Pliny the Elder, a Roman statesman and scholar (first century) is supposed to have come up with the famous saying Cum grano salis or more accurately Addito salis grano (Ref.1) which means ‘with the addition of a grain of salt’ questioning a statement’s veracity. It is possible that expressions with salt have been there even before the time of the Bible. In the Thirukural , a book of 1330 couplet verses of 4 and 3 words in the Tamil language, (Time period:two hundred years BC to 8th century?), there is a reference to salt almost at the end! At the tip of India, in a city called Kanyakumari where the Bay of Bengal meets the Indian Ocean and the Arabian Sea, there is a splendid statue of Thiruvalluvar, who is credited with the poetry of Thirukural. (See third picture)
Common salt or table salt has the formula NaCl in Chemistry, and it is called sodium chloride.
Let us look at the structure of sodium chloride. We have already looked at covalent bonds where there is sharing of electrons, and we have seen hydrogen bonds that are weak bonds between oxygen and hydrogen in compounds like water. Now we will look at a third kind of bond here. Inorganic compounds form bonds in a different manner than organic compounds where electrons are shared. Inorganic compounds form ionic bonds, which involves a transfer of electrons from one element to another element or group of elements. Many of the elements in the Periodic Table can form these ionic bonds. Today we will look at the simplest ionic bond involving two elements : sodium and chlorine forming the compound sodium chloride. Such compounds are called binary compounds.
When a sodium atom reacts with a chlorine atom, the sodium atom gives up one electron and becomes a positive ion(cation),the sodium ion,Na+. (All atoms are electrically neutral) The chorine atom accepts the electron and becomes a negative ion(anion), the chloride ion,Cl–
The bond is actually the attraction of oppositely charged particles :Na+Cl– , so technically it is not a bond like the covalent bond where two shared electrons constitute a bond. Since these are charged particles, a solution of sodium chloride or molten sodium chloride will conduct electricity. In fact, all ionic compounds in the molten state or in a solution will be conductors of electricity.Other properties of ionic compounds include high boiling and melting points, solubility in water (for some ionic compounds) and defined crystal shapes.(Ref. 2)
What is a crystal? It is a definite arrangement of atoms, molecules or ions in a periodic pattern in three dimension. (Ref.3) Let us look at a crystal of sodium chloride or more accurately Na+Cl–
Every sodium ion is surrounded by 6 chloride ions and each of the chloride ions are surrounded by six sodium ions as shown in the second picture. The crystal structure of sodium chloride is cubic.The larger green circles in the second picture are chloride ions and the smaller blue circles are sodium ions. Why are the sizes different?
First let us look at the position of Na in the Periodic Table. It is the 11th element in the Periodic Table. and then it loses an electron to become a cation,Na+. It loses the outer electron in the second shell and also has one more proton than electron and the attraction to the nucleus is more and hence becomes much smaller. Meanwhile the chloride ion,Cl– .has 18 electrons and only 17 protons, nuclear attraction to each electron is decreased and allows the Cl atom to expand as it forms the Cl– This makes the Cl– to be considerably larger than a Na+.(Ref.4)
Many ionic compounds form crystals and the order and arrangement of the atoms can be studied by crystallography. Using X-rays, or neutrons , a single crystal of any ionic compound is studied; a pattern of spots are seen and using mathematical models, the arrangements of the atoms/ molecules/ ions can be deduced.(Ref.5) Just as visible light is part of the electromagnetic spectrum, X- rays with much shorter wavelength are part of this same spectrum. (Ref.6)
This ionic compound, sodium chloride, has played a critical role in the history of the world.Before industrialization and definitely before refrigeration,salt was a much-needed commodity for food preservation. We will talk about the spice trade where wars were fought over the pepper trade, but everyone forgets the importance of salt, which is fundamental for living and caused upheavals in many countries.
When the British ruled India for over 200 years, they imposed a salt tax and made it very difficult for poor Indians to get their daily intake of salt In fact, there is evidence that it resulted in the death of many people due to a lack of salt in their systems. Mohandas K Gandhi, the freedom fighter, who used non-violent methods to achieve India’s independence refused to buy salt from the British. He started a long march, called the Dandi March to walk to the seashore on the west coast of India to evaporate salt from the sea and use it instead.(Ref. 7)
In France, from the 14th through the 18th century, high salt taxes called la gabelle were levied and caused a lot of anger and resentment among the people.(Ref.8) Even during the Civil War in the United States in the latter part of the 19th century, the South’s supply of salt was destroyed by the Union forces; tactics used to cripple the South’s effectiveness in the battlefield.(Ref.9)
No wonder so many sayings throughout the world mention salt, this small ionic compound called sodium chloride.
