BPGHS SJChO Atomic Structure I

Multiple Select

1. (Lv. 1) An early of the atom (the plum-pudding model), as shown below, was proposed by J. J. Thomson in 1904. He described the structure of the atom as a positively charged sphere with small, negatively charged electrons embedded inside. These electrons are free to circulate within the sphere. Which of the following provide a correct comparison of Thomson’s plum-pudding model with the currently accepted Bohr's model of the atom?

• Both currently describe the atom as consisting of electrons and protons.

• Both correctly describe the electrons as freely circulating inside the atom.

• The currently accepted model describes the positive charges as being concentrated in a tiny core of the atom.

• The currently accepted model describes the presence of neutral particles which contribute partially to the mass of the atom.

Answer/Feedback:

Do note that, when J.J. Thomson proposed this model of the atom, the proton was not yet discovered. J.J. Thomson was credited with the discovery of the electron and thus you see him justifying his idea when questioned by other chemists "where is the positive charge then??". It will not be until Ernest Rutherford discovered the nucleus with is alpha particle experiment that knowledge will continue to evolve!

Multiple Choice

2. (Lv. 1) Which of the following species has more electrons than neutrons, yet lesser protons than neutrons?

Answer/Feedback:

(c) No. of neutrons = 16, no. of electrons = 18

Answer/Feedback:

4(

Answer/Feedback:

4(12) + 2(1) + 16 = 66.

Do you think we are missing something in the calculation of the molecular weight? Why do you think we deliberately ignore it/not factor it in then?

Multiple Choice

4. (Lv. 1) Different species of chemical entities, can bear the same molecular weight! As an example, [C₄H₂O]⁺ in question 3 bears the same molecular weight as chemical entities [C₄H₄N]⁺ and [C₄H₂O]⁺. These species which would be considered "same" in your "O" Levels can actually be differentiated via a very sensitive mass spectrometer. Given that the mass spectrometer is accurate up to 6 decimal places and returns the molecular weight as 66.034374, what is the species present? (Atomic Weights C=12.000000, H=1.007825, O=15.994915, N=14.003074)

• [C₄H₄N]⁺

• [C₅H₆]⁺

• [C₄H₂O]⁺

• None of the above are the correct chemical entities.

Answer/Feedback:

4(12.0000) + 4(1.007825) + 14.003074) = 66.034374

What is a mass spectrometer? What are 2 types of mass spectrometers? What is the principle of how the mass spectrometer achieves what it is capable of doing?

Multiple Choice

5. (Lv. 1) Hydrogen is known to exist in 3 isotopes which are ¹H, ²D and ³T.

Which one of the following statements is true?

• ¹H is the most active species among the 3 isotopes listed above.

• All 3 isotopes have the same physical properties.

• The natural abundance of 1H is near 100%.

• ²D and ³T are unable to form bonds with oxygen like ¹H does (e.g., H₂O).

Answer/Feedback:

Isotopes have the same chemical properties (same no. of valence shell electrons), but different physical properties - e.g., mass.

Multiple Choice

6. (Lv. 1) The table on the left shows the relative abundances of the isotopes of a certain element. By calculating its atomic weight, which of the following properties is most likely true for that element?

• It is a liquid at room temperature and pressure.

• It is a magnetic material.

• It is found at the "staircase" region of the Periodic Table.

• It is unable to conduct electricity at all conditions.

Answer/Feedback:

Is there a faster way of solving the above question though? If yes, how?

Multiple Choice

7. (Lv. 2) An imaginary element X has 3 isotopes. The 1st isotope has a mass number of 32 and an abundance of 25.00%. The 2nd isotope has 16 protons. The 3rd isotope has 19 neutrons. The relative atomic mass of X is 35.9.

Which of the following pairs of values is not possible?

• No. of neutrons in 2nd isotope = 23, abundance = 41.25%

• No. of neutrons in 2nd isotope = 24, abundance = 33.00%

• No. of neutrons in 2nd isotope = 25, abundance = 27.50%

• No. of neutrons in 2nd isotope = 26, abundance = 24.25%

Answer/Feedback:

If no. of neutrons in 2nd isotope = 26, atomic mass = 42.

