NJC 3 Atomic structure tutorial ans 2023

National Junior College SH1 H2 Chemistry 1 Tutorial – Atomic Structure Discussion Questions Atomic Number and Mass Number 1. Use of the Data Booklet is relevant to this question. Nuclear magnetic resonance (NMR) spectroscopy is an analytical technique that uses the magnetic properties of certain atomic nuclei in order to elucidate the structure of an organic molecule. Atomic nuclei with an even number of protons and an odd number of neutrons (or vice versa) are most suitable for NMR spectroscopy. Which of the following nuclei is least suitable for NMR spectroscopy? A 28Si B 31P C 103Rh D 19F A: no. of protons 14, no. of neutrons = 28 – 14 = 14 (even no. protons and neutrons) B: no. of protons 15, no. of neutrons = 31 – 15 = 16 C: no. of protons 45, no. of neutrons = 103 – 45 = 58 D: no. of protons 9, no. of neutrons = 19 – 9 = 10 A 2 Use of the Data Booklet is relevant to this question. Tritium, 3 1 H, a radioactive isotope of hydrogen, slowly turns into a helium isotope 3 2 He. Which statements about the two isotopes are incorrect? 1 Both isotopes have more neutrons than electrons. 2 Both isotopes have the same number of protons in their nuclei. 3 Both isotopes have the same number of charged sub–atomic particles. A 2 only B 1 and 3 only C 2 and 3 only D 1, 2 and 3 H3 1 contains 1 proton, 1 electron and 2 (3 – 1) neutrons. He3 2 contains 2 protons, 2 electrons and 1 (3 – 2) neutron. 1 3He has less neutrons than electrons. 2 3H has one proton while 3He has two protons. They are isotopes of different element, hence they CANNOT have the same number of protons. 3 3H has two charged sub–atomic particles while 3He has four charged sub– atomic particles. D

National Junior College SH1 H2 Chemistry 2 3 Some isotopes are unstable and undergo nuclear (radioactive) reactions. In one type of reaction, an unstable nucleus assimilates an electron from an inner orbital of its electron cloud. The net effect is the conversion of a proton and an electron into a neutron. Which of the following describes this type of reaction? A 11C → 12C B 111I → 111Te C 76Br → 75Br D 76Kr → 75Br The conversion results in the loss of a proton. So proton no decrease by 1. The reactant and the product cannot be the same element. Therefore options A and C are NOT the answers. Since the proton is converted into a neutron, mass no (nucleon no do not change) B 2015 P3 Q5 5 Lithium metal and its compounds have many uses, ranging from nuclear chemistry, rechargeable batteries, organic reagents and pharmaceuticals. (a) Naturally occurring lithium contains isotopes 6Li and 7Li. (i) What is meant by the terms • proton number, • nucleon number? [1] proton number: number of protons in the nucleus of an atom nucleon number: total number of protons and neutrons in the nucleus of an atom (ii) The isotopes 6Li and 7Li have accurate isotopic mass of 6.015 and 7.016 respectively. The Ar of lithium is 6.942. Calculate the relative percentage abundances of the two isotopes, to two decimal places. [2] let relative percentage abundance of Li7 = a 7.016 ×a +6.015 ×(100-a) 100 = 6.942 a = 92.61  relative percentage abundance of Li7 = 92.61%, relative percentage abundance of Li6 = 7.39% (2 dp)