Activities for Middle School Teachers:
Students should have a discussion about sayings with salt in their cultures. It would be interesting to see how vital a role salt has played in our lives for so long.
Let students sharpen their idioms in English by thinking of all the sayings that are common using salt; eg, “rubbing salt to one’s wounds”, ” worth your weight in salt” etc.
Let students grow crystals of salt in the laboratory. let them examine the little crystals called seed crystals that may be found at the bottom of the beakers using magnifying glasses, what are their shapes?
Confluence of geometry and atomic arrangement: The Na+Cl– crystal has perfect symmetry among all the 14 Bravais crystal lattice structures. (Students need to understand the different types of symmetry, mirror image symmetry, rotational symmetry etc.)(Ref.10) Here a=b=c and the angles are all equal to 900. Students could construct the 14 possible arrangement of atoms/molecules/ ions among the various crystal structures.
How did people look for salt long ago besides going to the salt water areas? Let students search for salt- water lakes all over the world as a geography project.
Students can read the following books about salt:”Men of Salt” by Michael Benanav,”Salt” by Neil Morris and “Salt” by Mark Kurlansky.(Refs.11,12,13) The third book is considered a must for anyone interesting in cooking/food.
Nuggets of Information:
Crystals of sodium chloride are also called halite crystals.
Why do some elements like to give up electrons and others prefer accepting them? Look at the Periodic Table and you will notice that elements on the left side of the Periodic Table in the first two groups and the Transition Elements give up electrons to attain the electronic configuration of Noble Gas elements like He, Ne, etc.that are before these elements These elements are called metals
On the other hand, Cl, Br, F as well as O, S, N etc, on the right-hand side of the Periodic Table, like to take electrons to attain the electronic configuration of the Noble Gas that follows them. These elements are called non-metals.
Even though salt, NaCl, is essential for all of us, today we are dealing with health problems because of too much salt in our foods especially processed foods like canned soups. Obesity, high blood pressure are some of the negative effects of too much salt in our diets.(Ref.14)
In Chemistry, when we use the word ‘salt’ we are not just talking about NaCl. A reaction between an acid and a base creates salt and water. There are many salts that are created this way and one of them besides rock salt, (NaCl) can be used on the roadways to melt ice and snow during winter. This is a salt of calcium Ca, or magnesium, Mg.(Ref.15)
How does this salt melt the ice? It lowers the freezing point of ice when it mixes with the solid ice.
In Poland, there is a famous salt mine called Wieliczka where there are salt sculptures.(Ref.16)
Look at a tribute to Martin Gardner, who wrote regularly in the Scientific American; he was obsessed with symmetry and you can make a hexaflexagon! (Ref.17)
References:
1.Knowles, Elizabeth(editor),The Oxford Dictionary of Quotations (Oxford University Press,2001)
2. http://www.answers.com/Q/What_are_the_caracteristics_of_ionic_bonds
3. http://www.chemicool.com/definition/crystal.html
4. Hein, Morris & Arena, Susan, Foundations of College Chemistry, (John Wiley and sons, 2007)
5.http://www.acs.org/content/acs/en/careeres/college-to-career/chemistry-careers/cystallography.html
6. http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html
7. Sinha, Sarojini, A Pinch of Salt Rocks an Empire, (Children’s Book Trust, New Delhi,1985)
8. http://www.personal.psu.edu/sjh11/TCTaxBits/CuriousTaxFacts/SaltTax.shtml
9. http://www.encyclopediavirginia.org/Saltville_During_the_Civil_War
10. http://www.bing.com/videos/search?q=images+and+lectures+on+bravais+lattice&FORM=VIRE3#view=detail&mid=A84B06CECC099451B679A84B06CECC099451B679
11. Benanav, Michael, Men of Salt,(Lyons Press, 2006)
12.Morris, Neil, Salt, (London Franklin Watts,2005)
13. Kurlansky, Mark, Salt,(Penguin Books,2003)
14. Moss, Michael, Salt, Sugar and Fat,(New York, Random House,2013)
15. http://www.usroads.com/journals/p/rmj/9712/rm971202.htm
16. https://en.m.wikipedia.org/wiki/Wieliczka_Salt_Mine
17. https://www.youtube.com/watch?v=-6Q966FMVbQ
Tags: common salt, crystallography, Dandi march, ionic bonds, la gabelle, sodium chloride
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