Melting and Boiling Points
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Melting and boiling points of the alkali metals Alkali metal Melting point Boiling point Lithium 454 !453.69 K (180.54 °C) 1615 !1615 K (1342 °C) Sodium 371 !370.87 K (97.72 °C) 1156 !1156 K (883 °C) Potassium 337 !336.53 K (63.38 °C) 1032 !1032 K (759 °C) Rubidium 312 !312.46 K (39.31 °C) 0961 !961 K (688 °C) Caesium 302 !301.59 K (28.44 °C) 0944 !944 K (671 °C) Francium 300 !? 300 K (? 27 °C) 0950 !? 950 K (? 677 °C)
The melting point of a substance is the point where it changes state from solid to liquid while the boiling point of a substance (in liquid state) is the point where the vapor pressure of the liquid equals the environmental pressure surrounding the liquid and all the liquid changes state to gas. As a metal is heated to its melting point, the metallic bonds keeping the atoms in place weaken so that the atoms can move around, and the metallic bonds eventually break completely at the metal's boiling point. Therefore, the falling melting and boiling points of the alkali metals indicate that the strength of the metallic bonds of the alkali metals decreases down the group. This is because metal atoms are held together by the electromagnetic attraction from the positive ions to the delocalised electrons. As the atoms increase in size going down the group (because their atomic radius increases), the nuclei of the ions move further away from the delocalised electrons and hence the metallic bond becomes weaker so that the metal can more easily melt and boil, thus lowering the melting and boiling points. (The increased nuclear charge is not a relevant factor due to the shielding effect.)
Read more about this topic: Alkali Metal, Characteristics, Periodic Trends
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