Melting and Boiling Points

The outermost shell of Group 1 elements has one electron, and unlike most other atoms, this electron may wander away from the nucleus. Consequently, elements in Group 1 have higher atomic radii than elements in their respective periods, and the greater atomic size weakens atomic forces. Because Group 1 elements have wide atomic radii between neighbouring atoms, the melting and boiling points are lower than other metals.

The atomic radius increases when added an extra shell to the previous element, resulting in lower melting and boiling points down the group. Increasing atomic radius weakens interatomic forces, lowering melting and boiling points.

What exactly does the melting and boiling points of alkali metals mean?

The mass of atoms increases as one moves down the periodic table’s rows or periods. However, the energy required to transform a solid alkali metal to a liquid or vaporise a liquid alkali metal decreases with atomic number. As the atomic number of a metal increases, the boiling and melting points of the metal decrease.

General properties of the group

Physical properties

Metals are ductile, conductive, lustrous, and malleable. An alkali metal’s outermost electron is single. More loosely bound than inner shell valence electrons. Consequently, when alkali metals react with nonmetals, they generate positive charge cations. The resultant compounds are hard crystals with high melting temperatures. When a half-filled valence band forms, valence electrons become delocalised and mobile. A partly filled valence band is a conduction band responsible for metal valence properties. From lithium to francium, an electron loses its strength. In general, the energy required to remove the outermost electron from an element’s atoms, called the ionisation energy, decreases as one moves left and downward in each vertical file of the periodic table, with francium being the most readily ionisable element in the whole table. The alkali metals’ ionisation energies vary from 124.3 kcal/mole for lithium to 89.7 kcal/mole for caesium. The alkaline-earth metals have greater ionisation energies ranging from 214.9 kcal/mole for beryllium to 120.1 kcal/mole for barium.

The electronegativity scale compares the capacity of different atoms to attract electrons. It ranges from 0.7 for caesium, the least electronegative element, to 4.0 for fluorine, the most electronegative and metals are elements with electronegativity values below 2.0. The alkali metals have electronegativities ranging from 0.7 to 1.0, whereas the alkaline earth has 0.9 to 1.5.

Each atom in this body-centred cubic crystallographic arrangement has eight nearest neighbours. The closest distance between atoms in crystals rises with alkali metal atomic weight. The alkali metals have a more disordered crystallographic structure than any other metallic crystal. Caesium has a greater interatomic distance than any other metal due to its higher atomic weight.

The vapour pressures of the alkali metals and two alloys created between elements in the group increase with atomic weight, and caesium has the highest alkali metal boiling point (671°C/1240°F). The boiling points of alkali metals drop with atomic number, with lithium having the highest at 1,317 °C (2,403 °F).

The alkali metals have the lowest melting points of any nongaseous group in the periodic table, ranging from 179°C (354°F) for lithium to 28.5°C (83.3°F) for caesium. Only mercury (38.9 °C, or 38.02 °F) has a lower melting point than caesium, and the low melting points of alkali metals are due to their vast interatomic distances and weak bond energies. Low densities, low fusion temps, and small changes in volume in metal fusion are all due to these same factors. Lithium, Sodium, and potassium are light.

Having just one weakly bound electron in the massive outer s-type orbital gives an alkali metal atom its huge size (and low density). The added electron in potassium relocated the huge 4s orbital rather than the smaller 3p orbital. However, increasing pressure (up to half a million atmospheres or more) causes phase changes in potassium, rubidium, and caesium metals. So these alkali metals resemble transition metals because they prefer d-type orbitals over s-orbitals. Under those conditions, transition metal alloys (like iron) may form, which is impossible at low pressures. The lower-than-expected density of the earth’s core may be due to a potassium-iron alloy forming under extreme pressures.

Alkali metals are crucial in quantum physics. Bosons are alkali metal isotopes like rubidium-87. Alkali metal isotope atomic vapours produce Bose-Einstein condensates when confined by magnetic fields or “laser mirrors” and cooled to near absolute zero. The atom cluster is in a single quantum state and behaves like an atomic-sized particle. These include interference and coherent motion of atoms’ entire “cloud”.

Chemical properties

Because alkali metals are the most electropositive (least electronegative) elements, they react with nonmetals. Compared to other metals in its group, lithium’s chemical reactivity is more like Group 2 (IIa) than any other. It is less reactive with oxygen, water, and halogens but more reactive with carbon, nitrogen, and hydrogen.

Following are the melting and boiling points of alkali metals.

Alkali metal with the highest melting point – Lithium

Alkali metal that melts if the room temperature rises to 30°C – Cesium

Conclusion

Positive and negative ions form a crystal lattice in ionic solids. Ionic solids have high melting temperatures because ionic bonds are difficult to break. On the other hand, metals are tightly packed networks of positive ions with a “sea” of mobile valence electrons that can freely move throughout the metallic crystal structure. Metals have a broad range of melting points throughout the periodic table. Tungsten melts at 3422°C, whereas Caesium melts at 28°C.

Separating metal atoms from one another requires a certain amount of energy. The bigger the atom, the greater the distance between neighbouring atoms in a metal crystal. The greater the distance between atoms, the simpler it is to separate them. The energy level of the valence shell rises as you move down the group, atomic size increases, and the melting point of an atom decreases as its size increases.

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