B.sc 1st year Book
General characteristics of metals :
1- Metals are good conductors of heat and electricity and show very high values of melting and boiling points.
2- Metals are shiny, bright, and have reflective surfaces,
3- They are malleable and ductile.
4- They have a crystal structure either cubic close pack (ccp), hexagonal close pack (hcp), or body-centered cubic (bcc).
5- They form alloys with different metals and non-metals.
6- They crystallize with a high coordination number of 12 to 14.
7- They have generally low ionization energy (such properties of metals can not be explained on the basis of normal covalent or ionic bonding).
Since all the metal atoms in a crystal are identical in nature. They have a limited number of the valence electrons in their outermost shell hence they do not form normal covalent bonds or ionic bonds with each other. However, the bonding in metal atoms takes place by a new type of cohesive force known as a ‘metallic bond’. It is the bonding that occurs between a number of identical atoms of pure metal or of different metal atoms like in alloys. Metallic bonds are non-directional. A metal shows all the metallic properties when it is dissolved in a suitable solvent e:g. Na in NH3 or it exists in a liquid state like Hg
How the metal atoms in a metallic crystal are bonded together? can be answered by the following theories.
(a) Free electron theory or Electrons-sea theory :
This theory is also known as electron pool theory or electron cloud theory. In 1900 Drude proposed this theory and later on in 1923 it is developed by Lorentz. Hence it is called the ‘Drude-Lowrentz theory’. According to this theory, all the metal atoms have low ionization energy and lose their valence shell electron easily. Such electrons are called mobile electrons which form a ‘sea of electrons’ or electron pool around the positive metal ions which are rigid spheres and called ‘Kernel’. The mobile electrons can move freely from one place to another through the vacant orbitals of metals hence they are delocalized. The number of free electrons around the Kernel is not certain. The Kernels are rigid spheres that do not float randomly in the sea of electrons but have a definite position at a measurable distance from each other in the crystal lattice as shown in figure 3.04.
In other words, the valency electrons do not belong to one atom in particular and they move freely from one kernel to another as gas molecules move freely throughout their container. Thus, according to this theory, metallic solids may be considered as a collection of positive atomic cores immersed in a fluid of mobile electrons or a sea of mobile electrons. Hence, the metallic bond may be defined as :
Related Topic | Chemical Bonding
“The cohesive force of attraction between Kernel and mobile electrons is termed as a metallic bond”, or the force that binds a metal ion to a number of electrons within its sphere of influence is known as or called ‘Metallic bond’.
1- It does not explain the high degree of hardness in osmium metals.
2- It does not explain why the conductivity of metals is different.
3- It is unable to explain why tungsten has a high melting point (3300∘C) while Hg has a melting point equal to−39∘C.
(b) Valence Bond theory or Resonance theory :
This theory was introduced by Pauling in 1937. According to this theory, the metallic bonding is essentially covalent in origin and the metallic structure involves a resonance of ordinary covalent bonds between each atom and its nearest neighbors. In other words, there is a resonance of a large number of canonical forms. For example: In the metallic structure of lithium, each L-atom is surrounded by eight other Li-atoms. This is confirmed by X-ray studies. The electronic configuration shows L-atom has one valence electron which is sufficient to form only one normal covalent bond with one of its nearest lithium atoms. This L−Li covalent bond is assumed to resonate mainly between all the eight neighbors and to some extent between the six next to the nearest neighbors. In this way each Li-atom would bind all its neighbors, Consequently, it is assumed that resonance takes place
throughout the lithium solid. If each atom of lithium retains its single valency electron, the resonance of pairs of bonds must be assumed to be synchronized throughout the solid. Thus, in two dimensions various resonance structures may be obtained as shown in figure 3.05,
Li—Li Li Li
↔ ↓ ↓
Li—Li Li Li
Figure 3.05 : Resonance in Li2 molecules
Here we see the resonance of the bond pair between four Li-atoms but in a true sense, it is widely spread all over the atoms present in three-generational structures. Considering this fact there is a possibility that one of the LJatom may louse its valence electron and becomes positively charged whereas other Li-atom which has vacant p-orbitals occupy the same electron and become negatively charged as shown in the following electronic configuration of Li-atom, Li+, and Li-ions.
|Li – atom =
|Li+ -ion =
|Li– – ion =
Thus, according to resonance theory, each Li-atom and its ion form four contributing structures a,b,c, and d in which L− ion is surrounded by two normal covalent bonds as shown in figure 3.06 and the actual structure is a hybrid form of all possible resonating forms. In metallic lithium, the cohesive energy is three-time greater than in covalent Li2 molecules.
L atoms has vacant p-orbitals (1s2,2s1,2px0,2py0,2pz0) of nearly equal energies. Since the energy of 2p-orbitals is not very different from 2s orbital therefore the electron of 2s orbital of one Li-atom is
transferred to another Li-atom which is occupied by 2px0 orbital due to which positive and negative charges are developed on it. Li-Ion shows the electronic configuration : (1s2,2s1,2px0,2py0,2pz0) and Li-ion shows the electronic configuration (1s2,2s0,2px0,2py0,2pz0). Thus, there is a possibility of sp hybridization in Li-ion. This anion utilizes them to form two covalent bonds with two Ll-atoms. The two vacant orbitals, 2py0and 2pz0 of each Li-Ion that accept the conducting electrons are termed metallic orbitals. They are responsible for the conduction of heat and electricity. The resonance of metallic bonds is qualitative in nature. It does not explain all the properties of metal.