gold

gold

Gold is the elemental form of the chemical element gold, which is a soft, golden yellow, corrosion-resistant precious metal. Gold is one of the rarer, more precious and highly valued metals. In the world, gold is usually measured in ounces. In ancient China, it was measured in ounces, which is a very important metal. It is not only a special currency for reserve and investment, but also an important material for jewelry industry, electronic industry, modern communication, aerospace industry and other sectors.

The chemical symbol of gold is Au, and the financial English code is XAU or gold. The name of Au comes from a story of Aurora, the goddess of dawn in Roman mythology, which means the shining dawn.

On August 5, 2020, the price of gold exceeded US $2033 / oz, setting a new record.

Physical and chemical properties

physical property

1. Color:

When gold is melted, the steam is yellow green; during smelting, the gold powder is usually brown; if it is cast into a thin piece, it can transmit green light.

2. Ductility:

Abnormally strong. An ounce of gold can be drawn 50 Li long. Its malleability makes it easy to cast. It is a good choice for making jewelry. Gold is the most powerful of all metals.

3. Malleability:

come first on the list. It can make extremely thin gold flakes that are easy to roll up. An ounce of gold can be hammered down to 4 million one foot thick and 100 square feet in area. Ancient people hammered it into thin slices to make temples and palaces for decoration. All these can explain the flexibility and malleability of gold.

The color of gold is golden yellow, metallic luster, difficult to decompose. Its density is 19.3g/cm3, melting point is 1064.4 ℃; it has good ductility, can be pressed into thin foil, high heat transfer and conductivity, and the resistance of pure gold is 2.4p. Pure gold has good chemical corrosion resistance and is the best electroplating material.

As a precious metal, gold has good physical properties. "Real gold is not afraid of fire" means that gold has high chemical stability, is not easy to react with other substances, and does not have to worry about oxidation discoloration. Even in the molten state, it will not oxidize and discolor, and it will still glitter after cooling. High density, heavy handle. Good toughness and ductility, good conductivity. Pure gold has bright yellow, but the color changes greatly when mixed with other metals, such as gold copper alloy is dark red, silver alloy is light yellow or gray white. Gold is easy to be ground into powder, which is the reason why gold is dispersed in nature. Pure gold jewelry is also easy to be worn to reduce its weight.

In Mendeleev's periodic table, the atomic number of gold is 79, that is, the nucleus of gold contains 79 protons, which are positively charged. At the same time, due to the semi full rule, the fittings have good chemical stability. In the metal market, gold, ruthenium, rhodium, palladium, osmium, iridium, platinum and other metals are collectively referred to as precious metals.

Although gold is a very soft metal, it is inferior to lead and tin. Using nails on pure gold can mark marks. This softness makes gold very easy to process. However, this is not ideal for ornament manufacturers, because it is easy to scratch the ornament, make it tarnish and even affect its beauty. Therefore, when making jewelry with gold, copper and silver are generally added to improve its hardness.

Gold is easy to forge and extend. It can be rolled into transparent and green gold foil with a thickness of 0.001 mm. 0.5g of gold can be drawn into 160m long gold wire.

chemical property

Gold (transition metal), the gold code is XAU_ USD, XAU are chemical element codes;

Crystal structure: face centered cubic structure

Atomic weight: 196.96654

Shell structure: 2,8,18,32,18,1

Electronic layout: 4f145d106s1

Combined price + 1, + 3

Melting point: 1064.43 ℃

Boiling point: 2808 ℃

Electronegativity: 2.54

Covalent radius: 1.34a

Ion radius: 0.85 (+ 3) a

Atomic radius: 1.79a

Atomic volume: 10.2cc/mol

The first ionization potential is 9.2257v

The second ionization potential is 20.521v

Oxidation state: (3), 1

The density at 293k is 19.32g/cm3

Specific heat capacity: 0.128j/gk

Heat of vaporization: 334.4kj/mol

Melting heat: 12.55kj/mol

Conductivity: 0.452x106cm · Ω

Thermal conductivity: 3.17w/cmk

Elastic coefficient: 78.3x103mpa

Coefficient of thermal expansion: 14.2x10-6/k

Lattice parameter: 4.0786a

Mohr: 2.5

Gold has stable chemical properties and strong corrosion resistance. It does not react with oxygen in air even at high temperature (except for pure oxygen under specific conditions). Gold will not react with oxygen and sulfur at high temperature. Its chemical properties are very stable and stronger than silver (silver will oxidize to silver oxide at 1500 ℃, and gold will not react with oxygen even above 3000 ℃) Kuang Jin will be oxidized.

