Science Makes Sense-Week34: The other Alkali metals

Growing up, I was under the impression that potatoes were not good for you. Of course, my mother made them often and I enjoyed eating potatoes especially when fried. But I do remember the National Geographic feature on the merits of a potato and then I realized its significance as a great source of vitamins and minerals. A baked potato is high in potassium, an essential element needed in our bodies.
Potassium, like sodium, lithium, are part of Group 1A in the Periodic Table and are called alkali metals. They all have one lone electron in the ‘s’ orbital which makes them metals since they are willing to give it up to be a cation and have the outermost electronic structure of the closest neighbor, the inert noble gases. There are six alkali metals: lithium,Li, sodium,Na, potassium,K, rubidium,Rb, cesium,Cs and francium,Fr. Since the alkali metals are so reactive, it is never found in the elemental form in nature. Sir Humphrey Davy discovered Na and K in the early 19th century, while Li was discovered ten years later by a Swedish chemist. Later, Robert Bunsen, who invented the Bunsen burner, discovered Rb and Cs in 1860. Meanwhile, Fr was found in trace amounts and very little is known about its behavior.(Ref.1)
The alkali metals have a lot of properties similar to other traditional metals(copper, iron) like high conductivity, form cations easily, and are malleable and ductile. They have low ionization energy, i.e., the energy required to remove the outermost ‘s’ electron is not high. However, they are softer and silvery-white in color except for cesium which is yellow. In addition, their densities are lower than most metals and they have low boiling and melting points.(Ref.2)
They react very readily with oxygen when exposed to the air/oxygen to form oxides. Another important reaction is with water, when it vigorously reacts with it to form the hydroxides or alkalis, (which is responsible for its name) along with the release of hydrogen gas. This reaction is well documented in several videos to illustrate the increase in the rate of reaction as well as the increase in the intensity of the exothermic reaction as you go down this group of alkali metals. This particular video uses the same number of atoms of each alkali metal to react with the same amount of water each time and you will clearly see that the rate and intensity of the reaction increases significantly as you go down the group making cesium one of the most violent reactions to observe.(Ref.3)
The alkali metals react very easily with halogens to form the various halides.(Ref.1) No wonder sodium and potassium chlorides are found abundantly in nature.(Ref.4) In addition, the oxides of sodium and potassium react with carbon dioxide to form the various carbonates.(Ref.1) The alkali metals as hydroxides can easily react with nitric/nitrous and sulfuric acids to form the nitrates/nitrites and sulfates respectively.
Once we see the properties and reactions of alkali metals it is easy to see what are the salts of alkali metals available on the earth’s crust. Besides the fact that sodium chloride is found abundantly on the earth, we also find sodium carbonate, sodium nitrate(saltpetre), and sodium sulfate. Sodium nitrite is used extensively in the manufacture of gunpowder. The pulp and paper industry uses the hydroxide, carbonate and sulfates of sodium. Sodium sulfate is also used in the manufacture of cardboard and brown paper. Sodium carbonate is used in power companies to absorb sulfur dioxide which is a serious pollutant. NaOH, sodium hydroxide is one of the top ten industrial chemicals, while sodium bicarbonate, baking soda, NaHCO3, is invaluable in the food industry.(Ref.4)
Meanwhile most of the potassium is used as a fertilizer. KOH, potassium hydroxide is used for detergents,KClO3 potassium chlorate is needed in the manufacture of explosives while KBr, potassium bromide is essential in the photography industry.
(Ref.4)
The next three alkali metals, rubidium, cesium and francium are much less common. Rubidium is sometimes used for the treatment of depression, while cesium is mainly used in the manufacture of certain kinds of glass and radiation detection equipment. It is also used to make the atomic clock which will be explained more under Nuggets. But the main use of francium is in research, along with rubidium as well.(Ref.4)
The role of two of the alkali metals, sodium and potassium in our bodies cannot be underestimated. Both these alkali metals function as cations in our body; in other words they are charged particles that carry an electric current across our cells causing many physiological processes. It is important to maintain a balance of the sodium and potassium ions; ideally we need 3 times as much potassium as sodium in our bodies. Sadly we tend to consume more sodium containing products versus potassium containing products.(Ref.5) (More under Nuggets)
Even though the higher group alkali metals like rubidium, cesium and francium have limited uses, our lives would be cut short without the salts and hydroxides of the other alkali metals.
Activities for Middle School Teachers:
Look at the various halides formed by the alkali metals. Does it follow the multiplication principle? How about oxides and hydroxides? Do they follow that same principle? Why or why not?
How do we lose sodium from our bodies? What are the main sources of sodium in our diet? What are the common diseases associated with high sodium in our diets? Study the increase or decrease in these illnesses based on diet in various different countries over a 50-year period.
Nuggets of Information:
Sir Humphrey Davy who discovered two of the alkali metals, was born in Penzance, Cornwall, England. He was known as a wild man in the laboratory, often smelling and tasting the products of his experiments, which almost certainly shortened his life.(Ref.1)
The colors of the alkali metal salts are distinctive and are often used in the flame test to determine the cations of lithium, sodium and potassium. Chlorides of these cations are mixed with a little hydrochloric acid and placed on the blue upper part of a Bunsen flame. Lithium shows a crimson color, sodium the characteristic golden-yellow color and potassium has a lilac color.(Ref.6) Rubidium gives a dark red color, while cesium has a blue color.(Ref.4)
Potassium exists as sylvite, KCl on the earth’s crust.(Ref.4)
Both cations of potassium and sodium are found inside and outside a cell membrane in our bodies. The difference in the concentrations of these ions on each side of the cell membrane is called a membrane potential. The membrane potential uses the charge differences to conduct electricity between cells. This electricity then helps perform the work of contracting muscle, transmitting nerve impulses and maintaining the heartbeat. Processed foods increases the amount of sodium in our bodies, while foods like baked potatoes, prunes, spinach, bananas are good sources of potassium.(Ref.5)
Cesium has been used in the manufacture of atomic clocks that keep the best time. How does it work? Prior to 1964, the International Standard second was based on the orbital period of the earth. However, the cesium clock period was found to be much more stable. Using the 133 atomic mass of cesium, we focus on the lone electron outside the 54 electron core. This electron interacts with the nuclear spin and has an energy splitting called hyperfine structure. This splitting allows us to measure time with a precision of one second in 1.4 million years!(Ref.7)
References:
1.http://chemwiki.ucdavis.edu/?title=Textbook_Maps/General_Chemistry_Textbook_Maps/Map:_Chemistry_(Averill_%26_
Eldredge)/21:_Periodic_Trends_%26_the_s-Block_Elements/21.3:_The_Alkali_Metals_(Group_1)
2.http://www.buzzle.com/articles/alkali-metals-properties-of-alkali-metals.html
3.http://www.bing.com/videos/search?q=reaction+of+alkali+metals+with+water&view=detail&mid=32
DA092AA85B40F0218232DA092AA85B40F02182&FORM=VIRE
4.http://www.scienceclarified.com/A-Al/Alkali-Metals.html
5.http://healthyeating.sfgate.com/right-balance-sodium-potassium-6341.html
6.https://www.youtube.com/watch?v=k-ZsaTXiz9w
7.http://hyperphysics.phy-astr.gsu.edu/hbase/acloc.html

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