When light is shone on to the surface of a metal, its electrons absorb small amounts of energy and become excited into one of its many empty orbitals. The electrons immediately fall back down to lower energy levels and emit light. This process is responsible for the high luster of metals. The American Platinum Eagle is the official platinum bullion coin of the United States and was first minted in The luster of a metal is due to its metallic bonds.
Recall that ionic compounds are very brittle. Application of a force results in like-charged ions in the crystal coming too close to one another, causing the crystal to shatter. When a force is applied to a metal, the free-flowing electrons can slip in between the stationary cations and prevent them from coming in contact. Imagine ball bearings that have been coated with oil sliding past one another.
As a result, metals are very malleable and ductile. They can be hammered into shapes, rolled into thin sheets, or pulled into thin wires. Use the link below to answer the following questions:. Each magnesium atom also has twelve near neighbors rather than sodium's eight. Both of these factors increase the strength of the bond still further.
Note: Transition metals tend to have particularly high melting points and boiling points. The reason is that they can involve the 3d electrons in the delocalization as well as the 4s. The more electrons you can involve, the stronger the attractions tend to be. Metals have several qualities that are unique, such as the ability to conduct electricity and heat, a low ionization energy , and a low electronegativity so they will give up electrons easily to form cations.
Their physical properties include a lustrous shiny appearance, and they are malleable and ductile. Metals have a crystal structure but can be easily deformed. In this model, the valence electrons are free, delocalized, mobile, and not associated with any particular atom. This model may account for:.
However, these observations are only qualitative, and not quantitative, so they cannot be tested. The "Sea of Electrons" theory stands today only as an oversimplified model of how metallic bonding works.
In a molten metal, the metallic bond is still present, although the ordered structure has been broken down. The metallic bond is not fully broken until the metal boils. That means that boiling point is actually a better guide to the strength of the metallic bond than melting point is. On melting, the bond is loosened, not broken. The strength of a metallic bond depends on three things:. A strong metallic bond will be the result of more delocalized electrons, which causes the effective nuclear charge on electrons on the cation to increase, in effect making the size of the cation smaller.
Metallic bonds are strong and require a great deal of energy to break, and therefore metals have high melting and boiling points. A metallic bonding theory must explain how so much bonding can occur with such few electrons since metals are located on the left side of the periodic table and do not have many electrons in their valence shells. The theory must also account for all of a metal's unique chemical and physical properties.
Previously, we argued that bonding between atoms can classified as range of possible bonding between ionic bonds fully charge transfer and covalent bonds fully shared electrons. When two atoms of slightly differing electronegativities come together to form a covalent bond, one atom attracts the electrons more than the other; this is called a polar covalent bond. In the case of semiconductors, the gap is small enough for electrons to jump to the conduction band due to thermal or some other excitation.
Energy Bands in Solids : The overlap or size of the gap between the valence and conduction bands determines the electrical conductivity of a substance. Because the band gap is so small for semiconductors, doping with small amounts of impurities can dramatically increase the conductivity of the material. When incorporated into the atomic lattice of a semiconductor, the valence electrons of n-type dopants can be easily excited to the conduction band.
When a p-type dopant is incorporated into the atomic lattice of a semiconductor, it is able to host electrons from the conduction band, allowing the easy formation of positive holes. When doping a semiconductor, such as the group IV element silicon Si , with arsenic As , a pentavalent n-type dopant from group V in the periodic table which has one more valence electron than the semiconductor , the dopant behaves as an electron donor.
When this occurs, an atom of dopant replaces an atom of silicon in the lattice, and therefore an extra valence electron is introduced into the structure. The fifth valence electron of As creates a surplus of electrons. When just a few atoms of the dopant replace silicon atoms in the lattice, an n-type semiconductor is created. The newly created semiconductor is better able to conduct current than the pure semiconductor.
Doping a Silicon Crystal with the n-Type Dopant Arsenic : Doping a pure silicon semiconductor with the group V dopant arsenic creates a surplus of conductive electrons. When a group IV semiconductor is doped with a p-type trivalent group III dopant such as boron, B , which has one less valence electron than the semiconductor, the dopant acts as an electron acceptor.
These positive holes accept electrons, rendering the semiconductor more effective at conducting current. Doping a Silicon Crystal with the p-Type Dopant Boron : Doping a pure silicon semiconductor with the group III dopant boron results in a deficit of conductive electrons and creates a positive hole.
When we place p-type and n-type semiconductors in contact with one another, a p-n junction is formed. While semiconductors doped with either n-type dopants or p-type dopants are better conductors than intrinsic semiconductors, interesting properties emerge when p- and n-type semiconductors are combined to form a p-n junction.
P-n junction diffusion and drift : Diagram of the diffusion across a p-n junction, with the resultant uncovered space charges, the electric field and the drift currents. The p-n junction forms between juxtaposed p- and n-type semiconductors. The free electrons from the n-type semiconductor combine with the holes in the p-type semiconductor near the junction.
There is a small potential difference across the junction. The area near the junction is called the depletion band because there are few positive holes and few free electrons in this region. If no electricity is being passed through the system, then no current passes through the junction between n- and p-type semiconductors.
In this scenario, the surplus of electrons from the n-type semiconductor and the deficiency in electrons from the p-type semiconductor combine to create a depletion region.
In this state, the system is said to be at equilibrium.
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