Introduction to Dentistry and Biomaterials By Dr. Hadeel A. Sabry Assistant Professor of Biomaterials

Structure of Matter To understand the properties of materials we need to understand the way atom can combine to form solids

Atomic structure The atom is the basic unit of the internal structure of any material

An atom consists of three elementary particles: It is the difference in atomic structure that gives the element its characteristic properties An atom consists of three elementary particles: Electrons: -ve particles. Protons: +ve particles. Neutrons: ±ve particles.

Function of valence electron: Responsible for the size of atom. Responsible for the chemical reaction. Responsible for the physical properties.

Interatomic attraction: Atoms achieve a stable state by having 8 electrons in their outermost shell “as inert gases” .

Inter-atomic attraction : 1- receiving extra electron (negative ion) Inter-atomic attraction : 1- receiving extra electron (negative ion). 2- releasing electrons (positive ion). 3- sharing of electrons.

This can be obtained by: Releasing electrons so that the outermost shell has 8 electrons and the atom become ion. positive

Receiving extra electrons so that the outermost shell has 8 electrons and the atom become ion. negative

Sharing of electrons so that the outermost shell of two or more atoms are complete

Types of bond Secondary bond Primary bond

Types of primary bond Covalent bond Ionic bond Metallic bond

Ionic bonds: It is the attraction of positive and negative ions (NaCl ).

Ionic bond +ve Na -ve Cl

Characteristics of ionic solids: Heat resistant. Strong, hard and brittle. Stable and insoluble in organic solvents. Soluble in ionizing solutions as water, acids and alkalis. Electrical and thermal insulators as solid. But in solutions becomes an electrical conductors. Non directional.

In dentistry, ionic bonding is the predominant bonding in ceramic materials e.g. Gypsum Phosphate cements.

Covalent bonds: It is the sharing of electrons e.g (hydrogen molecule )

Covalent bond H H

Characteristics of covalent solids: Directional. Strong and hard. Withstand high temperature. Insulators. Water insoluble, but soluble in organic solvents.

Metallic bonds: It is the attraction between +ve cores and free electrons or electron cloud (metals)

Metallic bond

Characteristics of metallic solids: Non-directional. Strong. Have high melting temperature. Good thermal and electrical conductors. Opaque and luster.

Secondary bonds Physical forces, weaker than primary ones, but they influence the physical properties of some materials. They arise from atomic or molecular dipoles. The bonding results from the attraction between the -ve and adjacent +ve dipoles.

Secondary bonds “van Der Waal forces” H-bond Fluctuating induced Permanent dipole dipole bonds bonds

Secondary bonds “van Der Waal forces” Fluctuating induced dipole bonds

Characteristic of secondary bonds: A solid whose molecules are bonded together by Van Dar Waal forces (waxes). 1-Low modulus of elasticity 2-Low melting point 3-High thermal expansion

Secondary bonds “H-bond” Permanent dipole bonds

Primary bonds control the properties of the atomic solids Secondary bonds control the properties in the molecular solids