What is a Metallic Bond? - Definition & Properties
All elements tend to try and get eight electrons in their outermost shell, or valence shell. To try and get eight electrons in their valence shell, atoms of all elements try to form bonds with other compatible atoms. They form bonds either by donating electrons or by sharing electrons. In case of simple metals like sodium, it donates an electron and gets a positive charge and then forms compounds with elements like chlorine which receives an electron and gets a negative charge. This kind of bond is called ionic bond. Sometimes atoms like hydrogen and oxygen share electrons to fill their valence shell. This kind of bond is called covalent bond. Sometimes, a large number of metallic atoms come together and many of the electrons in their valence shell is detached and roams around the remaining positive ions, in a way being shared among all of them. This kind of bond is called metallic bond.
Metallic bond is a bond that holds together many metallic atoms together in any metallic substance. Usually the outermost electron of each of the metallic atoms gets detached from the atom and overlaps with a large number of neighbouring metallic atom, without being associated with any specific pair of atoms. In other words, the valence electrons of metals that form metallic bond are nonlocalized and capable of freely wandering throughout the entire crystal formed by such a bond. The atoms after losing an electron become positive ion and thus interaction between such ions and electrons give rise to a cohesive binding force that holds the crystal together. In free state, metal does not exist as a single atom. It either forms metallic bond with similar atoms or form an ionic bond with any non-metal. Not all metals form metallic bonds while existing in free state. Mercury for example, forms a metal-metal covalent bond to exist in free state, and exists as Hg2+2.
In the 1900s, Paul Drude came up with a theory that metallic substances existed by forming bonds that had a “sea of electrons” and it is an accurate and accepted image with regards to metallic bonding.
This structure is sometimes also described as “an array of positive ions in a sea of electrons”.
Bulk Properties of Metals
Metals have several properties that they are known for. Some among them is their conductivity, malleability, ductility, lustre, high melting point, and high boiling point. Metals exist in the form of a solid structure, but they can be easily deformed. The properties of metals in free state is due to the arrangement of electrons in the metallic bond. Since valence electrons are free, delocalized, mobile and not associated with any particular atom, it is possible to explain several properties of metals.
In metallic bonds electrons are free to move about within the confines of the crystals they exist in. This makes it easy for electric charge to move in. If electrons from any outside force are pushed into a metal due to an electric circuit, the electrons can move through the electron sea and come out from the other end of the metal connected to the electric circuit.
Malleability and Ductility:
If an outside force is applied to the metal, the sea of electrons acts as a cushion for this force. The structure of the metals is thus neither harmed nor changed, the protons only are rearranged according to the force applied. The sea of electrons rearranges themselves to accommodate the new arrangement of protons and thus keep the metal intact. The crystal structure does not fracture, they only get deformed due to the external force. This is the reason for the malleability and ductility of metals.
Metals can conduct heat, and expand and contract when heated or cooled, allowing them to be used in a variety of ways. This heat capacity and the thermal expansion is also because free electrons can move around the solid, facilitating the conduction of heat, and expansion when electrons move vigorously.
Free electrons in the sea of electrons can freely absorb photons and hence metals are opaque looking. Electrons on the surface of the metal can bounce back light at the same frequency at which the light hits its surface, and thus metals appear shiny.
Metallic bond in Molten Metal
In molten metal, though the metallic bond is still present, the ordered structure is broken down. But the metallic bond which exists in the solid form of a metal does not completely break until the metal boils. Metallic bond is completely broken when a metal boils, but it only slightly loosens when it melts.
Strength of metallic bonds
The strength of any metallic bond depends upon three factors:
1) The number of electrons that gets delocalized from the metal
2) The charge of the metallic ion
3) The size of the metallic ion
Metallic bonds are very strong and require a large amount of energy to break, and hence they have a high melting point and a high boiling point. A strong metallic bond implies a larger number of delocalized electrons, which causes the effective charge on the cation to increase, which makes the size of the cation smaller.
Solubility and compound formation of metals
Unless they undergo a reaction with them, metals are not soluble in water or any organic solvents. The reaction which makes metal soluble is typically an oxidation reaction that robs the metal atoms of their itinerant electrons, thus destroying the metallic bonding. However, metals are easily soluble in each other and they maintain their metallic characteristics of their bonding. For example, gold can easily be dissolved in mercury even in room temperature. Sometimes they retain their original properties, but sometimes the fusion of two metals form a metallic compound with an entirely different structure. We call these as alloys. The domain of study concerning the fusion of metals into alloys is called metallurgy.
The sea of charge carriers that is electrons have a profound effect on the optical properties of metals. We can understand about this only if we consider the electrons as a collective rather than individual electrons.
We all known that light is a combination of an electrical and magnetic field. The electric field of light is usually able excite an elastic response from the electrons that exist in the metallic crystal. Thus, the photons are unable to penetrate deep into the metal and are usually reflected, while some may be absorbed. All photons of the visible spectrum exhibit this characteristic.
This is the reason metals are often silvery white or greyish white in colour. The balance between the absorption and reflection of light determines how grey or white they are. Silver for example, is a metal with high conductivity and is also one of the whitest in colour. Notable exceptions to this are reddish copper and yellow gold. This is due to the existence of upper limit of frequency that metallic electrons can respond to.