ALS8A - Section 8 Periodic Properties of the Elements in the Periodic Table Suggested solutions 1 SKH Bishop Baker Secondary School AL Chemistry

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Section 8/ Periodic Properties of the Elements in the Periodic Table / Suggested solutions / SKH Bishop Baker Secondary School AL Chemistry Section 8 Periodic Properties of the Elements in the Periodic Table 85 IIA Q.6 (a) The atomic radius decreases across the Periodic Table because the added electron is placed in the same quantum shell [1], the increased proton provided a stronger attraction on the electrons , thus casing a decrease in size. [1] Or, The increase in electrons will be added to the same shell and the nuclear charge increases (1), thus the effective nuclear charge has a greater attraction for the outer electrons drawing them closer together. (1) (b) For the metallic elements , the ionic radii are smaller than the atomic radii. This is because the electron cloud contracts due to the loss of electrons , it is pulled closer to the positive nucleus. [1] As aluminium has a nucleus that is more positive , the atomic radius and the ionic radius are less than those of sodium and magnesium. [1] For the non-metallic elements , ions are formed by gaining electrons . The greater repulsion by the electron cloud gives rise to a greater ionic radius. [1] As the electron cloud is distant from the nucleus , additional electrons do not greatly influence the atomic or ionic radius. [1] (c) There is a general increase in first ionization energy across the period. This is because there is an increase in nuclear charge giving a decrease in radius and [1] thus a decrease in the distance of the electron cloud. [1] Two discrepancies occur : (i) The first ionization energy of aluminium is lower than that of magnesium. The electronic configuration of Magnesium is 1s 2 2s 2 2p 6 3s 2 but Aluminium is 1s 2 2s 2 2p 6 3s 2 3p 1 It is easier to lose the 3p electron than one from a complete s-subshell. [1] (ii) The first ionization energy of S is less than that of P. The electronic configuration of Phosphorus is 1s 2 2s 2 2p 6 3s 2 3p 3 but Sulphur is 1s 2 2s 2 2p 6 3s 2 3p 4 The 3p sub-shell is half filled in P and this is a more stable arrangement. [1] Less energy is thus required to remove an electron from S than from P. (d) Sodium, magnesium and aluminium form metallic bonds . These bonds are fairly strong , but flexible and do not have to be substantially broken to form the liquid. The melting point of metals is reasonably high. [1] Silicon exists as a giant lattice covalent structure . Strong covalent bonds must be broken to form the liquid. The melting point is very high. [1] Phosphorus, sulphur and chlorine exist as discrete molecules held together in the solid by weak van der Waals’ forces . It is largely the breaking of these weak bonds that gives these non-metals a low melting point. [1]
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This note was uploaded on 08/17/2011 for the course CHEM 100 taught by Professor Linzhenyang during the Fall '10 term at HKUST.

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ALS8A - Section 8 Periodic Properties of the Elements in the Periodic Table Suggested solutions 1 SKH Bishop Baker Secondary School AL Chemistry

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