Copper, silver and gold form a subgroup (group 11); these mark the end of their respective d-block series. They are called coinage metals as they are traditionally used for making coins. These atoms have the configurations (n-1)d¹0ns1. However, their ions have incomplete d shells, and therefore these metals are considered transition metals.
Their first ionisation energies are much greater than those of the preceding alkali metals because their ten d electrons are ineffective in shielding the outer s electron from the coulombic field of the nucleus. However, their second ionisation energies are not high; therefore, they form their +II cations (Cu2+, Ag²+). They can take part in covalent bonding to give formal oxidation states as high as +III (Au³+).
Some properties of Cu, Ag and Au
| Properties | Mn | Tc | Re |
|---|---|---|---|
| Atomic Number | 29 | 47 | 79 |
| Electronic Configuration | [Ar] 3d10 4s1 | [Kr] 4d10 5s1 | [Xe] 4f14 5d1 6s1 |
| Metallic radius (nm) | 0.128 | 0.143 | 0.144 |
| M+ radius (nm) | 0.096 | 0.126 | 0.137 |
| I1 (kJ mol-1) | 745 | 731 | 889 |
| I2 (kJ mol-1) | 1958 | 2072 | 1980 |
| Melting point (K) | 1357 | 1234 | 1338 |
| Density | 8.93 | 10.5 | 19.3 |
Ag and Au have the same atomic radii and slightly differing ionic (M+) radii.
Cu²+ (+), Ag2+ (d9), Au³+(d8) form coloured aqueous species and salts and are paramagnetic.
Their melting points and boiling points are high, indicating strong metallic bonding; both their d and s electrons take part in bonding.
The strong metallic bonding inhibits their corrosion and gives them resistance to forming cations. Conversely, they are quickly deposited electrolytically from the aqueous solutions of their cations.
The metals have high densities due to strong metallic bonding and, consequently, short internuclear distances. The radius of Au is the same as that of Ag due to the effect of the lanthanide contraction.
The size of Cu+ is only 0.096 nm compared to 0.133 nm for K+; therefore, compounds of Cu have high lattice energies and are insoluble.
The metals have a ccp crystal structure. They are soft because of the regularity of the packing in the lattice. They are malleable and ductile. They are excellent conductors of heat and electricity. Their boiling points are high, > 2400 K. Silver has the highest electrical and thermal conductivity among all metals.
The three metals form alloys among themselves and also with other metals. These alloys are used in industries for the production of several articles.
Copper salts impart blue colour to the Bunsen flame.
Oxidation states of Cu, Ag, Au
All of them form +I, +II and +III oxidation states.
| Oxidation States | Cu | Ag | Au |
|---|---|---|---|
| +I |
Cu2O, CuI Cu(CN)43- CuCl2- CuBr2 |
AgCl, Ag(CN)2- Ag(NH3)2+ AgSCN |
Au(CN)2- Au2S AuX (X = Cl, Br, I) |
| +II |
CuO, CuCl2 CuF42- Cu(py)42+ |
Ag(py)42+ AgO AgF2 Ag(H2O)42+ |
CsAuCl3 |
| +III |
CuF63- KCuO2 K3CuF6 |
Ag(py)42+ AgO AgF2 Ag(H2O)42+ |
AuBr4-, Au(CN)4- [AuPh4]- AuBr63- |
These metals do not form any species in the zero or negative state.
In the simple molecules, the binding is predominantly covalent.
CN– and NH3 form water soluble complex species such as Cu(CN)43- and Ag(NH3)2+
The +II (d9) state is common only with copper. Ag2+ is not common, and it is a strong oxidising agent. Au²+ does not exist in simple compounds.
All three form M³+ (d8) complexes; such complexes are common; they are oxidising agents.
Cu has a d10 configuration and is therefore expected to be more stable than Cu²+ (d9); however, it is not so, and Cu²+ is more stable than Cu+. This is explained based on the enthalpy of hydration of Cu²+; it is so much more negative than that of the Cut that the dipositive ion is much more stable than the unipositive ion in an aqueous solution.
Uses
Copper is used as an electrical conductor, alloys in coinage, Fehling’s solution in analytical chemistry, CuSO4 as a fungicide in agriculture, vessels, frying pans, plumbing and heating components, and coins.
Silver is used as Sterling silver (Ag + Cu) for tableware, silvering mirrors, storage batteries (Ag-Zn, Ag-Cd), photography (AgBr), coins, artificial rain (AgI), and dental alloys.
Gold is used in jewellery, electronics, coins (alloys), coating for satellite panels, photography (HAuCl4), nanotechnology.
