2. Group I
Properties and trends of alkali metals
ALKALI METALS
PHYSICAL PROPERTIES OF ALKALI METALS
Alkali metals are the bigger elements in each row of the Periodic table because of the increase of the effective nuclear charge as we move across the period which attract closer the atom’s outermost electrons to the nucleus and thereby decrease the overall size. Additionally, as we move down within the group, the principal quantum number of the outermost electrons increases resulting in larger orbitals and therefore larger radius. The increase in size in the group is not linear because the increase of nuclear charge is not the same for the upper elements as it is for the elements at the bottom of the group. On the contrary, the big sizes of alkali metals contrast with their low melting and boiling point. These values decreased as we move down in the group, Cesium being a liquid metal at temperatures above 29º as illustrated in Table 1. Additionally, they all have low density values in agreement with its larger sizes and they do not adopt a close-package structure but they all have a body-centered structure.

CHEMICAL PROPERTIES OF ALKALI METALS
The chemical properties of elements of Group 1 are related with their valence electron configuration ns1 and their high tendency to form +1 ion by losing the outermost electron. The trend found in atomic radii correlates with the trend found in the values of ionization energy. As we move down in the column the first ionization energy decreases and therefore metals are more reactive. Their reaction with water illustrates this behavior well from the gentle reaction of lithium to the explosive reaction found with cesium. The general equation for the reaction of alkali metal with water is the following:
This is a redox reaction where alkali metal acts like reductant agent liberating hydrogen gas in the process. Their reduction potentials of the couples M+/M are quite negatives and with the exception of lithium there are more negative as you descend in the group. The small size of lithium and its very large hydration energy of free gaseous lithium ion make it the more electropositive of the alkali metals (E0Li+/Li)=−3.045V) . The negative value contrasts with the less vigorous reaction with water compared with the other alkali as it was explained above which is attributed to a kinetically less favorable process.
All metals react with hydrogen, oxygen, halogens and most nonmetal elements. The anomalous behavior of lithium is also observed in the reaction of alkali with dinitrogen because is the only element of the group that reacts spontaneously with nitrogen to form the corresponding nitride compound. The high energy required to break the triple bond in the dinitrogen molecule is only compensated by the high lattice energy released in the formation of lithium nitride due to the small size of the lithium cation. Some of these reactions are summarized in the following equations:
As we can see in the previous equations the product of the reaction when alkali metal are heated in excess of air or oxygen is different depending on the metal, so lithium tends to form the oxide, sodium the peroxide and potassium, rubidium and cesium the superoxide. Oxides with other stoichiometry can be obtained although the above described are the most stable for each metal.
ALKALI METALS IN THE LAB
Francisco Javier Cervigon Ruckauer
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