycle for the formation of from and . Use Hess's Law to find the unknown lattice enthalpy. Remember to account for the stoichiometry, especially for chlorine.
Compare the number of electron shells and the nuclear charge (number of protons) for each ion.
The strength of ionic bonding depends on the charge of the ions and the distance between them (ionic radii). Compare these factors for and .
Manganese is in the first row of the d-block. Remember the filling order of the 4s and 3d sub-levels.
The colour of transition metal compounds is related to the electronic structure of the transition metal ion. What is special about the d-sublevel in coloured ions?
The colour we see is the complementary colour to the one that is absorbed. Use the colour wheel in the data booklet to find the complementary colour of the absorbed light.
In electroplating, the object to be plated is the cathode, and the metal used for plating is the anode. Oxidation occurs at the anode and reduction occurs at the cathode.
Consider the relative oxidizing strengths of the halogens. A more reactive halogen will displace a less reactive halide from its salt solution.
Question 3
MediumPaper 2 · calculator8 marksSilicon, a key component in electronics, can be purified to an exceptional degree, making it one of the purest elements commercially available. It reacts with oxygen to form silicon dioxide, a compound with a vast range of applications from glassmaking to fibre optics.
(a) Distinguish between the terms element and compound.
(b) Justify, using electronegativity values from the data booklet, why the Si-O bond is polar.
(c) Contrast the electrical conductivity of silicon and silicon dioxide. Relate the difference to their bonding and structure.
(d) The first ionization of a gaseous silicon atom can be initiated by electromagnetic radiation. Calculate the wavelength, in m, of the radiation corresponding to the first ionization energy of silicon. Use the data booklet.
Think about the fundamental particles that make up each type of substance. What is the key difference in their composition at the atomic level?
Find the electronegativity values for silicon and oxygen in the data booklet and calculate the difference. A significant difference indicates a polar bond.
Consider the type of bonding and structure for each substance and how this affects the mobility of electrons.
First, find the ionization energy for a single atom in Joules by converting from kJ mol⁻¹ using Avogadro's constant. Then, use the equations relating energy, Planck's constant, and wavelength.
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Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
