Prepared by: Mahmoud Kanan Mohsin MSc Candidate, Orthodontics Hawler Medical UniversitySupervised by: Dr. Bayan A. Hassan BDS, MSc, PhD Hawler Medical University
Has many definitions, but the mostcommon is that it is any substance which hasmass and occupies space.
Is a unit of matter, the smallest unit ofan element, having all the characteristics ofthat element and consisting of adense, central, positively charged nucleussurrounded by a system of electrons.
An atom itself is made up of three tiny kinds of particles called subatomic particles;1. Protons2. Neutrons3. Electrons
1) All matter is made of atoms, atoms are indivisible and indestructible.2) All atoms of a given element are identical in mass and properties.3) Compounds are formed by a combination of two or more different kinds of atoms.4) A chemical reaction is a rearrangement of atoms.
An ion is an atom or molecule in whichthe total number of electrons is not equal tothe total number of protons, giving the atoma net positive or negative electrical charge. Ifa neutral atom loses one or more electrons, ithas a net positive charge and is known as acation. If an atom gains electrons, it has a netnegative charge and is known as an anion.
Is an electrically neutral group of two ormore atoms held together by chemical bonds.Molecules are distinguished from ions bytheir lack of electrical charge.
An alloy is a mixture or metallic solid solution composed of two or more elements, e.g. Amalgam, Bronze, gold alloy,…ect.
Is an attraction between atoms thatallows the formation of chemical substancesthat contain two or more atoms.
Description of the structure of atomsproposed (1911) by the British physicistErnest Rutherford. The model described theatom as a tiny, dense, positively charged corecalled a nucleus, in which nearly all the massis concentrated, around which thelight, negative constituents, calledelectrons, circulate at some distance, muchlike planets revolving around the Sun.
Primary Bond A bond that forms between atoms andthat involves the exchanging or sharing ofelectrons.Secondary Bond A bond that involves attraction between molecules. Unlike primary bonding, there is no transfer or sharing of electrons.
: The forces that hold atoms together are called cohesive forces. These interatomic bonds may be classified as primary or secondary. The strength of these bonds and their ability to form after breakage determine the physical properties of material.
Interatomic Bonding Primary SecondaryMetalic Covalent Van der Hydrogen Ionic BondsBonds Bonds Waals Forces Bonds
Interatomic primary bonding may be of three different types:1. Ionic Bonds: Result from the mutual attraction of positive and negative charges. The classic example is sodium chloride (Na+Cl-). In dentistry, ionic bonding exists in certain crystalline phases of some dental materials, such as gypsum and phosphate based cement .
In many chemical compounds, two valence electrons shared by adjacent atoms. The hydrogen molecule H2, is an example of covalent bonding. Covalent bonding occur in many organic compounds, such as dental resin, in which the compound link to form the backbone structure of hydrocarbon chains. The carbon atom has four valence electrons and can be stabilized by combining with hydrogen.
Covalent bond formation- characterized by electron sharing
It is the attraction force between positive metal ions and the delocalized (freely moving) electrons, gathered in an electron cloud. These free electrons are responsible for the high electric and thermal conductivities of metals also for their ability to deform plastically. Found only in metals.
In contrast with primary bonds, secondarybonds don’t share electrons. Instead, chargevariations among molecules or atomic groupsinduce polar forces that attract the molecules.
Bonds between hydrogen atom and atoms of the most electronegative elements (N, O, F) are called hydrogen bonds. When a water molecule intermingle with other water molecules, the hydrogen (positive) portion of one molecule is attached to the oxygen (negative) portion of its neighboring molecule and hydrogen bridges are formed. Polarity of this nature is important in accounting for the intermolecular reaction in many organic compounds, such as the absorption of water by synthetic dental resins.
Van der Waals Forces form the basis of a dipole attraction. E.g. in a symmetric molecule, such as an inert gas, the electron field constantly fluctuates. Normally, the electrons of the atoms are distributed equally around the nucleus and produce an electrostatic field around the atom. However this field may fluctuate so that its charge becomes momentarily positive and negative. A fluctuating dipole is thus created that will attract other similar dipoles. Such interatomic forces are quite weak .
