Alkali Metals

Characteristics

Lithium was discovered in 1817. It isfound in most igneous rocks and is used in batteries as an anode because it has a very low reduction potential. Lithium is soft and is stored in oil or kerosene to prevent it from reacting with the air.

Sodium derives its name from the word soda. It was first isolated in 1807 from the electrolysis of caustic soda, NaOH. Sodium is soft enough to be cut with a knife. It is shiny until it reacts with oxygen, which causes the surface to lose its luster.

Potassium was first isolated in 1807 from the electrolysis of caustic potash, KOH.

Common Reactions


With Water to Form Bases and Hydrogen Gas


Example: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Li, K, Rb, and Cs also follow this pattern.


With Acids to Form Salts and Hydrogen Gas


Example: 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g)
Li, K, Rb, and Cs also follow this pattern.


With Halogens to Form Salts

Example: 2Na(s) + F2(g) → 2NaF(s)
Li, K, Rb, and Cs also follow this pattern in reacting with F2, Cl2, Br2, and I2.


With Oxygen to Form Oxides, Peroxides, or Superoxides

Lithium forms an oxide.
4Li(s) + O2(g) → 2Li2O(s)


Sodium also forms a peroxide.
2Na(s) + O2(g) → Na2O2(s)
Alkali metals with higher molecular masses can also form superoxides.
K(s) + O2(g) → KO2(s)
Rb and Cs also follow this pattern.

Alkali-Metal Oxides with Water to Form Bases

Oxides of Na, K, Rb, and Cs can be prepared indirectly. These basic anhydrides form hydroxides in water.


Example: K2O(s) + H2O(l) → 2KOH(aq)
Li, Na, Rb, and Cs also follow this pattern.

A small piece of potassium dropped into water will react explosively, releasing H2 to form a strongly basic hydroxide solution. The energy of the reaction ignites the hydrogen gas that is produced.

Sodium reacts vigorously with chlorine to produce NaCl. Most salts of Group 1 metals are white crystalline compounds.

Analytical Test


Alkali metals are easily detected by flame tests because each metal imparts a characteristic color to a flame. When sodium and potassium are both present in a sample, the yellow color of the sodium masks the violet color of the potassium. The violet color can be seen only when the combined sodium-potassium flame is viewed through a cobalt-blue glass. The glass blocks the yellow flame of sodium and makes it possible to see the violet flame of potassium.

Lithium

Sodium

Potassium

Cesium

Properties

Quality Li Na K Rb Cs Fr
Melting Point (C) 180.5 97.8 63.25 38.89 28.5 27
Boiling Point (C) 1342 882.9 760 691 668 677
Density (C) 0.534 0.971 0.862 1.53 1.87 —
Ionization energy (kJ/mol) 520 496 419 403 376 —
Atomic radius (pm) 152 186 227 248 265 270
Ionic radius (pm) 76 102 138 152 167 180
Common oxidation number in
compounds		 +1 +1 +1 +1 +1 —
Crystal structure bcc* bcc bcc bcc bcc —
Hardness (Mohs’ scale)` 0.6 0.4 0.5 0.3 0.2 —
             

Application

Sodium Vapor Lighting
The flame test for sodium shows two bright lines at 589.0 and 589.6 nm, which is the yellow range of the emission spectrum. Sodium can be vaporized at high temperatures in a sealed tube and made to give off light using two electrodes connected to a power source. Sodium vapor lighting is often used along highways and in parking lots because it provides good illumination while using less energy than other types of lighting. Sodium vapor lighting comes in both lowpressure and high-pressure bulbs. Low-pressure lamps reach an internal temperature of 270°C to vaporize the sodium under a pressure of about 1 Pa. High-pressure lamps contain mercury and xenon in addition to sodium. These substances reach an internal temperature of 1100°C under a pressure of about 100 000 Pa. The high-pressure lamp provides a higher light intensity. The design of both types of lamps must take into account the high reactivity of sodium, which increases at high temperatures. Because ordinaryglass will react with sodium at 250°C, a special sodium-resistant glass is used for low-pressure lamps. High-pressure lamps use an aluminum oxide material for the column containing the sodium, mercury, and xenon. Both types of lamps contain tungsten electrodes. The light intensity per watt for sodium vapor lamps far exceeds that of fluorescent lamps, high-pressure mercury vapor lamps, tungsten halogen lamps, and incandescent bulbs.

Application

Health

Table 1A

Sodium-Potassium
Concentration in Body Fluids

    





Cation
Inside cells
(mmol/L)		 Outside cells or in
plasma (mmol/L)
Na+ 12 145
K+ 140 4
       
Electrolyte
 Normal Range
(mmol/L)		 Excess Deficiency
Sodium, Na+ 135–145 hypernatremia (increased urine
excretion; excess water loss)
 hyponatremia (dehydration;
diabetes-related low blood pH;
vomiting; diarrhea)
Potassium, K+ 3.5–5.0 hyperkalemia (renal failure; low
blood pH)
 hypokalemia
(gastrointestinal conditions
Hydrogen
carbonate, HCO3
-		 24–30 hypercapnia (high blood pH;
hypoventilation)
 hypocapnia (low blood pH;
hyperventilation; dehydration)
Chloride, Cl- 100–106 hyperchloremia (anemia; heart
conditions; dehydration)
 hypochloremia (acute infection;
burns; hypoventilation)

