a) What is the ground state configuration of Sc^(+)\mathrm{Sc}^{+}ion? Justify.
Answer:
The ground state electron configuration of a neutral scandium (Sc\mathrm{Sc}) atom is [Ar]3d^(1)4s^(2)[\mathrm{Ar}]\,3d^1\,4s^2, where [Ar][\mathrm{Ar}] represents the electron configuration of the noble gas argon (1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)1s^2\,2s^2\,2p^6\,3s^2\,3p^6). Scandium is the first element in the transition metals series, with an atomic number of 21, indicating it has 21 electrons as a neutral atom.
When scandium forms a +1+1 ion (Sc^(+)\mathrm{Sc}^+), it loses one electron. The removal of an electron from a transition metal typically starts with the outermost ss electrons before the dd electrons. This is because the ss electrons are generally at a higher energy level than the dd electrons of the same period in the transition metals, making them easier to remove.
Therefore, the electron configuration of the Sc^(+)\mathrm{Sc}^+ ion, after losing one electron, would be:
[Ar]3d^(1)4s^(1)[\mathrm{Ar}]\,3d^1\,4s^1
This configuration shows that the Sc^(+)\mathrm{Sc}^+ ion has one electron in the 3d3d orbital and one electron in the 4s4s orbital after the loss of one electron from the 4s4s orbital of the neutral scandium atom. This configuration is justified based on the general principle of electron removal from the outermost shell first, which, for transition metals, means the 4s4s electrons are removed before the 3d3d electrons.
b) Why zine and cadmium are soft metals?
Answer:
Zinc and cadmium are considered relatively soft metals compared to other transition metals. This softness can be attributed to several factors related to their electronic structure and bonding:
1. Electronic Configuration
Zinc (ZnZn) has the electronic configuration [Ar]3d^(10)4s^(2)[Ar] 3d^{10} 4s^2, and cadmium (CdCd) has the electronic configuration [Kr]4d^(10)5s^(2)[Kr] 4d^{10} 5s^2. In both cases, the dd orbitals are completely filled. The filled dd orbitals contribute to the stability of the electron configuration but do not significantly contribute to metallic bonding compared to elements with partially filled dd orbitals. The effectiveness of metallic bonding is a key factor in determining a metal’s hardness; stronger metallic bonds usually result in harder metals.
2. Metallic Bonding
The strength of metallic bonds in zinc and cadmium is relatively weaker than in other transition metals with partially filled dd orbitals. This is because the delocalized electrons in zinc and cadmium are primarily from the ss orbital, as the dd orbitals are fully occupied and do not contribute much to the bonding. The weaker metallic bonds make these metals softer and less rigid.
3. Crystal Structure
Both zinc and cadmium crystallize in the hexagonal close-packed (hcp) structure. While this structure is quite efficient in packing atoms, the particular arrangement in zinc and cadmium, combined with their electronic configurations, does not favor the formation of very strong metallic bonds. The directional nature of some of the metallic bonds in the hcp structure can also contribute to anisotropy in mechanical properties, which might influence the perceived softness.
4. Cohesive Energy
The cohesive energy, which is a measure of the strength of the bonds holding a metal together, is relatively lower for zinc and cadmium compared to other transition metals. Lower cohesive energy correlates with lower melting points and softer materials.
Conclusion
The relative softness of zinc and cadmium compared to other metals is primarily due to their filled dd orbital configurations, which lead to weaker metallic bonding and lower cohesive energy. Their crystal structures also do not favor the formation of very strong metallic bonds, contributing to their softness. These properties make zinc and cadmium suitable for applications where malleability and ductility are advantageous, but they are less suited for applications requiring high strength and hardness.
What is the composition of brass? Is it harder than pure copper? Give its uses.
Answer:
Brass is an alloy primarily composed of copper (Cu) and zinc (Zn). The proportions of copper and zinc can vary depending on the specific type of brass and its intended use, with copper content typically ranging from about 55% to 90%. The addition of zinc to copper results in an alloy with enhanced properties compared to pure copper, such as increased strength, hardness, and corrosion resistance.
Composition
Standard Brass: The most common form of brass, known as "cartridge brass," contains about 70% copper and 30% zinc. This composition is well-balanced, offering good strength and ductility, making it suitable for a wide range of applications.
Special Brass Alloys: Other elements may be added to brass in small amounts to achieve specific properties. For example, lead (Pb) is often added (up to 3%) to improve machinability, while aluminum (Al), tin (Sn), and nickel (Ni) can be added to enhance corrosion resistance, strength, and color.
Hardness Compared to Pure Copper
Brass is generally harder than pure copper. The addition of zinc to copper results in a material that is stronger and more resistant to wear and deformation under stress. The exact hardness of brass can vary depending on its zinc content and any additional elements that have been added. The hardening effect is due to the solid solution strengthening and, in some cases, the formation of a more complex microstructure than that of pure copper.
Uses of Brass
Brass is used in a wide variety of applications due to its excellent machinability, corrosion resistance, and attractive appearance. Some common uses include:
Musical Instruments: Brass is used to make horns, trumpets, trombones, and other brass instruments due to its acoustic properties.
Decorative Items: Its gold-like appearance makes brass a popular choice for decorative items such as door handles, plaques, and jewelry.
Plumbing and Water Fittings: Brass’s corrosion resistance makes it suitable for water pipes, faucets, and fittings.
Ammunition Casings: The alloy known as cartridge brass (70% copper, 30% zinc) is commonly used for ammunition casings due to its workability and durability.
Gears and Bearings: Brass is used in gears, bearings, and valves where low friction is required.
Marine Hardware: The addition of elements like tin can make brass resistant to saltwater corrosion, making it suitable for marine hardware.
The versatility of brass, stemming from its various compositions, allows it to meet the requirements of many industrial, decorative, and musical applications, making it a valuable material in numerous fields.