Introduction

Of the 118 elements in the periodic table, 95 are metals. These metals are located to the left of the periodic table. Each group of the periodic table has a different name. Among these groups in the periodic table, the most notable group is the transition group. Some of the metals in this transition group are noble metals. Initially, all these elements in the periodic table were arranged according to their atomic mass.  But later with the discovery of isotopes, it was found that the same element could have multiple atoms with the same chemical properties and different atomic mass. Thus the fundamental properties of the element are not controlled by atomic mass.

Then in 1912, the scientist Moseley’s experiment showed that the atomic number controls the qualitative properties of the element. The periodic table is a list of elements sorted according to these atomic numbers. Based on their physical and chemical properties, the elements in their tables are divided into metals, nonmetals, and metalloids (1) & (2).

What are noble metals?

There are some metals in the transition metal section in the periodic table that are resistant to oxidation and corrosion in moist air such metals are known as noble metals. Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Osmium (Os), Iridium (Ir), Platinum (Pt), and Gold (Au) are noble metals.

Some of the other elements in the periodic table are also called noble meals, these are mercury, copper, and rhenium. Noble metals are not easily attacked by acids. Some metals in noble metals are not exposed to acid at normal temperatures but dissolve in the aqua regia (a mixture of nitric and hydrochloric acid). Examples are platinum and gold that dissolved in aqua regia (1) & (2).

Description of noble metals

S.no	Name	Symbol	Atomic no.	Atomic mass	Discovery year	Discoverer	No. of Electron per shell
1.	Ruthenium	Ru	44	101.07	1844	Karl Ernst Claus	2, 8, 18, 15,1
2.	Rhodium	Rh	45	102. 90	1803	William Hyde  Wollaston	2, 8, 18, 16, 1
3.	Palladium	Pd	46	106.42	1802	William Hyde  Wollaston	2, 8, 18, 18, 0
4.	Silver	Ag	47	107.87	3000/ 4000 BC	–	2, 8, 18, 18, 1
5.	Osmium	Os	76	190.23	1803	Smithson Tennant	2, 8, 18 32, 14, 2
6.	Iridium	Ir	77	192.22	1803	Smithson Tennant	2, 8, 18, 32, 15, 2
7.	Platinum	Pt	78	195.09	1735	Antonio de Ulloa	2, 8, 18, 32, 17, 1
8.	Gold	Au	79	196.99	1848	James W.  Marshall	2, 8 , 18, 32, 18, 1

There is no record of who discovered silver. About 4000 BC silver was found in Greece and later in turkey. About 3000 BC silver artifacts were found in the Sumerian city of Kish. Gold is also found in ancient periods (1).

Location in the periodic table of noble metals

There are currently 118 elements in the periodic table. Of these 118 elements, 95 are metals, 17 are nonmetals and 6 are metalloids. Metals, nonmetals, and metalloids are located in different periods and groups of the periodic table. Each group in the periodic table is divided into different names. One of these groups is the transition group. The elements in this group are not filled with electrons in the shell before the outermost shell of a valence layer. These elements are called transition metals. Transition metals are located in the middle of the periodic table. Transition metals include some metals that are not corroded by water, air, and acid at normal temperatures. These are noble metals. According to the IUPAC modern long-form periodic table, the noble metals are located in the 5^th and 6^th periods and 8,9 10,11 groups (1).

Why are noble metals called noble?

Of all the noble metals in the periodic table, gold, platinum, and silver are mainly called noble metals. Because these are unreactive, stable, and do not react with any metals. These metals have resistance to oxidation and corrosion. Also, gold and platinum jewelry is always in high demand. Moreover, these metals do not easily form any hydride, oxide, phosphate, and nitride. So they are called noble (1).

Properties of noble metals

1. Physical state

All of the noble metals are solid at room temperature. Example Silver, Gold, etc.

2. Luster

Metals have the quality of reflecting light from their surface. The property of a metal having a shining surface is called “metallic luster”.  All noble metals have a shiny surface.

3. Weight

Noble metals are heavier in weight.

4. Resistance

These metals have resistance to oxidation and corrosion. Noble metals are not easily corroded by air, water, and acids.

5. Malleability and Ductility

All noble metals are malleable and ductile.

6. Color

Noble metals are colored. For example, ruthenium is silvery-white in color, osmium is bluish-white in color, etc.

