structure of matter; mahalaxmi

Question 1

hydrogen bonding

Answer 1

The hydrogen bond is a special case of dipole attraction of polar compounds. It can be understood by studying water molecules. Attached to the oxygen atom are two hydrogen atoms. These bonds are covalent. Therefore, the protons of the hydrogen atoms pointing away from the oxygen atom are not shielded efficiently by the electrons. They become positively charged. On the opposite side of the water molecule, the electrons
that fill the outer shell of the oxygen provide a negative charge. The positive hydrogen nucleus is attracted to the unshared electrons of neighboring water molecules. This type of bond is called a hydrogen bridge. Polarity of this nature is important in accounting for the intermolecular reactions in many organic compounds—for example, the sorption of water by synthetic dental resins.

Question 2

van der Waal’s forces

Answer 2

Van der Waals forces of attraction arise from dipole attractions. In the case of polar molecules, dipoles are induced by an unequal sharing of electrons. In the case of nonpolar molecules, the random movement of electrons within the molecule creates fluctuating dipoles. Dipoles generated within these molecules will attract other similar dipoles. Such interatomic forces are quite weak compared with the primary bonds.

Question 3

interatomic secondary bonds

Answer 3

In contrast with primary bonds, secondary
bonds do not share electrons; instead, charge
variations among molecules or atomic groups
induce polar forces that attract the molecules.

Question 4

interatomic primary bonds

Answer 4

1. ionic bonds

Question 5

ionic bonds

Answer 5

The classic example of ionic bonding is the bond between the Na+ and Cl– of sodium chloride. 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 in gypsum and phosphate-based cements.

Question 6

covalent bonds

Answer 6

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. By virtue of sharing electrons, the two atoms are held together by covalent bonds to form a molecule that is sufficiently stable and electrically neutral in a definite arrangement.
Covalent bonding occurs in many organic compounds, such as dental resins, where they link to form the
backbone structure of hydrocarbon chains.

Question 7

metallic bonds

Answer 7

It is found in elements with one, two, or
three valence electrons. The outer-shell valence electrons can be removed easily from metallic atoms and form positive ions. The free valence electrons can move about in the metal space lattice to
from what is sometimes described as an electron “cloud” or “gas.” The electromagnetic attraction between the electron cloud and the positive ions in the lattice provide the force that bonds the metal atoms together as a solid. The free electrons give the metal its characteristically high thermal and electrical conductivity. These electrons absorb light energy, so all metals are opaque to transmit light. The metallic bonds are also responsible for the ability of plastic deformation of metals. Plastic deformation means a material can be reshaped by force without fracture. The free electrons can move through the lattice, whereas their plastic deformability is associated with slipping along crystal planes. During slip deformation, electrons easily regroup to retain the cohesive nature of the metal.

Question 8

classes of dental biomaterials

Answer 8

1. Metals; metallic bonds; ex. gold, silver, palladium
2. Polymers; covalent bonds; ex. resins
3. Ceramics; ionic bonds; ex. porcelain