Physical Metallurgy & Structure of Solids

Introduction

Solid materials are made up of large collection of atoms where the individual atoms are bonded together. The type of bonding which exists between atoms is different for all material.

An atom with a completely filled outer electron shell has a high degree of stability. The atoms of other elements with incomplete outer shells attempts, by combination with other atoms, to satisfy this condition for stability. The principal types of inter atomic bonds which may be formed are the ionic, covalent, metallic, Van Der Wall’s or molecular bonds.

The solids are also classified as per the bond structure found in them. They are Ionic, Metallic, Covalent and Molecular.

Ionic Solids

Ionic Bond. An ionic bond is formed when the outer most orbit electrons from one atom are transferred to another atom as a result of which each atom acquires 8 outermost electrons (octet configuration). In this process, the atom which loses an electron, develops a positive charge and the atom that gains an electron develops a negative charge. As a result of these charges, an electrostatic force of attraction develops between the atoms, which bond the atoms together.

Ionic Bond in NaCl
Fig 1 Ionic Bond in NaCl

The above images schematically show the process that takes place during the formation of an ionic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it would like to give up so that it would become stable with a full outer shell of eight. Also note that chlorine has seven valence electrons and it would like to gain an electron in order to have a full shell of eight. The transfer of the electron causes the previously neutral sodium atom to become a positively charged ion (cation), and the previously neutral chlorine atom to become a negatively charged ion (anion). The attraction for the cation and the anion is called the ionic bond. (Refer Fig 1)

Ionic Solids. This solid consist of alternating arrangements of positive and negative ions in a uniform pattern throughout the crystal lattice. Ionic solids are generally rigid and crystalline since the ions present are tightly held by electrostatic force of attraction.

Properties of Ionic Solid. Ionic solids are bad conductor of electricity in the solid state while good conductor of electricity at the liquid or molecular state. Ionic solids generally have high melting and boiling points. In ionic solids, the electrostatic force of attraction holds the charged atoms very strongly. Hence large amount of energy has to be supplied for breaking the bond and so the boiling point of ionic solid is found to be high. Ionic solids are freely soluble in polar solvent like water or alcohol. Example of ionic solids are NaCl, BaCl, AlCl3, etc.

Metallic Solids

Metallic Bond. In all the metal atoms, valence electron is loosely held by their nuclei. So, valence electron requires very less energy to detach themselves from their nuclei. The metal atom after leaving their outer most electrons becomes positive charged ions. These atoms are surrounded by a large number of free electrons. So, the electrostatic force of attraction between the electron cloud and positive ion, form a bond which is known as metallic bond. In such bond, electrons are shared by all most all the atoms. There is no way of determining to which nucleus any particular valence electrons belong.

Metallic Bond in Aluminium
Fig 2 Metallic Bond in Aluminium
Aluminium Atom
Fig 3 Aluminium Atom

When aluminium atoms are grouped together in a block of metal, the outer electrons leave individual atoms to become part of common “electron cloud.” In this arrangement, the valence electrons have considerable mobility and are able to conduct heat and electricity easily. Also, the delocalized nature of the bonds, makes it possible for the atoms to slide past each other when the metal is deformed instead of fracturing like glass or other brittle material.(Refer Fig 2 & 3)

Since the aluminium atoms lose three electrons, they end up having a positive charge and are designated Al3+ ions (cations). These ions repel each other but are held together in the block because the negative electrons are attracted to the positively charged ions. As a result of sharing of electrons, the cations arrange themselves in a regular pattern. This regular pattern of atoms is the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals called grains that touch at grain boundaries.

Metallic Solids. Metallic solids are those in which positive ions occupy the lattice sites. The atoms of a metal assume nearly fixed position relative to each other. A solid metal usually is composed of a multitude of crystals. Within any one crystal, the atomic arrangement is repeated by adjacent atoms.

Properties of Metallic Solid. As these solids have unlimited number of free electrons, the electrical conductivity of metallic solids is very high. Due to the presence of free electrons which serves as carriers of heat, these solids are also having high thermal conductivity. These solids have high optical reflection and absorption co-efficient. It is having high tensile strength and ductility. Examples of metallic solids are Al, Mg, Si, etc.

Covalent Solids

Covalent Bond. A covalent bond is formed when the electron or electrons of an atom in its outermost orbit are shared by the electrons of another atom in its outermost orbit in order to fulfil the octet configuration (8 electrons)

Where a compound only contains non-metal atoms, a covalent bond is formed by atoms sharing two or more electrons. Non-metals have 4 or more electrons in their outer shells (except boron). With this many electrons in the outer shell, it would require more energy to remove the electrons than would be gained by making new bonds. Therefore, both the atoms involved share a pair of electrons. Each atom gives one of its outer electrons to the electron pair. Consequently, both atoms are held near each other since both atoms have a share in the electrons. (Refer Fig 4)

Atoms Covalent Bonding
Fig 4 Atoms Covalent Bonding

More than one electron pair can be formed with half of the electrons coming from one atom and the rest from the other atom. An important feature of this bond is that the electrons are tightly held and equally shared by the participating atoms. The atoms can be of the same element or different elements. In each molecule, the bonds between the atoms are strong but the bonds between molecules are usually weak. This makes many solid materials with covalent bonds brittle. Many ceramic materials have covalent bonds.

Covalent Solids. It consists of atoms that share electrons with neighbouring atoms so that atoms are linked with one another by a continuous system of covalent bond. Diamonds, SiC, SiO2 are some example of covalent solids.

Properties of Covalent Solid. The covalent solids are electrically neutral so they are bad conductors of electricity (except graphite). Because of the strongly bonded rigid structures, most covalent solids are very hard and melt at high temperature.

Molecular Solids

Molecular bond or Van Der Wall’s Bond. Inert gases and molecules like methane, which have no valence electrons available for crystalline binding, obtain a weak attractive force as a result of polarization of electrical charges. Polarization is displacement of the centres of positive and negative charges in an electrically neutral atom or molecule when it is brought close to its neighbouring atoms. Its neighbors also become polarized. The resulting weak electrical attraction between neighbouring atoms or molecules is the molecular or van der wall’s bond. Take water as an example. Research has determined that the hydrogen atoms are bonded to the oxygen atoms at an angle of 104.5°. This angle produces a positive polarity at the hydrogen-rich end of the molecule and a negative polarity at the other end. A result of this charge imbalance is that water molecules are attracted to each other. This is the force that holds the molecules together in a drop of water. (Refer Fig 5)

Molecular Bonding in Water
Fig 5 Molecular Bonding in Water

Molecular Solids . Molecular or Van Der Wall’s solids are those solids created by the molecular bond. These solids can have both crystalline and non-crystalline structure. Molecules are their structural units.

Properties of Molecular Solid . They have low density as the crystals are formed of elements with low atomic number. These solids are goods insulator as no valence (free) electrons are available in the solids. Molecular solids are soft, have low melting and boiling points and high vapour pressure. These solids are usually transparent to light. Examples of molecular solids are inert gases like helium, neon, argon etc. in solid state.

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