Metals constitute about three-fourth of all the known elements. They have characteristic properties such as bright lustre, high electrical and thermal conductivity, malleability and ductility and high tensile strength. The attractive force which binds various metal atoms together is called metallic bond. The metallic bond is neither a covalent bond nor an ionic bond because neither of these bonds are able to explain the known properties of metals. For example, neither ionic nor covalent compounds conduct electricity in the solid stale but metals are very good conductors of electricity. In order to explain bonding in metals different theories have been put forward. We shall be studying here electron gas model or electron sea model for metallic bonding.
ELECTRON GAS MODEL OR ELECTRON SEA MODEL
This is the simplest model that explains the properties of metals. This model was proposed by Lorentz. The main features of this modal are:
1. A metal atom is supposed to consist of two parts, valence electrons and the remaining part (the nucleus and inner shells) which is called kernel.
2. The metallic crystal consists of crystal packed metal atoms in three dimensions. The kernels of metal atoms occupy fixed positions called Lattice sites while space between the kernels is occupied by valence electrons. The arrangement of kernels and valence electrons is shown in Fig. 7.16.
Fig. 7.16. Arrangement of metallic kernels.
3. Due to smaller ionisation energy, the valence electrons of metal atoms are not held by the nucleus very firmly. Therefore, they can leave the field of influence of one kernel and enter the field of influence of the other. This movement can take place through the vacant valence orbitals. Thus, the valence electrons are not localised but are mobile or delocalised. As the movement of electrons in metallic crystal is just like gas molecules, hence, the model is called electron gas model.
4. The simultaneous force of attraction between the mobile electrons and the positive kernels is responsible for holding the metal atoms together and is known as metallic bond.
The metallic bond is non-directional and is weaker than the covalent bond.
STRENGHT OF METALLI C BONO
Strength of metallic bond depends on the magnitude of attractive force between positive kernels and mobile valence electrons.
The average attractive force and metal bond strength increases with the decrease in atomic radius and increase in number of valence electrons. It must be noted carefully that both these factors at the same time decrease the metal character because of the tendency to form metallic crystal decreases. For example when we move along the period from left to right metallic character decreases. Among the elements of 3rd period metal character decreases from left to right. The metallic elements are only Na, Mg, AI, but strength of metallic bond increases from Na -7 AI. It is reflected from their melting points.
Similarly, as we move down the group among alkali metals, the atomic radius increases. Consequently metal bond strength decreases and this causes decrease in the melting points among alkali metals from top to bottom, i.e., from LiàCs
Li Na K Rb Cs
Melting points (K) 454 371 336 312 302
FACTORS THAT FAVOUR THE METALLIC BONDING
Metallic bonding is generally favoured by the following factors:
1. The atomic size of the element should be large.
2. Ionisation energy of the element should be low
3. Electron affinity of the element should be low
4. The Umber of valence electrons should be small (usually 1-2)
5. The number of vacant orbitals in the valence shell should be large.
EXPLANATION ON OF PHYSICAL PROPERTIES OF METALS
1. Metallic Lustre. When light falls on the surface of the metal, the free electrons absorb the photons of light and are set into vibrations. These vibrating electrons immediately emit energy and become a source of light. Thus, incident light appears to be reflected from the surface of the metal. Consequently the metallic surface acquires a shining appearance which is referred to as metallic lustre.
2. Electrical Conductivity. Whenever a difference is applied across the metallic strip, the free mobile electrons in the metal start moving towm-ds positive terminal At the same time the electrons from the negative terminal enter into the metallic crystal. Thus, metallic crystal maintains flow of electron from negative to positive terminal.
At high temperature, the metallic kernels start due to increase of the kinetic energy. This restricts the free movement of the electrons. Consequently, the resistance of metals increases with the increase in the temperature.
Fig. 7 .17. Electrical conductivity of metals.
3. Thermal Conductivity. The conduction of through the metals can also be explained on the basic electron gas mode\. On heating a part of the metal, the kinetic energy of the electrons in that region increases. These energetic electrons move rapidly to the cooler parts and transfer their kinetic energy by means of collisions with other electrons . In this way, the heat travels from hotter to cooler parts of the metals.
4. Malleability and Ductility. Malleability is the property of metals by virtue of which they can be beaten into sheets whereas ductility is the property by virtue of which they can. be drawn into wires. These properties are exhibited by metals on account of the of non-directional nature of metallic bond. Whenever any stress is applied on metal, the position of metallic kernels is altered without destroying the crystal The crystal lattice gets deformed by slippage of the layers of kernels moving past to another as shown in fig 7.18. whenone layer of kernels moves past another, the positive on metal ions are shielded from each other by the electrons.
The electron sea model could explain the properties metals qualitatively. However, the properties of metals be explained more quantitatively by molecular orbital which is beyond the scope of this book.
Fig. 7.18. Malleability and ductility of metals.
The general properties associated with three primary interatomic bonds are being summarized in tabular form as follows.
General Characteristics of Substances with different Interatomic Bonding
I .Objective type Questions
Select the most appropriate choice from the options given as
(a), (b), (c) and (d) after each question:
1. Which of the following species the constituent atoms are held by non-directional bonds?
(a) NH3 (b) CsCl
(c) NF3 (d) BeF2.
2. X and Y atoms have 2 and 6 valance electrons in their outermost shells respectively, the compound which X and Y are likely to form is:
(a) XY 2 (b) XY
(c) YX2 (d) YX3.
3. Which of the following substance is a polar covalent molecule?
(a) Hydrogen sulphide (b) Nitrogen
(c) Potassium chloride (d) Oxygen.
4. Which of the following statement is/are correct?
I. Energy is absorbed when a chemical bond is formed
II. Energy is released when a chemical bond is formed
III. SF 6 is a super octet molecule
(a) I and III (b) III and II
(c) II only (d) III only