The following sketch shows the variation in a physical property of third period elements against their atomic numbers: What physical property is plotted in this sketch?
Correct answer: B. Melting point
- A. Ionization energy
- B. Melting point
- C. Ionic radius
- D. Atomic radius
Explanation
Na, Mg, Al, Si are all GIANT LATTICE structures involving either metallic (Na, Mg, Al) or giant covalent macromolecule (Si) bonding. Whereas, P, S, Cl are COVALENT MOLECULES, with Ar being a monatomic nonmetal gas. Melting is achieved by disrupting the considerably weaker van der Waals intermolecular forces that exist between the molecules. Melting point steadily increases between Na and Al due to: increased metal cation charge (more protons), more delocalised e− per atom, smaller sized metal ions, hence stronger electrostatic attraction between cations and delocalised e− therefore stronger metallic bonding. Silicon has a significantly higher melting point because Silicon is a giant covalent macromolecule. In order to melt Silicon there is a need to break down many strong covalent bonds between Si atoms, that can only be achieved by applying considerable energy. Melting point of S8 > P4 > Cl2 > Ar because there is decreasing strength of vdW intermolecular forces of attraction as there is a decreasing size of molecule. This explanation is based on the known correlation between molecular size (molecular mass) and magnitude of vdW force of attraction between molecules.
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About Periodic Properties and Trends
Periodic properties arise from electron configuration and effective nuclear charge, producing trends in atomic and ionic radius, ionization energy, electron affinity, electronegativity, metallic character and reactivity. Comparisons run across periods and down groups, with attention to common exceptions. The topic also relates these trends to the behavior of s-block and p-block elements.
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