32(0.25) + 42(0.2425) + 35(0.5075) > 35.9

Multiple Choice

8. (Lv. 2) Most elements consist of 2 or more isotopes. Of the many known isotopes of elements, only about 275 of these are stable. The rest are unstable and disintegrate spontaneously to form other elements. The stability of an atom depends on its neutron-to-proton ratio. The graph on the left shows the plots of number of neutrons to protons for the known stable isotopes of elements. Which of the following is not a valid conclusion drawn with respect to the graph?

• Unstable isotopes have too many protons and neutrons in the nuclei.

• Heavier isotopes with are stable have a higher number of neutrons than protons.

• For stable isotopes, as the atomic number increases, the number of neutrons also increases proportionately.

• Ligher isotopes are most stable when they have an approximately equal number of neutrons and protons.

Answer/Feedback:

The orange line of the graph does not increase in a straight line - it looks very slightly exponential. Note - you have to discern the BEST option to choose.

Multiple Choice

9. (Lv. 2) By observing the trend of the behaviour of stable isotopes from the graph in the question 7, which of the following isotopes is likely to be unstable?

Answer/Feedback:

Only (a) very obviously lies away from the orange line of the graph in question 8.

Multiple Choice

10. (Lv. 2) The table on the left lists the possible types of radioactive decay that an unstable isotope can undergo. Based on the answer in question 8, state the type of decay that the unstable nuclei is most likely to undergo.

• α-decay

• β-decay

• Position Emission

• Electron Capture

Answer/Feedback:

From seeing the graph, it seems that (a) from option 9 is above the orange line. Thus, it has more neutrons than usual.

Is there another way of deducing this if you don't know how to interpret the graph?

Multiple Choice

11. (Lv. 3) Based on the type of decay that the unstable nuclide has undergone (as chosen in question 9), what is the correct identity of the decayed product?

• Sc-48

• Ne-23

• Pb-207

• Ge-73

Answer/Feedback:

If neutron converts to proton, the atomic number will +1, while the nucleon number will stay the same (neutron -1 proton +1 net effect = 0). Note what when no. of protons change, the element also changes.

Multiple Select

12. (Lv. 2) The Ge-68 isotope is medically useful because it undergoes electron capture to give an isotope of a different element, H; which can be used to detect tumors. Referring to the table in question 7, check off all correct statement(s) below which are correct regarding the composition of an atom of element H.

• H has the same nucleon number as Ge-68.

• H has an atomic number of 32.

• H has 37 neutrons.

• H bears a charge – it is now a positive ion.

Answer/Feedback:

Net effect = proton -1, neutron +1. As 1 electron has been consumed, the entitiy H should now bear a charge.

Multiple Choice

13. (Lv. 3) Copper exists as 2 naturally occurring isotopes Cu-63 and Cu-65, with percentage abundances 69.17% and 30.83% in nature, respectively; while that of Chlorine exists as 2 naturally occurring isotopes Cl-35 and Cl-37, with percentage abundances 75.76% and 24.24% in nature, respectively. How many possible molecular weights for the ionic compound Copper(II) Chloride, CuCl₂, are there?

• 6

• 5

• 4

• 3

Answer/Feedback:

Possible molecular weights = 133, 135, 137, 139

Multiple Choice

14. (Lv. 3) Based on the data given in question 13, what is the probability of forming a Copper(II) Chloride compound with a molecular mass of 137?

• 3.97%

• 1.81%

• 9.73%

• 6.57%

Answer/Feedback:

Probability=0.6917x0.2424x0.2424+0.3083x0.7576x0.2424=0.09725

Multiple Choice

15. (Lv. 3) Based on the data given in question 13, rank the following isotopes in terms of increasing percentage abundance: ⁶³Cu³⁷Cl₂ vs. ⁶⁵Cu³⁵Cl³⁷Cl vs. ⁶³Cu³⁵Cl³⁷Cl.

