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Zinc, Cadmium and Mercury - Chemwiki
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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > d-Block Elements > Group 12: Transition Metals
> Zinc, Cadmium and Mercury
Zinc, Cadmium and Mercury
Group 12 of the Periodic Table include Zinc, Cadmium and Mercury. These metals tend to have properties characteristic of elements with full
subshells, which includes having low melting and boiling points (due to the weak metallic bonding of the ns2 electrons) and charges of +2 or +1.
Introduction: The Properties of Group 12
Zinc (Zn) is a blue-white metal of moderate strength, hardness and ductility. Zinc is one of the least common elements and is mostly produced
through electrolysis of aqueous zinc sulfate. About one third of all metallic zinc is used to manufacture galvanized nails. Because of its low melting
point and its ability to form bonds with iron or steel, it serves to coat the metal and protect it from corrosion. Metallic zinc is also used to make dry
cell batteries.
Zinc is being compared with a penny. Zinc is used to galvanize nails.
Zinc has many unique characteristics. For example, its vapor burns in air with a green flame, forming zinc oxide. Zinc oxide is a common zinc
compound that is used in paints, cosmetics, plastics and more. Metallic zinc reacts with weak acids very slowly. Sulfur has a strong affinity for zinc.
When heated, the two powders react explosively to form zinc sulfide. Zinc sulfide is used to make television screens and fluorescent light bulbs. Zinc
also reacts with halogens. However, as the electronegativity decreases among the halogen group, the reactivity with zinc decreases. Thus, the most
electronegative of the halogens (Fluorine) reacts to zinc violently, while the less electronegative halogen (Iodine) only generates a small amount of
heat. Interestingly, properties of zinc are strongly affected by impurities such as lead, cadmium and iron. Also, zinc is most often used as a reducing
agent in chemical reactions and it forms complex ions with ammonia and cyanide ions. Zinc plays a huge part in the production of alloys. One of the best known zinc alloy is brass, which contains between 55-95% copper. Zinc also takes
part in manufacturing solder, which has a relatively low melting point. Solder is used to join electrical components, as well as pipes and other metals.
Brass
Cadmium (Cd) is a unique metal that was first found in zinc dust. Cadmium usually occurs in zinc ores and it even less common than zinc.
Cadmium has never been found by itself in nature; it is always combined with other elements, usually in oxycarbonate and cadmium oxides.
Cadmium is more volatile than zinc and it is a fairly soft, white metal. It also has lower melting points and boiling points than those of zinc.
Cadmium Nickle Cadmium Battery
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Cadmium is often used for electroplating, which is done to make small parts on radios, nuts and screws. Cadmium is used in making alloys such as
aluminum solders and control rods for nuclear reactions. Like zinc, it forms complex ions with ammonia and cyanide ions. Interestingly, cadmium
hydroxide is more basic than zinc hydroxide. Cadmium burns in air to form brown cadmium oxide. Although cadmium is most often found in the +2
oxidation state, the +1 oxidation state can found by dissolving cadmium in a solution of cadmium chloride and aluminum chloride. This forms Cd22+,
which is similar to the Hg22+ in mercury (I) chloride.[1]
Cadmium is also toxic to humans. Poisoning occurs by inhaling its fumes or ingesting cadmium compounds. Symptoms of cadmium poisoning
include soreness in the chest, difficulty breathing, abdominal pain, nausea and vomiting.
Mercury (Hg) was one of the first known metals to man. In the past, many alchemists tried to turn Mercury into silver or gold. It is a silvery white
liquid and below its freezing point becomes a white malleable metal. Mercury is the only metal that remains liquid at room temperature and is
nicknamed quicksilver.
Most metals dissolve in Mercury. Mercury is not oxidized by air at room temperature, but when heated slowly it forms HgO. Mercury also has little
tendency to combine with oxygen, which means that HgO is thermally unstable. Most mercury compounds are water soluble and many compounds
are covalent. Mercury is also used in amalgams, which are mercury alloys. Amalgams are typically used for commercial industry such as silver
dental fillings. Mercury does not react with most acids. Interestingly, oxidizing acids such as concentrated sulfuric acid or nitric acid dissolve it to
give sulfate and nitrate salts. Mercury is also very toxic to humans. Inhalation of mercury vapor leads to severe poisoning and it can be absorbed through unbroken skin.
Symptoms of mercury poisoning includes salivation, ulceration, diarrhea, tremors, nausea and headaches. Interestingly enough, Mercury (II) chloride
is a violent poison, however, Mercury (I) chloride is a medicine. Mercury was used in thermometers because of its abilities of having a uniform
expansion rate and the fact that it does not wet glass. Generally the mercury (I) compounds are fairly stable in comparison with other forms of
mercury.
In the past, mercury was used in thermometers, however, it is no longer used due to health hazards.
These are some properties of group 12 metals
Zinc
Cadmium
Atomic Number
30
48
Mass
65.39
112.4
Density (g/cm3)
Melting Point (ºC)
Boiling point (ºC)
Electron Configuration
Principal Oxidation States
Mercury
80
200.59
7.14
8.64
13.59 (liquid)
419.6
907
320.9
765
-38.87
357
[Ar]3d104s2
+2
[Kr]4d105s2
+2
[Xe]4f145d106s2
+2 or +1
References
1. Petrucci, Ralph H., Harwood, William S., Herring, Geoff E. and Madura, Jeffry D. General Chemistry : Principles and Modern Applications,
Ninth Edition. Macmillan Publishing Company. Upper Saddle River, New Jersey: Pearson Prentice Hall, 2007.
2. Sneed, Cannon M. and Brasted, Robert C. Comprehensive Inorganic Chemistry, Volume Four: Zinc, Cadmium and Mercury, Howard M. Cyr
and the Editors, Scandium, Yttrium and the Lanthanide Series, Thomas D. O'Brian and the Editors. D. Van Nostrand Company, INC. Princeton,
New Jersey. 1955.
Outside Links
http://en.wikipedia.org/wiki/
http://education.jlab.org/itselemental/ele030.html
Video Mercury in a Precipitation Reaction: http://www.youtube.com/watch?v=pFovlKpPCbI
Video Displacement of Zinc and Hydrochloric Acid: http://www.youtube.com/watch?v=DTiU4...eature=related
Gorodetsky, Malka; Singerman, Ammon. "Electroforming: An improvised experiment on electroplating." J. Chem. Educ.1980, 57, 514.
Problems
Highlight for the answers.
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1. ZnO + CO →Zn + CO2
2. CdCl2 → Cd2+ + 2Cl3. HgCl4 →HgCl2 + 2Cl4. What is the electron configuration of Zn2+? Zn2+= [Ar] 3d10 (Zinc is oxidized so electrons are removed from the 4s orbital)
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