 

 

Electrolyte Balance in the Body The elements of Group 1 are important to a person’s diet and body maintenance because they form ionic compounds. These compounds are present in the body as solutions of the ions. All ions carry an electric charge, so they are electrolyte solutes. Two of the most important electrolyte solutes found in the body are K+ and Na+ ions. Both ions facilitate the transmission of nerve impulses and control the amount of water retained by cells. The sodium and potassium ion concentrations in body fluids are shown in Table 1A. Sodium ions are found primarily in the fluid outside cells, while potassium ions are largely found in the fluid inside cells. Anions are present in the fluids to balance the electrical charge of the Na+ and K+ cations. Abnormal electrolyte concentrations in blood serum can indicate the presence of disease. The ion concentrations that vary as a result of disease are Na+, K+, Cl−, and HC O 3− . Sodium ion concentration is a good indicator of the water balance between blood and tissue cells. Unusual potassium ion levels can indicate kidney or gastrointestinal problems. Chloride ion is the anion that balances the positive charge of the sodium ion in the fluid outside the cells. It also diffuses into a cell to maintain normal electrolyte balance when hydrogen carbonate ions diffuse out of the cell into the blood. Table 1B shows medical conditions associated with electrolyte imbalances. Sodium-Potassium Pump in the Cell Membrane The process of active transport allows a cell to maintain its proper electrolyte balance. To keep the ion concentrations at the proper levels shown in Table 1B, a sodium-potassium pump embedded in the cell membrane shuttles sodium ions out of the cell across the cell membrane. A model for the action of the sodium-potassium pump is shown in the figure below. Nerve Impulses and Ion Concentration The difference in Na+ and K+ concentrations inside and outside nerve cell membranes is essential for the operation of the nervous system. This unequal concentration of ions creates a voltage across nerve cell membranes. When a nerve cell is stimulated, sodium ions diffuse into the cell from the surrounding fluid, raising voltage across the nerve cell membrane from −70 mV to nearly +60 mV. Potassium ions then diffuse out of the cell into the surrounding fluid, restoring the voltage across the nerve cell membrane to −70 mV. This voltage fluctuation initiates the transmission of a nerve impulse. The amount of Na+ inside the cell has increased slightly, and the amount of K+ outside the cell has decreased. But the sodium- potassium pump will restore these ions to their proper concentrations.

During situations where the body is losing water rapidly through intense sweating or diarrhea for a prolonged period (more than 5 hours), a sports drink can hydrate the body and restore electrolyte balance. However, using sports drinks regularly when not exercising can have a negative effect on electrolyte balance in the body. (See Table 1B)

The sodium-potassium pump is a protein embedded within the cell membrane that allows the passage of Na+ and K+ into and out of the cell. Each figure depicts the action of a single protein.

Though sodium is an important mineral in your body, a diet that is high in sodium is one of several factors linked to high blood pressure, also known as hypertension. High Na+ levels cause water retention, which results in increased blood pressure. Sodium is not the direct cause of all hypertension, but reducing sodium levels in the diet can affect individuals with a condition known as salt-sensitive hypertension. Therefore, the Dietary Guidelines for Americans recommend consuming salt and sodium in moderation. Test your knowledge about sodium in foods with the questions below. 1. Which of the following condiments do you think has the lowest salt content? a. mustard c. catsup e. vinegar b. steak sauce d. pickles 2. One-fourth of a teaspoon of salt contains about of sodium. a. 10 mg c. 500 mg e. 1 kg b. 100 g d. 500 g
3. According to FDA regulations for food product labels, a food labeled salt-free must contain less than mg of sodium ion per serving. a. 100 c. 0.001 e. 0.00005 b. 5 d. 0.005 4. The Nutrition Facts label for a particular food reads “Sodium 15 mg.” This is the amount of sodium ions per . a. package c. serving e. RDA b. teaspoon d. ounce 5. The recommended average daily intake of sodium ion for adults is 1500 mg. For a lowsodium diet the intake should be . a. 200 mg c. 750 mg e. 150 mg b. 2000 mg d. 500 mg 6. Each of the following ingredients can be found in the ingredients lists for some common food products. Which ones indicate that the product contains sodium? a. trisodium phosphate d. sodium sulfate b. sodium bicarbonate e. MSG c. sodium benzoate f. baking soda 7. Which of the following spices is NOT a salt substitute? a. caraway seeds c. ginger b. dill d. onion salt 8. Most salt in the average American diet comes from salting foods too heavily at the dinner table. a. true b. false 9. Which of the following foods are high in sodium? a. potato chips c. doughnuts e. figs b. pizza d. banana 10. Your body requires about 200 mg of sodium ion, or 500 mg of salt, per day. Why do these numbers differ? What’s Your Sodium IQ? Answers 1.e; 2. c; 3. b; 4. c; 5. c; 6. all of them; 7. d; 8. b, processed foods can contain very high levels of sodium; 9. a, b, c; 10. Salt is not pure sodium. Alkali Metals