• ⁶³Cu³⁷Cl₂ < ⁶³Cu³⁵Cl³⁷Cl < ⁶⁵Cu³⁵Cl³⁷Cl

• ⁶³Cu³⁷Cl₂< ⁶⁵Cu³⁵Cl³⁷Cl < ⁶³Cu³⁵Cl³⁷Cl

• ⁶⁵Cu³⁵Cl³⁷Cl < ⁶³Cu³⁷Cl₂ < ⁶³Cu³⁵Cl³⁷Cl

• ⁶³Cu³⁵Cl³⁷Cl < ⁶⁵Cu³⁵Cl³⁷Cl < ⁶³Cu³⁷Cl₂

Answer/Feedback:

Probability of ⁶³Cu³⁷Cl₂ = 0.6917x0.2424x0.2424 = 0.040642

Probability of ⁶⁵Cu³⁵Cl³⁷Cl = 0.3083x0.7576x0.2424 = 0.056616

Probability of ⁶³Cu³⁵Cl³⁷Cl = 0.6917x0.7576x0.2424 = 0.12702

Will the probabilities be different if I were to compare ⁶⁵Cu³⁵Cl³⁷Cl vs. ⁶⁵Cu³⁷Cl³⁵Cl? Do you have a shorter method to solve the question?

Multiple Choice

16. (Lv. 1) Which of the following might be classified as both being isotones and isoelectronic?

Answer/Feedback:

For (c), both species have 20 neutrons and 18 electrons.

Multiple Choice

17. (Lv. 1) The following sketches of graphs A and B (x and y-axes not properly labelled) are from the journal of a Chemistry student who is studying the 1st 12 elements of the Periodic Table. Which of the following likely represents the x- and y-axis of the above graphs respectively?

• Graph A: x-axis: Total number of electrons in elements across 1st 12 elements, y-axis: Respective nucleon numbers

  Graph B: x-axis: Proton number across 1st 12 elements, y-axis: Respective cation charge

• Graph A: x-axis: Number of neutrons across 1st 12 elements, y-axis: Respective total number of e-

  Graph B: x-axis: Nucleon number across 1st 12 elements, y-axis: Respective atomic weights

• Graph A: x-axis: Number of electrons in elements across 1st 12 elements, y-axis: Respective anion charge

  Graph B: x-axis: Number of neutrons across 1st 12 elements, y-axis: Respective proton numbers

• Graph A: x-axis: Proton number across 1st 12 elements, y-axis: Respective number of neutrons

  Graph B: x-axis: Total number of electrons in elements across 1st 12 elements, y-axis: Respective number of valence e-

Multiple Choice

18. (Lv. 1) The following diagram on the left are flight paths of charged particles when accelerated in an electric field. Which of the following are correct positions for the particles?

• R= ¹⁶O⁺ S= ¹⁴C⁺ T= ¹⁴N⁺

• R= ¹⁵O^– S= ¹⁴C⁺ T= ²⁸Si⁺

• R= ¹⁴N^– S= ¹⁵O⁺ T= ²⁸Si²⁺

• R= ¹⁴N^– S= ¹⁴C⁺ T= ²⁸Si⁴⁺

Answer/Feedback:

R should be a negative ion, while S and T are positive ions. Angle of deflection of R and S are the same, just in opposite directions, so their q/m ratio better be equal. T could have higher q or lower m than S or both.

Multiple Choice

19. (Lv. 1) A plasma is a gaseous mixture in which the atoms have been completely stripped of their electrons, leaving bare nuclei. Because of possible use in controlled nuclear fusion reactions, plasma behavior has been intensively studied. The ⁴He nuclei were deflected as shown on the picture in the left. Based on the data, what is the difference between the angle of deflection of the ³H vs. ³He nuclei?

• 0.7^o

• 1.1^o

• 1.3^o

• 1.6^o

Answer/Feedback:

4/2k=2, k=4.

For ³He, angle of deflection = 4x2/3 = 2.666^o

For tritium, angle of deflection = 4x1/3 = 1.333^o

Multiple Choice

20. (Lv. 1) In an experiment, a sample of Einsteinium element (Es) was vaporised, ionised and passed through an electric field. Analysis revealed that a beam of ²⁵²Es⁺ gives an angle of deflection of +1.000^o. What would be the angle of deflection for a sample of ⁴⁰Ca⁺ ions?

• +12.600^o

• +6.300^o

• +3.150^o

• +1.575^o

Answer/Feedback:

1/252(k) = 1, k=252.

For ⁴⁰Ca+, angle of deflection = 1/40x252=6.3^o

Open Ended

Which questions do you want the trainer to go through/clarify stuff?