By Dr.Reham Mohammed Abdallah Structure of matter By Dr.Reham Mohammed Abdallah

Items to be covered Types of Atomic Bonding I. Primary bonds Ionic bond Covalent bond Metallic bond II. Secondary bonds Hydrogen bond VanderWaal bond Arrangement of atoms in solids Crystalline solids Non-crystalline solids

All materials are built up from atoms and molecules and there is a strong relationship between atomic basis of a material and its properties. To understand the properties of materials, we need to have an understanding of the way atoms can combine to make solids.

Types of Atomic Bonding I. Primary bonds 1. Ionic bond 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. (Fig.1)

Ionic materials are characteristically hard and brittle. Electrically and thermally insulators. In dentistry ionic bond is found in gypsum investment, phosphate cement and ceramic. Fig.1: Ionic bond.

2. Covalent bond Sharing of electrons between adjacent atoms. Example: a molecule of methane(CH4). The carbon atom has four valence electrons, whereas each of the four hydrogen atoms has a single valence electron (Fig.2).

Covalently bonded materials are typically electrical and thermal insulators. In dentistry: monomer molecules of dental resin are held by covalent bonds within the polymer chain. Fig.2: Covalent bond.

3. Metallic bond “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 positively charged cores. The metallic bond is weaker than the ionic and the covalent bonds (Fig.2.3).

This structure gives the metal: 1. Excellent thermal and electrical conductivity 2. Opacity(due to absorption of light by free electrons). 3. Lustrous appearance. 4.The ability to be plastically deformed. Fig.2.3: Metallic bond.

II. Secondary bond In contrast with primary bonds, secondary bonds don’t share electrons. Instead, charge variations among molecules or atomic groups induce polar forces that attract the molecules. These forces are physical, weak and arise from the polarization of molecules i.e. formation of electrical dipoles.

1. 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.

Hydrogen bond 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” (Fig.4).

Fig.4: Hydrogen bridges formed between water molecules. Example: Absorption of water by synthetic resin. Fig.4: Hydrogen bridges formed between water molecules.

2. Van der Waal bond (fluctuating dipole) “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 (Fig.5).

Chemisorption of gases by alloy liquids is followed by attraction due to vander Waal, forces. Fig.5: Fluctuating dipoles.

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

STRUCTURAL ARRANGEMENT OF ATOMS IN SOLIDS All solid materials may be classified on the basis of their atom arrangements as: Crystalline Non crystalline or amorphous Semi crystalline

Fig.6: Space lattice of crystalline solid material. 1. 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. (Fig.2.6). Fig.6: Space lattice of crystalline solid material.

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.(fig.7)

Fig.8: Bravais Lattices.

II. Non-crystalline 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”(Fig.9).

Fig.9: Crystalline and amorphous arrangement of atoms.

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.