Solid State Electronics EC 210 – EC 211 Prof.Dr. Iman Gamal Eldin Morsi 1

Chapter 2 : Crystal Binding It is known that the atomic structure of any element is made up of a positively charged nucleus surrounded by electrons revolving around it. Atoms like to have a balanced electrical charge. Therefore, they usually have negatively charged electrons surrounding the nucleus in numbers equal to the number of protons. Atoms are at their most stable when they have no partially- filled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to empty the shell. 2

Types of crystal bonds Metallic Bond (Na). Ionic Bond (NaCl). Covalent Bond (C, Ge, Si). Van der Waals Bond (Ar). 3

Metallic Bond What is a Metallic Bond? - A metallic bond occurs in metals. A metal consists of positive ions surrounded by a “sea” of mobile electrons. - A common characteristic of metallic elements is they contain only one to three electrons in the outer shell. - When an element has only one, two or three valence electrons (i.e. electrons in the outer shell), the bond between these electrons and the nucleus is relatively weak. So, for example, when aluminum 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. 4

1.1. Good conductors of heat and electricity. 2.Great strength. 3.Malleable and Ductile. 4.Luster. Characteristics of a Metallic Bond Electron cloud 5

Ionic Bonds What is an Ionic Bond? What is an Ionic Bond? - An Ionic Bond is a chemical bond resulting from the TRANSFER of electrons from one bonding atom to another. 6

characteristics of an ionic bond 1.Crystalline at room temperatures 2.Have higher melting points and boiling points compared to covalent compounds 3.Conduct electrical current in molten or solution state but not in the solid state 4.Polar bonds 7

Covalent Bonds What is a Covalent Bond?What is a Covalent Bond? - A covalent bond is a chemical bond resulting from SHARING of electrons between 2 bonding atoms. - A covalent bond is formed between two nonmetals. Water H2O Oxygen Atom Oxygen Molecule (O 2 ) Oxygen Molecule (O 2 ) 8

Van der Waals Bond What is a Van der Waals Bond? What is a Van der Waals Bond? -The van der Waal bonds occur to some extent in all materials but are particularly important in plastics. -These materials are made up of a long string molecules consisting of carbon atoms covalently bonded with other atoms, such as hydrogen, nitrogen, oxygen, fluorine. -The covalent bonds within the molecules are very strong and rupture only under extreme conditions. The bonds between the molecules that allow sliding and rupture to occur are called van der Waal forces. 9

Van der waals bond When ionic and covalent bonds are present, there is some imbalance in the electrical charge of the molecule. Take water as an example. Research has determined 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 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 10

Binding Energy U(R) Solids are stable structures, and therefore there exist interactions holding atoms in a crystal together. For example a crystal of sodium chloride is more stable than a collection of free Na and Cl atoms. Solids are stable structures, and therefore there exist interactions holding atoms in a crystal together. For example a crystal of sodium chloride is more stable than a collection of free Na and Cl atoms. This implies that the Na and Cl atoms attract each other, i.e. there exist an attractive inter-atomic force, which holds the atoms together. This also implies that the energy of the crystal is lower than the energy of the free atoms. This implies that the Na and Cl atoms attract each other, i.e. there exist an attractive inter-atomic force, which holds the atoms together. This also implies that the energy of the crystal is lower than the energy of the free atoms. The amount of energy which is required to pull the crystal apart into a set of free atoms is called Binding Energy. The amount of energy which is required to pull the crystal apart into a set of free atoms is called Binding Energy. 11

A typical curve for the potential energy (binding energy) representing the interaction between two atoms. A typical curve for the potential energy (binding energy) representing the interaction between two atoms. It has a minimum at some distance R=R0. For R>R0 the potential increases gradually, approaching 0 as R, while for R R0 the potential increases gradually, approaching 0 as R, while for R<R0 the potential increases very rapidly, tending to infinity at R=0. Since the system tends to have the lowest possible energy, it is most stable at R=R0, which is the equilibrium inter-atomic distance. The corresponding energy U0 is the binding energy. Since the system tends to have the lowest possible energy, it is most stable at R=R0, which is the equilibrium inter-atomic distance. The corresponding energy U0 is the binding energy. Cont’d 12

Cont’d when F(R) is the inter-atomic force 13