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ALKALI METALS @7:25 AM
Alkali metals
- Lithium
- Sodium
- Potassium
- Rubidium
- Cesium
- Francium

Brief introduction
Group 1 elements are soft metals. They have low densities and low melting and boiling points. All the alkali metals react vigorously with cold water. They react with water to produce a solution of metal hydroxide and hydrogen gas. The more reactive the metal, the more vigorous the reaction (i.e. Li <>
Atomic radii of Group 1 elements





Being the first elements of each period, alkali metals have the largest atomic and ionic radii in their respective periods. On moving down the group, the number of shells increases and therefore, atomic and ionic radii increase.









Alkali metals have only 1 valence electron and have the lowest ionization energy in each period. As we go down the group, the ionization energy decreases. This is because in large atoms, the valence electrons are held loosely by the nucleus, thereby losing their electrons readily to attain the noble gas configuration. Therefore, alkali metals have high reactivity and the reactivity increases on going down Group I as the size of the atom increases.




Melting and boiling points of Group 1 elements






All alkali metals are soft and have low melting and boiling points. The atoms in a metal are held together by the attraction of the nuclei to the delocalised electrons. As the atoms get bigger, the nuclei get further away from these delocalised electrons, and so the attractions fall. That means that the atoms are more easily pulled apart to make a liquid and finally a gas.
As alkali metals have only one valence electron per metal atom and the size of the atoms is large, the energy binding the atoms in the crystal lattice of the metal is low and the metallic bonds in these metals are not very strong. Thus, less energy is required to overcome the bonds, resulting in low melting and boiling points.



Density of the Group 1 elements



Notice that these are all light metals - and that the first three in the Group are less dense than water (less than 1 g cm-3). That means that the first three will float on water, while the other two sink.
Alkali metals have low densities as compared to other metals, Even though they do have close packaging of metal atoms in their lattice, the large size of their atoms cause them to have low densities. As we move down the group from lithium to cesium, even though there is an increase in atomic size, the simultaneous increase in atomic mass compensates more than the increase in atomic size. As a result, densities increase as we move down the group.

Reaction with water

Alkali metals are famous for their vigorous, exothermic and fast reactions with water or even more explosively with cold water. The more reactive the metal, the more vigorous the reaction (i.e. Li <> 2LiOH (aq) +H2 (g)
When lithium is placed in water, it floats, and may melt and move around the surface of the water with “fizzling”. It gradually reacts with water and disappears to produced hydrogen gas and forming a colourless solution of lithium hydroxide. The reaction generates heat too slowly and lithium's melting point is too high for it to melt.

Sodium + Water à Sodium hydroxide + hydrogen
2Na (s) + H2O (l) --> 2NaOH (aq) + H2 (g)

Sodium also floats on the surface, but enough heat is given off to melt the sodium (sodium has a lower melting point than lithium and the reaction produces heat faster) and it melts almost at once to form a small silvery ball that dashes around the surface. A white trail of sodium hydroxide is seen in the water under the sodium, but this soon dissolves to give a colourless solution of sodium hydroxide.

Potassium + Water à Potassium hydroxide]
2K (s) + H2O (l) --> 2KOH(aq) + H2 (g)

Potassium behaves rather like sodium except that the reaction is faster and exothermic enough heat is given off to set light to the hydrogen. This time the normal hydrogen flame is contaminated by potassium compounds and so is coloured lilac (a faintly bluish pink).

Rubidium, caesium and francium are very explosive with water.
Rubidium is denser than water and so sinks. It reacts violently and immediately, with everything spitting out of the container again. Rubidium hydroxide solution and hydrogen are formed.
Caesium explodes on contact with water, quite possibly shattering the container. Caesium hydroxide and hydrogen are formed

Here are two videos showing the reactions between group 1 elements and water:






Alkali metals’ reducing power

Alkali metals are powerful reducing agents. The tendency of alkali metals to act as strong reducing agents is evident from the fact that these metals can liberate H2 from H2 O and acids. The high valves of oxidation potential show that alkali metals can lose their valence electron quite readily and hence have a strong tendency to act as reducing agents.

SUMMARY OF THIS TOPIC:

Group 1 metals(alkali metals)-physical properties
· Soft
· Have low densities
· Melting point of metals decrease down the group.
· Boiling point of metals decrease down the group.
· Densities of metals increase down the group.

Group 1 metals( alkali metals)-Reactions with water
· All group 1 elements react vigorously with cold water. This means that a small amount of metal in a large amount of water will produce a large reaction.
· All the group 1 elements react with water to produce a solution of metal hydroxide and hydrogen gas.
· Equation for reaction of group 1 element with alkali is as follows:

2X(s) + 2H2O(l) --> 2XOH(aq) + H2(g) ( * X is any group 1 metal)

· Metals are more reactive down the group.
· The more reactive the metal, the more vigorous the reaction (i.e. Li <>powerful reducing agents.



Sunday, February 28, 2010
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