SRJC_H2_CHEM_P2 ANS

SRJC 2009 9746/02/PRELIM/2009 [Turn Over SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/02 Preliminary Examination 26 th August 2009 Paper 2 Structured 1 hr 30min M A R K S C H E M E CONFIDENTIAL DOCUMENT Teachers to keep this document under lock and key until 26 August 2009, 12pms RECOMMENDED F ORMAT OF PRINTING 2 PER PAGE This document consists of 15 printed pages and 1 blank page

SRJC 2009 9746/02/PRELIM/2009 [Turn Over BLANK PAGE

3 SRJC 2009 9746/02/PRELIM/2009 [Turn Over 1 The atomic and cationic radii of the Period 3 elements, Na to Cl, are plotted in the graph below, in order of increasing atomic number. Na Mg Al Si P S ClNa+ Mg2+ Al3+ Si4+ P3- S2- Cl- 0.00 0.05 0.10 0.15 0.20 0.25 10 11 12 13 14 15 16 17 18 Radius / nm A tomic & Ionic Radii vs. Proton no. for Elements in Period 3 Proton no. Atomic Radii Cationic Radii Anionic Radii (a) Explain each of the following: (i) The atomic radius decreases across the period from Na to Cl. Across the period from Na to Cl,  nuclear charge increases [1/2M];  but increase in shielding effect is negligible as electron is added to the same quantum shell; [1/2M]  Hence, the effective nuclear charge increases OR the electrostatic forces of attraction between the nucleus and the outermost/ valence electrons increases across the period. [1M]  Therefore, the atomic radius decreases across the period from Na to Cl. [2] (ii) The cationic radius of Na+ is smaller than its atomic radius. Na+ has one electron less than Na or Na + has one quantum shell less than Na. Nuclear charge remains the same . [1M] The electrostatic forces of attraction between the nucleus and the remaining electrons are greater for Na+. [1M] [2]

4 (b) (i) Indicate, on the graph, the relative positions of the ions, P3–, S2– and Cl–. Refer to Graph [1M] [1] (ii) Explain your answer in (b)(i). The anionic radii of P 3–, S 2– and C l– are larger than their respective atoms. This is due to electrons are being added to the same valence shell , resulting in greater inter-electronic repulsion s between the valence electrons. [1M] Across the period from P 3–, S 2– and C l–, the anionic radii decrease. The nuclear charge increases across the period with negligible increase in shielding effect resulting in the corresponding increase in the effective nuclear charge OR electrostatic forces of attraction between the nucleus and the valence electrons. [1M] [2] (c) (i) Write down the electronic configuration in the ground state for an atom of sulphur. 1s