2017 SRJC H2 Chemistry 9729 Preliminary Examination Paper 2 Solution 1 The size of an atom can be measured using the distance between the nuclei of two atoms. For example, the ‘metallic radius’ of the Na atom is half the distance between two Na atoms in the crystal lattice of the metal. The ‘covalent radius’ of the Clatom is taken to be half the distance between the nuclei in a Cl2molecule. Finally, the ‘van der Waals’ radius’ of the Aratom is assumed to be half the distance between two atoms in the solid state. These three types of radius are commonly known as ‘atomic radii’ and thecorresponding values for Period 3 elements can be found in the Data Booklet. (a) (i) Explain the general trend in atomic radius across Period 3. Across Period 3,nuclear charge increases as no. of protons increases. Shielding effect remains relatively constant due to thesame no. of electron shells. Effective nuclear charge increases. Stronger electrostatic forces of attraction between nucleus and valence electrons lead to decrease in atomic radius.  (ii) Suggest a reason for the anomaly in the measurement of atomic radius for argon. The instantaneous dipole-induced dipole interactions between the two Ar atoms aremuch weaker than the strong metallic bonds for metals and strong covalent bonds for molecules. Thus, the two atoms for Ar will befurther apart leading to a much higher measurement. (b) (i) State the general relationship between atomic radius and first ionisation energy across Period 3. The first ionisation energy is inversely proportional to atomic radius. (ii) Explain why the relationship does not hold from P to S.It is easier to remove an electron from S due to theinter-electronic repulsion between the paired electrons in thesame 3p orbital of S. Thus 1stIE decreases from P to S.
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