7. Melting and boiling

These metals have high melting and boiling points. For example, the ruthenium melting point is 2334°C and the boiling point is 4150°C. Palladium melting point is 1554.9°C and the boiling point is 963°C.

8. Conduction

All noble metals are good conductors. These metals have good conduction of heat and electricity (3) & (4).

Uses of noble metals

Noble metals have been used in various fields since ancient times. For example, in ancient periods the Egyptians and Greeks used silver pots to keep the water pure. There was also the use of silver and gold coins in the past. However, the use of noble metals is still widespread. The use of noble metals is described below (3) & (4).

1. Ruthenium

• It is one of the rarest metals in the world. This metal is used in the metallic electron industry and in the chemical industry.
• In the chemical industry, it is used in electrochemical cells to produce chlorine in anodes.
• This metal is combined with titanium in small amounts to help prevent corrosion.
• Ruthenium is used to paint glass.
• It is sometimes used to produce ammonia from natural gas and acetic acid from methanol.
• Used to make jewelry, solar cells, etc.

2. Rhodium

• This metal is used as a catalyst converter for cars.
• It is used to make jewelry.
• Rhodium is used to clean some amount of exhaust gases.
• Used in electroplating and optical instruments.
• The metal is used as a catalyst in industry, in laboratories, and in thermocouple components, etc.

3. Palladium

• The largest use of palladium is in catalytic converters for automobiles.
• Palladium is used as a catalyst for hydrogenation/ dehydrogenation reactions and petroleum cracking.
• It is used to purify hydrogen.
• This metal is used in making watches, surgical instruments, and electrical contacts.

4. Silver

• It has long been used in the manufacture of coins, jewelry, and ornaments for its corrosion and oxidation resistance.
• Silver is used in medicine.
• In ancient periods silver pots to keep the water pure.
• Silver is widely used in both industrial and electronic applications.
• It is used in batteries, mobile cellular phones, automobile switches, and in making pharmaceutical creams.

5. Osmium

• Platinum and osmium alloys are used in surgical replacements such as pacemakers and replacement heart valves.
• It is used with other metals of the platinum group to make very hard alloys.
• Used in fingerprint identification.

6. Iridium

• Radioactive isotopes of iridium are used in radiation therapy to treat cancer.
• Also, this metal is used as a catalyst. Such a type of iridium catalyst absorbs sunlight and is able to convert it into chemical energy.
• Spark plugs used in helicopters are made of platinum-iridium alloy.
• It is used to make electronic communications.

7. Platinum

• Platinum is widely used as a catalyst for chemical reactions.
• It is used in making jewelry and decoration.
• Used in fine resistance wires and laboratory instruments.
• A mixture of platinum and cobalt is used to produce strong permanent magnets.
• This metal is used to make sealed electrodes of glass.

8. Gold

• This metal has been used since ancient times. Gold was used to make jewelry about 6,000 years ago.
• Gold was used for coins and other financial assets in ancient periods.
• Nowadays gold has long been used in the manufacture of jewelry and ornaments for its corrosion and oxidation resistance.
• Gold has played an important role in the dental industry for nearly 3,000 years. It is also used in medicines.

Noble gases form what type of bond with metals?

Noble gases are located in group zero of Mendeleev’s periodic table and in group 18 of the IUPAC modern long-form periodic table. These gases do not participate in any chemical reactions under normal conditions. Noble gases do not react with any metals to form compounds at normal temperatures. Because they are chemically inert as their outermost orbit or valence orbit is completely filled with electrons.

That is, elements do not usually react with any other elements to form compounds. So their valence is zero. And they are chemically inert. So they do not form chemical bonds.  But there are exceptions. Among these noble gases contain some gases that react with certain elements at extreme temperatures. Xenon and krypton, two of these noble gases react with fluorine and oxygen to form fluoride and oxide compounds. Covalent bonds are formed in these reactions. Noble gas has extreme ionization energies. So these gases do not form ionic bonds. (5) & (6)

• Xenon reacts with fluorine and produces xenon difluoride. When xenon reacts with fluorine they form a covalent bond. XeF₂ is a covalent molecule.

Xe + F₂ → XeF₂ (xenon difluoride)

• Krypton reacts with fluorine and produces krypton difluoride. The molecule krypton difluoride has 2 covalent bonds between Krypton and fluorine.

Kr + F₂ → KrF₂ (krypton difluoride)