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Why are metallic bonds the strongest?

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Abstract

The positive ions of atoms in a metal are immersed in a sea of electrons. A metal is strong because when its structure is deformed by applied stress and the positive ions move, the delocalized electrons move as well, thus maintains their attraction to the positive ions. Many metals are strong and can be deformed without breaking; many are malleable (can be hammered) and ductile (can be drawn out under tension). They are shiny when freshly cut and good conductor of heat and electricity. Any theory of metallic bond must account for all these physical properties.

Metallic bond theory

In a metal, the outer shell electrons (the valence electrons) are relatively easily removed with the formation of metal cations. When two metal atoms approach closely, as in a metal structure, their outer shell orbital overlap to form molecular orbitals. If a third atom approaches, its atomic orbital can overlap with those of the first two atoms to form another molecular orbital. For a large number of atoms, a large number of molecular orbitals are formed extending over three dimensions.

Delocalized electrons

When large number of molecular orbitals forms with the multiple overlapping of atomic orbitals, the outer electrons from each atom come under the influence of very large number of atoms. They are free to move through the structure and are no longer located in the outer shell of any one atom – they are delocalized. The removal of electrons leaves behind metal cations. The reason why cations are not pushed apart by the repulsion between them is that in a pair of cations each cation is attracted to a delocalized electron cloud between them.

Explanation for unique physical properties of metals

Metals bend but do not fracture easily

When a stress is applied to a metal, the structure of metal can change in shape without fracturing. When planes of metal are 'bent' the delocalized electrons can run in between the ions and maintain a strong bonding situation with ions without letting the structure to fracture. The packing of metal ions surrounded by sea of electrons become a cushion for the metal to absorb stress. The strong bonding is retained even when the metal is stressed (at least up to a point!). The layers of atoms can slide over each other without fracturing the structure. The strong bonding in metals gives them a high tensile strength. To summarize, the positive ions of atoms in a metal are immersed in a sea of electrons. A metal is strong because when its structure is deformed by applied stress and the positive ions move, the delocalized electrons move as well, thus maintains their attraction to the positive ions.

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High thermal conductivity of metals

Metallic bond theory explains why metals have high thermal conductivity. When heat is applied to one end of a piece of metal, the kinetic energy of electrons is increased. The increase is transmitted through the system of delocalized electrons to other parts of the metal.

High electrical conductivity of metals

When a potential difference is applied between the ends of a metal, the delocalized electron cloud will flow towards the positive potential.

Shiny appearance of metals

As explained, metal contains large number of molecular orbitals at large number of different energy levels. When light falls on to the metal, electrons are excited. At this stage large number of transitions of electrons happens between different energy levels with a whole range of frequencies being absorbed. As electrons return to lower energy level, light is emitted and makes the metal shine.

High melting point and high boiling point

Metals have high melting point and high boiling point because the attraction between delocalized electrons and metal ions is very strong. The greater the numbers of delocalized electrons the stronger this attraction. So magnesium with atomic number 12 has a higher melting point than sodium, atomic number 11.

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