National Junior College SH1 H2 Chemistry 3 (iii) A nuclear reaction is a reaction in which there is a change to an atomic nucleus. An experimental nuclear reactor uses 6Li and deuterium, 2H, as fuel. Three nuclear reactions between these two atoms are described below (P+ is a proton; n is a neutron). 6Li + 𝐻 → 2 1 2 4He 6Li + 𝐻 → 1 2 4He + X + n 6Li + 𝐻 → 1 2 Y + P+ Suggest the identities of X and Y. [2] Li6 3 + H2 1 → 2 He4 2 Li6 3 + H2 1 → He4 2 + He3 2 + n1 0 Li6 3 + H2 1 → Li - 3 7 + +p1 1 X: He3 2 Y: Li - 3 7 6 Beams of charged particles are deflected by an electric field. When a beam of protons passes through an electric field of constant strength, the angle of deflection is +12 . In another experiment under identical conditions, particle Y is deflected by an a ngle of – 4 . What could be the composition of particle Y? protons neutrons electrons 1 1 2 2 2 3 3 5 3 4 5 1 A 1, 2 and 3 B 1 and 2 C 1 only D 3 only Answer: B Magnitude of angle deflected  charge/mass charge/mass for 1H+ = 1  deflected by 12 o for 1 unit of charge/mass To be deflected through angle of – 4 o (i.e. opposite side of protons), particle Y should be negatively-charged, with charge/mass = – 1/3 Option 1: charge/mass = – 1/3 ✓ Option 2: charge/mass = – 2/6 = – 1/3 ✓ Option 3: charge/mass = + 3/9 = + 1/3 

National Junior College SH1 H2 Chemistry 4 2011 P1 Q31 7 Which statement about relative molecular mass are correct? 1 It is the sum of the relative atomic masses of all the atoms within the molecule. 2 It is the ratio of the average mass of a molecule to the mass of a 12C atom. 3 It is the ratio of the mass of 1 mol of molecules to the mass of 1 mol of 1H atoms. A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 1 only [Only option 1 is correct] so answer is option D Option 2: It is the ratio of the average mass of a molecule as compared to 1 12 of the mass of a 12C atom, NOT the mass of a 12C atom. Option 3 is wrong as the mass of atoms or compounds (1 mole/ 1 particle) must be compared against 1 12 of (1 mole/ 1 particle) of 12C, NOT 1H. 12C is used as the reference because of its abundance as well as stability. Electronic Configuration 8 Gallium has the electronic configuration [Ar] 3d104s24p1, where [Ar] represents the electronic configuration of argon. In which order are the electrons lost in forming the Ga4+ ion? A 3d, 4p, 4s, 4s B 4s, 4s, 4p, 3d C 4p, 3d, 3d, 3d D 4p, 4s, 4s, 3d Ga is a group 13 element. The sequence of electrons removed should start from electron of highest energy to electron of lower energy. The energy of electron 1) increases with the quantum number n. 2) Within a quantum shell, energy of subshell decreases in the following order: f > d > p > s highest lowest Hence the sequence is from 4p subshell first where there is only 1 e, followed by 2 e in the 4s subshell. The 4th one from the 3d subshell. 9 An isolated gaseous species has paired electrons in at least one of its 3d orbitals and a fully filled 4s subshell. What could be the identity of the species? A Cu B Fe3+ C Ni2+ D Sr2+ Cu: 1s2 2s2 2p6 3s2 3p6 3d104s1 Fe3+: 1s2 2s2 2p6 3s2 3p6 3d5 Ni2+: 1s2 2s2 2p6 3s2 3p6 3d8 Sr2+: 1s2 2s2 2p6 3s2 3p6 3d10 4s24p6 D

National Junior College SH1 H2 Chemistry 5 10 Which ion has the most number of unpaired electrons? A Cr3+ B Ni2+ C Ca2+ D Co3+ Cr3+: 1s2 2s2 2p6 3s2 3p6 3d3 (3 unpaired e− in 3d orbitals) Ni2+: 1s2 2s2 2p6 3s2 3p6 3d8 (2 unpaired e− in 3d orbitals) Ca2+: 1s2 2s2 2p6 3s2 3p6 (0 unpaired e−) Co3+: 1s2 2s2 2p6 3s2 3p6 3d6 (4 unpaired e− in 3d orbitals) 11 Use of the Data Booklet is relevant to this question. What do the ions 23Na+ and 24Mg2+ have in common? A Both ions have more electrons than neutrons. B Both ions have 12 neutrons in their nuclei. C Both ions contain the same number of nucleons in their nuclei. D Both ions have an outer electronic configuration of 3s2 3p6. Option A is wrong as they are cations, not anions. Option C is wrong as their nucleon number (mass number) are 23 and 24 respectively. Option D is wrong because they assume Ne noble gas configuration where the outer electronic configuration is 2s22p6. Option B is correct as shown i