Wednesday 3 August 2011

Chemistry: Periodicity

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Periodic variation in physical properties from Li to Ar
1)       s-block elements and p-block elements represents group I, II and Group III ~ 0 respectively. They are known as main group elements. Group I elements are known as alkali metals (Li, Na, K, Rb…), Group II elements are known as earth alkaline metals, Group VII elements are halogens while Group 0 elements are known as noble gases.
2)       d-block elements, placed between Group II and Group III starting on period 4 on the periodic table, are known as transition elements. Different from main group elements, they occupy the outermost d-orbit and they don't have a significant physical variation.
3)       f-block elements, lanthanide and actinide series are known as inner-transition metals.
Variation in atomic radius in main group elements
-          Atomic radius describes the size of atom. Covalent radius is half of the internuclear distance between the two covalently bonded atoms while metallic radius is half of the internuclear distance between in atoms in a metallic crystal.

-          Trend: Decrease across the period (e.g. Li > Be > B) and increase down the period (e.g. Br > Cl)
-          Explanation: The electrostatic attraction in each bonding obeys Coulomb's law, so the larger net charge on the two agents, the larger attractive force, and hence smaller radius. The charge of one agent is being fixed (pairs of shared electrons, delocalized electrons), so the net charge on the atom is being concerned. Screening effect states that the electrons in inner shell repel the electrons in the outermost shell, hence reducing the attractive force. Effective nuclear charge is used to describe the order of attractive force under screening effect and since number of outermost shell electron increase across the period, the effective nuclear charge increases, so atomic radius decrease across the group. Similarly, when there's one more fully filled inner electron shell, the screening effect strengthens a lot, so effective nuclear charge decrease, atomic radius increase.
Variation in melting point in main group elements
-          Trend: In period 2 it increases from Group I to Group IV, then sharply decrease in Group V, and decrease slightly to Group 0 (no significant change); in period 3, it increases from Group I to Group IV and then sharply decrease in Group V (P). The m.p. of Group V ~ 0 is arranged as S>P>Cl>Ar.
-          Explanation on Group I ~ III: They are in giant metallic structure, in order to melt them not all strong metallic bond needed to be broken, so their m.p. is higher than Group V ~ 0 but lower than Group IV. Across group I ~ III, number of delocalized electrons per atom increase, so their charge density increase, metallic bond strengthens, hence increasing m.p.. Note that the b.p. has a larger different with m.p. for metals because all metallic bonds have to be broken to vaporize metal.
-          Explanation on Group IV: They are in giant covalent structure (Silicate and diamond), all strong covalent bond has to be broken, and each atom forms 4 bonds, so they have the highest m.p..
-          Explain on Group V ~ 0: they are in simple molecular structure, only weak van der Waals' force has to be broken to melt it, hence very low melting point. On period three, since phosphorus (white) exist in P4 while sulphur exist in S8, Cl exist in Cl2, Ar exist in monatomic form, molecular size S8 > P4 > Cl2 > Ar, so the polarizability is arranged in S > P > Cl > Ar, magnitude of intermolecular force S > P > Cl > Ar, so m.p. has the same order.
Electroconductivity on main group elements
-          It depends on the existence of delocalized electrons or ions, here we only focus on delocalized electrons.
-          Trend: Increase from Group I to III, sharply decrease on Group IV (net zero), zero for Group V – 0.
-          Explanation: Group I ~ III are bonded by strong metallic bond, so there's delocalized electron to conduct electricity. For Group IV (Si), under r.t. very few electrons gain enough energy to delocalize, so very few delocalized electrons are used to conduct electricity, hence it's a semiconductor. Group V ~ 0 are insulators because they don't have delocalized electrons, hence it can't conduct electricity.
Density of elements depends on atomic mass, atomic radius and structure, so generally metals across the group has increasing density, and non-metal elements, solid form in r.t. has lower density. Density of gaseous non-metal in r.t. has very low density.

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