Structure of Matter

1. Structure of Matter

2. Introduction: All materials are built up form atoms and molecules ,so it is not really surprising that there is a close relationship between the atomic bases of a material and its propertied.

3. Interatomic attraction: Atoms achieve a stable state by having eight electrons in their outer shell. This can obtained by: 1- receiving extra electrons to complete the outer shell electrons (and becomes negative ions). 2- releasing electrons so that the outer shell has eight electrons (and the atom becomes positive ions) 3- sharing of electrons so that the outer shell of two or more atoms are complete

4. In solids, atoms are held together by either: 1- primary bonds 2- secondary bonds

5. 1- Types of primary bonds: a) Ionic bonds: It is the attraction of positive and negative ions. The classic example is sodium chloride (Na Cl ) because the sodium atom contains one valence electron in its outer shell and the chlorine atom has seven electrons in its outer shell, the transfer of the sodium valence electron to the chlorine atom results in the stable compound Na Cl. In dentistry, ionic bonding exists in some dental materials, such as gypsum and phosphate cements.

6. Ionic bonds

7. Characteristics of ionic solids: A) Heat resistant. B) Insoluble in organic solvents. C) Easily dissolved in ionized solvents such as water, acids and alkalis. D) Dissociate into their constituent ions in solutions which in their turn can conduct an electric current.

8. Covalent bonds: “It is sharing of electrons”. In many chemical compounds, two valence electrons are shared by adjacent atoms. The hydrogen molecule, H2 ,is an example of covalent bonding. The single valence electron in each hydrogen atoms is shared with that of the other combining atom, and the valence shell become stable. Covalent bonding occurs in many organic compounds, such as dental resin.

9. Covalent bonds

10. Characteristics of covalent bonds: A) Water insoluble. B) Very strong C) Insulator and with stand high temperature.

11. Metallic bonds: “It is the attraction between +ve cores and free electrons or electron cloud”. It occurs in metals, because they easily give up the electrons in their valence shells giving positives cores. The electrons move freely through the metal from atom to atom and from electron cloud. There is attraction between free electrons and the positive charged cores.

12. Metallic bonds

13. Characteristics of the metallic bonds: The free mobility of electrons contributes to the following properties of metals: A) High thermal and electrical conductivity. B) Opacity (due to absorption of light by free electrons). C) Metallic luster D) High ductility and malleability. N.B: sometimes more than one type of bond can exist in a material. An example is CaSo4(it has ionic and covalent bonds).

14. II-Secondary bonds (Van Der Waal forces): These forces are physical, weak and arise from the polarization of molecules i.e. formation of electrical dipoles.

15. a) Fluctuating dipole or dispersion forces: “it is the attraction between the +ve pole of one atom and the –ve pole of another atom”. In symmetric molecule, such as in inert gases, the electron field is constantly fluctuating. 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 and the electrons may concentrate toward one side of the atom and as a result its charge becomes momentarily positive and negative. A fluctuating dipole thus created which will attract other similar dipoles. Such interatomic forces are quite weak.

16. b) Permanent dipole: An electrical dipole or polarization exists in a molecule that has an electrical imbalance and hence has a center of positive charge and a center of negative charge. This type of attraction forces occurs in asymmetric molecules where one atom can attract the electrons and becomes negative, and the other atom becomes positive in relation to it resulting in a permanent dipoles.

17. The hydrogen bond is an important example. In H2O there is a covalent bond because oxygen and hydrogen atoms share electrons. However, the electrons around oxygen nucleus are more than those around the hydrogen nucleus and as a result the hydrogen portion of the water molecule is positive in relation to the oxygen portion. Therefore “attraction will take place between the positive hydrogen portion of one water molecule and the negative oxygen portion of another water molecule”

18. Permanent dipole

19. Characteristics of secondary bonds: A solid whose molecules are bonded together by Van Der Waal forces has: 1- Low modulus of elasticity. 2- Low melting point. 3- High thermal expansion e.g. waxes, acrylic resin

21. Types of solids: a) Crystalline solids. b) Amorphous solids.

22. a) Crystalline solids: Solid dental materials are termed crystalline when their atoms are regularly arranged in a space lattice. A space lattice is the regular arrangement of atoms in the space so that every atom is situated similary to every other atom.

24. Types of space lattices: There are about 14 different types of space lattice but only few are of dental interest. The simplest way to study these types, is to consider a unit cell which is the smallest repeating unit in the space lattice.

25. 1) The cubic system: 1- The length of the axes a,b,c are equal. 2- The interfacial angles α =β =µ =90 There are three types of the cubic system. a) Simple cubic space lattice (S.C.). b) Body centered cubic (B.C.C) c) Face centered cubic (F.C.C)

26. a) Simple cubic space (S.C): The unite cell has one atom at each corner. Each atom is surrounded with eight unit cells. Therefore each atom has 1\8 of its volume in each of these eight cells, so S.C. contains 8x1\8 = one atom.

27. b) Body centered cubic (B.C.C) Each corner is occupied by 1\8 of an atom with one atom at the center of the cube. Therefore, the number of atoms in a B.C.C unit cell is (8x1\8 = 1 + 1 in the center) = 2 atoms

28. c) Face centered cubic (F.C.C): Each corner is occupied with an atom and one in the center of each of the six faces. Atoms at each face shared by two adjacent unit cells. So F.C.C. contains (8 x 1\8 +6 x1\2) = 4 atoms.

29. 2) The Hexagonal system: The axis a=b but ≠ c. The angle α =β =90 but the angle µ equal 120. a) The simple hexagonal system (S.H) contains: 6x1\6 (at corners of top surface) + 6x 1\6(at corners of bottom surface) +2 x 1\2 (at top and bottom surfaces)= 3 atoms.

30. The simple hexagonal system (S.H)

31. b) The hexagonal closed packed system (HCP) contains: 12 x 1\6 (at corners) + 2x 1\2 (at top and bottom surfaces) + 3 at the center = 6 atoms, so this structure has the densest packing of atoms.

32. Atomic packing factor (APF): Volume of atoms inside the unit cell APF = Volume of the unit cell Examples of APF for : S.C. = 0.54 This indicates that nearly 50% of the space is free so that other atoms can occupy this free space without causing too much disruption to the crystalline structure.

33. B.C.C. = 0.68 and F.C.C.=0.74 With these larger atomic packing factors it is of course more difficult for smaller atoms to occupy the free space without disrupting the structure. N.B: Usually, the higher the APF The higher the strength, density and melting temp of the material.

34. B) Amorphous solids: Amorphous means without shape. Gases and liquids are amorphous substances. Some solids like glass and some polymers are amorphous because of the random arrangement of their atoms, yet their atoms may form a short localized range of order lattice with a considerable number of disordered units in between. Since such an arrangement may be considered typical of the liquid structure, these solids are sometimes called “super cooled liquids”.

36. Crystalline solids Amorphous solids 1) Have definite 1) No definite melting Melting temperature. Temp (gradually soften on heating and gradually harden on cooling) 2) Have regular unit 2) No regular unit cell Cell with repetition. But may have a short range of regularity but no repetition.

37. Isomerism (Allotropy and polymorphism) and Isomorphism Isomerism : it is the existence of elements or compounds in more than one crystalline form. Isomorphism : it is the existence of similar crystals by substances having different compositions. It is opposite to isomerism. Allotropy: it is the existence of an element in more than one crystalline form. E.g. iron is F.C.C above 910C and BCC below it. Polymorphism: it is the existence of a compound in more than one crystalline form. E.g. Silica (sio2)can exist in four different crystalline forms.