Atomic Structure and Interatomic Bonding

• The structure of matter will be described
• Dr/Diaa Elmwafy is the presenter

Atomic Structure

• Atoms comprise a nucleus containing positive protons and uncharged neutrons, along with negative electrons.
• Atoms are the fundamental units of internal structures.
• Physical and chemical properties depend on the interaction between valence electrons furthest from the nucleus.
• Elements, excluding inert gases, strive for a stable configuration of 8 outer electrons.
• This is achieved by gaining (receiving -ve charge), losing (giving +ve charge), or sharing electrons between atoms to complete the outer shell.

Interatomic Bonding

• Interatomic bonding is a primary, strong intramolecular bond
• Covalent, ionic, and metallic bonds are types of interatomic bonds.
• Intermolecular bonding is a secondary, weak bond

Bonding Types

• Covalent bonds involve sharing electrons.

• An example is the H2 gas molecule.

• Covalent bonds are very strong and are insulators, forming basic polymer bonds

• Ionic bonds involve electrostatic attraction between unlike charges.

• An example is NaCl forming Na+ cations and Cl- anions.

• Ionic bonds are insulators, heat resistant, and are commonly used in ceramics and glasses.

• Metallic bonds have metal atoms that lose electrons freely moving in valence shells.

• Metallic bonds consist of positive ion cores held by diffused electron clouds, enabling metal deformability.

• Metals have high thermal and electric conductivity and are opaque because free electrons absorb light.

• Metals are reflective or lustrous because electrons re-emit light.

• Van der Waals forces occur, for example, in H2O molecules.

• Combinations include ionic & covalent bonds as in Caso4

• Combinations include metallic & ionic bonds as in Dental Amalgam

• Combinations have high conductive properties, and are hard & brittle

Interatomic Distance

• Interatomic distance is caused by interatomic repulsive & attractive forces
• Factors affecting Interatomic Distance (IAD):
• Temperature: Increased temperature increases energy and IAD.
• Number of adjacent atoms: Increased number of adjacent atoms decreases IAD.
• Covalent bonding: Increased shared electrons decreases IAD.
• State of matter impacts IAD from gas to liquid to metal.
• Free electron movement increases energy and IAD.

Structure of Solids

• Solids are classified by internal atomic structure and atom or molecule regularity in three spatial directions.
• Crystalline solids have atoms arranged regularly & repetitively in 3 dimensions.
• Space lattice describes the 3D arrangement of atoms, with each atom in a similar position to others.
• Amorphous (non-crystalline) solids have atoms or molecules positioned randomly with no regularity, like gases and liquids
• Semi-Crystalline (Mesomorphous) solids have atoms or molecules arranged with regularity in one or more directions.

Crystalline Structures

• Atomic arrangement patterns: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral.
• The unit cell is the smallest repeated unit in crystal lattice.
• Space lattice type is defined by the length of 3 unit cell axes and the angles between them.

Cubic Unit Cells

• Simple Cubic (SC): Each corner atom is associated with 8 surrounding cells, with 1/8 of its volume in each cell.
• SC contains one metal atom per unit cell and has 8 atoms * 1/8 at each corner = 1 atom.
• Body Centered Cubic (BCC): One atom at each of the 8 corners of the cube and another at the unit cell's center = 2 atoms per unit cell.
• Its atoms are 8 atoms * 1/8 at each corner + 1 atom in the center = 2 atoms.
• Examples are iron and chromium.
• Face Centered Cubic (FCC): One atom at each of the 8 corners of the cube and another at the center of each face of the unit cell.
• There are 4 atoms per unit cell in FCC
• Its atoms are 8 atoms * 1/8 at each corner + 6 atom * 1/2 in the center = 4 atoms.
• Examples are copper, gold, platinum, and silver.

Non-Crystalline Structures

• Amorphous structures such as gases or liquids, have random molecules

Properties of Non-Crystalline Structures

• Higher internal energy exists
• Amorphous structures lack a definite melting temperature; they soften gradually as temperature increases and harden when cooled.
• They possess a glass transition temperature where the coefficient of thermal expansion sharply increases, indicating higher molecular mobility.

Semi-Crystalline Structures

• Semi-crystalline structures have definite regularity in one or more directions, such as pyrolytic carbon.

Polymorphism

• Polymorphism occurs when materials with identical chemical compositions exhibit different polymorphic or allotropic forms.
• An example of allotrophy is silica in dentistry.
• Quartz (hexagonal) transforms into tridymite (rhombohedral) at 870°C, then into cristobalite (cubic) at 1470°C, and finally into fused quartz (amorphous) at 1713°C.

Imperfections in Crystalline Solids

• Point defects involve:
• Vacancies where atoms are missing due to imperfect packing during crystallization or thermal vibrations at high temperatures, increasing the likelihood of atoms jumping out.
• Interstitial impurities occur when extra atoms are within a crystal structure.
• The presence of point defects causes atomic distortion within the crystal lattice.
• Line defects involve the displacement of a row of atoms from their normal lattice positions, known as dislocation.
• Plastic deformation in metals results from dislocation movement.
• Plane defects include grain boundaries in metals.

Explanation of atomic structure, focusing on the nucleus, protons, neutrons, and electrons. It describes interatomic bonding as a primary intramolecular bond, including covalent, ionic, and metallic types. Covalent bonds are exemplified with the H2 molecule.