Metallic Bond

Metallic Bonding There are more than 80 elements in periodic table, which are metals. Generally all the metals are solid at room temperature and pressure except mercury and gallium. The force, which binds together the atoms of metals, is called Metallic Bond. Ionic and covalent bonds can’t explain the properties of metals like high thermal and electrical conductivity, lustrous nature, malleability and ductility. There are evidences, which show the absence of ionic, and covalent bond between the metal atoms. 1) Evidence showing the absence of ionic bond – It is known the transfer of electrons between 2 atoms (+ive ion to –ive ion) forms the ionic bond. Such type of bond is not possible between the similar atoms. Metals conduct electricity in the solid state whereas ionic compounds can’t conduct. Metals are malleable while ionic compounds are brittle. So ionic bond is not present between the metal atoms. 2) Evidence showing the absence of covalent bond – As we know covalent bond is formed by mutual sharing of electrons between two atoms so that atoms acquire noble/inert gas configuration. But metal atoms have small number of valence electrons and are not able to complete their octet by mutual sharing of electrons. Covalent compounds are generally liquid s and gases while metals are solids. Covalent compounds are bad conductors of heat and electricity while metals are good conductors. It, therefore, shows the absence of covalent bond between the metal atoms From the above evidences we can say metals are neither held by ionic bonds nor by covalent bonds. There must be different type of bond present between the metal atoms.

Electron Sea Model for Metallic Bonding Lorentz proposed this model and it is based on the properties of metals like low ionization enthalpies and large number of empty orbitals as explained below: 1) Low ionization enthalpies - The valence electrons of the metal atoms are weakly held by the nucleus because of the low ionization enthalpies of metals. As a result, the valence electrons can move freely out of the

influence of their kernels (remaining part of an atom after loosing valence electrons) e.g. Na+, Mg2+, Al3+ are kernels.

2) Large number of empty orbitals – Metals generally have less number of valence electrons as compared to number of valence orbitals. So there are large numbers of vacant orbitals in the valence shell. Example

Features of electron sea model proposed by Lorentz: 1) The positively charged kernels of metal atoms are arranged in a regular pattern in a metallic lattice.

2) Each kernel is surrounded by loosely held by valence electrons.

3) The valence electrons in the metals are termed as mobile electrons because there is a complete freedom to the electrons in the metallic lattice as these are loosely held to the kernels.

A metal is thus an ordered arrangement of kernels surrounded by mobile valence electrons. The name electron sea model is derived on account of the presence of positively charged kernels immersed in a sea of valence electrons.

Consider Lithium metal. Its electronic configuration is 1s22s1. It has 1 valence electron and after loosing it the kernel of lithium is Li+. The arrangement of kernels of lithium atom, (+) and valence electrons (•) in its metallic lattice is as shown above. So we can define a metallic bond according to Lorentz – The simultaneous force of attraction between the mobile electrons and +ive kernels, which binds the metal atoms together, is called metallic bond.

Differences between Metallic and Covalent bond

Physical Properties of Metal and their Explanation 1) Metallic Lustre – Metals are lustrous due to the presence of delocalized mobile electron. When metal surface is exposed to light then loosely held electrons absorb photons of light and start oscillating with the same frequency as that of incident light. These oscillating electrons emit radiations in the form of light. As a result the incident light appears to be reflected from the metal surface and it acquires a shining appearance i.e. gains metallic lustre.

2) Electrical Conductivity – Metals are good conductors of electricity due to the presence of mobile electrons. When a potential difference is applied across the metal sheet then free mobile electrons start moving towards positive electrode. The electrons coming from the negative electrode simultaneously replace these electrons. Thus metallic sheet maintains the flow of electrons from negative electrode to the positive electrode and behaves as a good conductor.

3) Thermal Conductivity – It is also due to the presence of mobile electrons. On heating a metallic sheet, these electrons gain kinetic energy and move rapidly to the cooler parts. Hence they transfer their kinetic energy to the other molecules by means of collision. 4) Malleability and Ductility – Malleability means beaten into sheet and ductility means drawn into wires. These properties are due to the nondirectional nature of the metallic bond. When any force is applied on the metal the position of kernels is changed without destroying the crystal. The metallic lattice gets deformed due to the slippage of the adjacent layers of the kernels from one part to another. It doesn’t change the environment of the kernels. It simply moves the kernel from one lattice to another.

5) High Tensile Strength – This property of metals is due to the presence of strong electrostatic attraction between the positively charged kernels and mobile electrons surrounding them. High tensile strength means metals can resist stretching without breaking. 6) Hardness of Metals – Hardness of metals is due to the strength of the metallic bond which defends upon a) Number of valence electrons; greater the number of valence electrons for delocalization, stronger the metallic bond. b) Size of the kernel; smaller the size, greater the attraction for delocalized electrons and stronger is the metallic bond e.g. alkali metals have only 1 valence electrons and large kernels. Thus they form weak metallic bond and are soft. -----------------------------------------------------------------------------