Gold is a precious metal which has been in human use since 2000 BC. It is unreactive and is highly resistant to corrosion which makes it favorable for wide range of medicinal, ornamental and structural use.
History and Discovery
Gold is the metal of prehistoric times and it was collected from water streams in form of particles. The Egyptians in around 2000 BC started mining of gold. And gold made items, such as masks, graves etc. have been found to contain gold from civilization of 3800 to 2000 BC. Pure gold has been found from around 6000 years ago Israel kingdom [1]. Gold coins were made in the reign of King Croesus of Lydia (modern Turkey) (561-547 BC). Gold has been considered as perfect and precious metal since ancient times, and long relentless but in vain efforts have been made over centuries to transform various metals into gold, a concept known as alchemy. The word gold has been originated from the word “geolo” used by Anglo-Saxon civilization that inhabited England in the 5th century. Geolo (Sanskrit origin meaning to shine) means yellow. The symbol of gold comes from a word of Latin origin, aurum, which is originated from Aurora, which is the goddess of morning glow [2].
Gold
| Periodic Table Classification | Group 11 Period 6 |
|---|---|
| State at 20C | Solid |
| Color | Metallic yellow |
| Electron Configuration | [Xe] 4f14 5d10 6s1 |
| Electron Number | 79 |
| Proton Number | 79 |
| Electron Shell | 2, 8, 18, 32, 18, 1 |
| Density | 19.32 g.cm-3 at 20°C |
| Atomic number | 79 |
| Atomic Mass | 196.97 g.mol -1 |
| Electronegativity according to Pauling | 2.54 |
Occurrence
Gold is widely present in the Earth’s crust (0.3 ppm by weight). It is also present in river beds as rock bound gold is released by erosion of rock by running water. Gold also exists in the form of alloy, such as amalgam (with mercury) or with silver. Gold is found in association of pyrites deposits and from quarts and gravels. Naturally, most of the gold present in the Earth’s crust is in combination with silver. The term “electrum” is used for gold ore that have silver content of more than 20%. The biggest producer of gold is China from around two third of the gold (around 455 tons) in world is produced. The other countries where gold is being mined include USA, Canada and Russia. The annual production of gold in the world is 2500 tons per year. Oceans, including the Northeast Pacific and Atlantic contain about 10–30 parts per quadrillion, that makes about 10–30 g/km3 of gold in form of flakes or nuggets [3].
Physical Characteristics
Gold is a reddish yellow metal. It is highly ductile and malleable. Gold is a soft metal and is usually used in alloyed form to provide strength and durability. About 1 ounce of gold can be hammered or pressed into 300 square feet sheet. The unit carat is used for the percent purity of gold, especially in jewellery. Twenty-four carats gold is considered as pure gold. Gold has the ability to conduct heat and electricity with great efficiency. And is considered as a fairly dense metal with density of 19.32 g/cm3 [4].
Chemical Characteristics
Gold is an inert metal and considered as the most noble metal. However, these are many diverse forms of gold, with oxidation number ranging from -1 to +5. The dominant form of gold is Au(I) and Au (III). Gold is resistant to attack by oxygen at any temperature. Gold is unreactive and resistant to most acids. It can dissolve in a mixture of hydrochloric acid and nitric acid, known as aqua regia. Gold is also soluble in alkaline solutions of cyanide. Gold forms amalgam with mercury but it does not involve a chemical reaction. Gold is highly resistant to corrosion. Gold can react with certain halogens, such as fluorine to form gold (III) fluoride. Gold in powdered form can react with chlorine to form gold chloride. Various alloys of gold are formed to alter the strength and hardness of gold and create exotic colors [5].
Uses and Significance
- Gold is a precious metal and widely used in making of jewellery, coinage, crowns and decorative items.
- It is widely used in making components of computerized devices, such as corrosion resistant electrical conductors.
- Gold is used in the glass industry for making colored-glass.
- It is used as fillers in tooth restoration.
- Certain salts of gold are used for medicinal purpose.
- Flakes of gold are used in various drinks and sweets.
- Gold plating is used in helmets used by astronauts as gold due to its inert nature can provide protection against dangerous and harmful effects of solar radiations.
- Radioactive isotope of gold (Au-198) has been used to treat various cancers including prostate and bladder.
Health Hazard
Gold is a non-toxic metal. It may cause irritation in eye or skin and prolonged exposure can lead to irritation in lungs. There is no ecotoxicity of gold as it is insoluble in water. Gold is used for therapeutic purpose in a treatment called Chrysoteraphy, for relieving pain in rheumatoid arthritis. Recently, gold particles based anti-cancer drugs have been investigated.
Isotopes of Gold
There are 35 isotopes of gold, with mass numbers ranging from 171 to 205. These are the artificially produced isotopes of gold. The natural gold consists of one stable isotope, Au-197, which is the only stable isotope. The isotopes with atomic masses above 197 decay by emission of β rays [6].
REFERENCES
[1]. Gopher, A.; Tsuk, T.; Shalev, S. & Gophna, R. (August–October 1990). “Earliest Gold Artifacts in the Levant”. Current Anthropology. 31 (4): 436–443. doi:10.1086/203868. JSTOR 2743275.
[2]. Eric. J. Holmyard, Makers of Chemistry., 1931, Oxford at the Clarendon Press. p163
[3]. Kenison Falkner, K.; Edmond, J. (1990). “Gold in seawater”. Earth and Planetary Science Letters. 98 (2): 208–221. Bibcode:1990E&PSL..98..208K. doi:10.1016/0012-821X(90)90060-B.
[4]. https://www.webelements.com/gold/physics.html
[5]. https://sciencestruck.com/chemical-properties-of-gold
[6]. “Nudat 2”. National Nuclear Data Center. Retrieved 12 April 2012