1. It is located at the bottom of the river and has been rubbed by the sand at the bottom for many years.

2. When the river freezes in winter, the water near the river bottom is liquid, and the oxygen pressure in the water is several times higher than that without ice.

3. The river freezes and melts year after year.

4. The oxygen at the bottom of the river is relatively pure (4000 ℃, high pressure, catalyst), insoluble in single strong acids such as hydrochloric acid, nitric acid and sulfuric acid, and only soluble in the mixed acid of hydrochloric acid and nitric acid (namely aqua regia) to form chloroauric acid H

.

In the presence of oxygen at room temperature, gold can be dissolved in the solution containing potassium cyanide or sodium cyanide to form a stable complex m; gold can also be dissolved in the solution containing thiourea; it can also be dissolved in the acidic solution containing chlorine. Gold does not react with alkali solution, but forms naauo < sub > 2 < / sub > with sodium peroxide in molten state. The valence of gold is - 1, - 2, + 1, + 2, + 3, + 5, + 7, etc., the oxide is gold trioxide Au < sub > 2 < / sub > o < sub > 3 < / sub >, the chloride is gold trichloride AuCl < sub > 3 < / sub >.

In acidic medium, chloroauric acid h or complex M can be reduced to elemental gold powder by metal zinc (zinc powder or zinc wire), sodium sulfite, hydrazine hydrate, etc. sulfide of alkali metal will corrode gold and form soluble gold sulfide. Humic acid in soil and metabolites of some bacteria can also dissolve trace gold.

Gold has high ionization potential, so it is difficult to lose outer electrons to form positive ions, and it is not easy to accept electrons to form anions. Its chemical properties are stable, and its affinity with other elements is weak. Therefore, most of them exist in the state of elemental gold in nature.

The average content of gold in the crust is about 1.1 parts per 100 million (0.0011ppm), and that in the seawater is about 1 parts per 100 billion (0.00001ppm). Due to the crustal movement and geological changes from hundreds of millions to billions of years, gold is enriched into gold deposits. The grade of gold in the gold deposits of general industrial value is 2-3 g / T, the rich ore is 5-50 g / T, the super rich ore is 50-500 g / T, and lump gold, single block gold The smallest one is more than ten grams, the largest one is dozens of kilograms, and the rare lump gold is hundreds of kilograms. Because some of them look like dog's head, they are commonly known as dog's head gold. Indian scientists have discovered two pieces of dog's head gold of nearly 2.5 tons. If the lean ore is 0.1-1 g / ton, and the dressing and smelting technology level is above 0.5 g / ton, it has industrial mining value.

Pure gold is rare in nature, and often contains associated elements such as silver, copper, iron, palladium, bismuth, platinum, nickel, tellurium, selenium, osmium, etc. in natural gold, those with more than 15% silver are called silver gold deposits, those with more than 20% copper are called copper gold deposits, those with 5% - 11% palladium are called palladium gold deposits, and those with more than 4% bismuth are called bismuth gold deposits.

The fittings are sulphophilic and often coexist with sulfides such as pyrite, arsenopyrite, galena and stibnite; they are easy to form intermetallic compounds with sulphophilic elements such as silver and copper.

Here is the chemical equation for gold

Gold can be oxidized by relatively pure chlorine and fluorine (if heated)

2Au+3Cl₂=2AuCl₃

2Au+3F₂=2AuF₃

Gold will not be eroded and discolored by sulfur and oxygen at high temperature, but it will be oxidized under specific conditions, or it will react very slowly with oxygen at high temperature of 4000 ℃ under pressure and with catalyst: it can be considered that it does not react, because in general, few flames can exceed 3500 ℃, and gold melts above 1000 ℃, so it is generally believed that real gold is not afraid of fire, That is to say, gold does not react with oxygen at high temperature. The reaction temperature of gold with sulfur, bromine, iodine and nitrogen is higher than that with chlorine, fluorine and aqua regia.

4Au + 3O < sub > 2 < / sub > = high temperature and high pressure, catalyst = 2au < sub > 2 < / sub > o < sub > 3 < / sub >

rust

Although gold itself is not subject to corrosion, its chemical affinity with sulfide is very strong, and it is easy to absorb hydrogen sulfide like acid gas in the air. It is possible for the surface to appear dark colors, such as orange, brownish red and so on. The production of gold products needs to go through many complicated processes.

Take the gold bar and gold medal as an example, they all have to go through the processes of smelting, pressing, cutting, holding, desalting, acid frying, polishing, pressing, edge holding and so on

Chinese PinYin : Huang Jin