In general, materials can be subdivided into two categories according to their atomic arrangement. In crystalline materials there is a three-dimensional periodic pattern of the atoms, whereas no such long range periodicity is present in noncrystalline materials, which possess only short-range atomic order.
Atoms are bonded to each other by either primary or secondary forces. In the solid state, they combine in a manner that ensures minimal internal energy. For example, sodium and chlorine share one electron. In the solid state, however, the atoms do not simply form only pairs; all of the positively charged sodium ions attract all of the negatively charged chlorine ions. The result is that they form a regularly spaced configuration known as a space lattice or crystal.
A space lattice can be defined as any arrangement of atom in space in which every atom is situated similarly to every other atom. Space lattices may be the result of primary or secondary bonds
There are 14 possible lattice types or forms, but many of the metals used in dentistry belong to the cubic system; that is, the atoms crystallize in cubic arrangements. All dental amalgams, cast alloys, wrought metals, gold foil are crystalline. Some pure ceramics, such as aluminia and zirconia core ceramics, are entirely crystalline. Other ceramics, such as porcelains, consists of noncrystalline glass matrix and crystalline inclusions that provide desired properties, including color, opacity, and increase in thermal expansion coefficients, radiopacity, strength, fracture toughness .
Structures other than crystalline forms can occur in the solid state. For example,waxes may solidify as amorphous materials so that the molecules are distributed at random.. A resin based composite consists of resin matrix, filler particles and an organic coupling agent that bond the filler particles to the resin matrix. In some cases, the filler particles are made from radiopaque glasses that are nancrystalline. Composites have a noncrystalline matrix and may or may not contain crystalline filler particles. The structural arrangements of the noncrystalline solids don’t represent such low internal energies as do crystalline arrangements of the same atoms and molecules. Noncrystalline solids do not have a definite melting temperature, but rather they gradually soften as the temperature is raised 6.
It’s appropriate to first consider the pure metals, which have the simplest composition and atomic arrangement, metals always have crystalline structures. There are seven crystal systems (cubic, tetragonal, orthorhombic, rhombohedral [trigonal] hexagonal, monoclinic, and triclinic). Space lattice is a geometric construct wherein each point has identical surroundings. Crystal structures of real materials are based upon space lattices, where there is a single atom or a group of atoms at each space lattice point.
Its most convenient to visualize the crystal structures of metals in term of their cubic cells, where a unit cell is the smallest portion that can be repeated in three dimensions to produce the crystal structure.
Ceramics, which consists of more than one atomic species, can have crystalline or noncrystalline structures, depending upon the materials and sometimes the mode of preparation. Important ceramics for orthodontic application are aluminum oxide (alumina) and zirconium oxide (zirconia), which are used as bracket materials. Other ceramics are found in the powder portion of cements. Silicon dioxide (silica) is important filler in composite restorative resins. The crystal structure of aluminum oxide is illustrative of the principles involved with ceramics having substantial ionic bonding character. The crystal structure consists of nearly hexagonal close packed (hcp) arrangement of the larger oxygen anions (O2), with the smaller aluminum cations (Al3+) located in two- thirds of the octahedral interstitial sites in the hexagonal close packed (hcp) structure .
A wide variety of polymeric materials are used in orthodontics: polyurethane elastomers for tooth movement, adhesive resins for bonding brackets to tooth structure, polycarbonate brackets, elastomeric impression materials and polycarbonate brackets. All of these polymeric materials are based on macromolecules with varying compositions, molecular weight and degrees of cross linking. The polymers have predominantly noncrystalline structures without long range periodicity. Schematic polymer structure of alginate impression material.
http://physics.about.com/ http://www.universetoday.com/ http://www.iun.edu/~cpanhd/C101webnotes/co mposition/dalton.html http://en.wikipedia.org/ http://www.britannica.com/EBchecked/topic/514 258/Rutherford-atomic-model Phillips’ Science of Dental Materials, Kenneth J. Anusavice, Eleventh Edition Orthodontic Materials, William A. Brantley