ATOMIC AND MOLECULAR STRUCTURE & BONDING

ATOMIC AND MOLECULAR STRUCTURE & BONDING Structure of atoms Protons, neutrons, and electrons Electron configurations: shells and subshells Valence states Atoms and the periodic table Types of bonding between atoms Ionic bonding Covalent bonding  Metallic bonding Secondary bonds Permanent dipoles and the hydrogen bond           Temporary dipoles and the van der Waals bond Influence of Bond Type on Engineering Properties Brittle versus ductile behavior Electrical conductivity Melting temperature of polymers

ATOMIC AND MOLECULAR STRUCTURE & BONDING Structure of Atoms Atomic number of an element is equal to the number of electrons or protons in each atom. Atomic mass of an element is equal to the average number of protons and neutrons in the atom. Avogadro number of an element is the number of atoms or molecules in a mole. Atomic mass unit of an element is the mass of an atom expressed as 1/12 the mass of a carbon atom. Quantum numbers are the numbers that assign electrons in an atom to discrete energy levels. A quantum shell is a set of fixed energy levels to which electrons belong. Pauli exclusion principle specifies that no more than two electrons in a material can have the same energy. The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions. Electronegativity describes the tendency of an atom to gain an electron.

Electron Quantum Numbers ATOMIC AND MOLECULAR STRUCTURE & BONDING Electron Quantum Numbers Principle, n:  Allowed values of n are 1, 2, 3… Roughly associated with electron's energy – higher values of n mean larger (less favorable) energies. The value of n also tells how "far" from the nucleus the electron is n is also called the electron shell (n=1 is the first shell, etc.) Angular momentum, l: Allowed values of l are 0 to (n-1) related to angular momentum of the electron in its orbit l is also called the electron orbital within a given shell Each value of l is assigned a letter of the alphabet: Magnetic, m: Allowed values of m are -l to +l  (2l + 1 distinct values) Spin, ms: Allowed values of ms are -1/2 (spin-up) and +1/2 (spin-down). Pauli Exclusion Principle: No two electrons in the same system can share the same four (n, l, m, ms) quantum numbers!

ATOMIC AND MOLECULAR STRUCTURE & BONDING Electron States shell n orbital (0 to n-1) m values (m = -l to l) spin (ms = ±1/2) # electrons in shell 1 0 (1s) 2 0 (2s) 1 (2p) 3 (2px, …) 1x2+3x2= 8 3 0 (3s) 1 (3p) 2 (3d) 3 (3px, …) 5 (3dxy, …) 1x2+3x2+5x= 18 4 0 (4s) 1 (4p) 2 (4d) 3 (4f) 7 (4fxyz,…) 1x2+3x2+5x2+7x2= 32

ATOMIC AND MOLECULAR STRUCTURE & BONDING Orbital Filling Example of Pauli Exclusion Principle no 2 electrons share the same quantum numbers for the same (n,l) at same E.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Summary of Periodic Table Trends

ATOMIC AND MOLECULAR STRUCTURE & BONDING Atomic Bonding Bonding: There are two types of bonds: primary and secondary. Primary bonds are the strongest bonds which hold atoms together. The three types of primary bonds are: Metallic bond, Covalent bond, and Ionic bond Metallic Bonds Elements in groups I and II of the periodic table, and some in group III form metallic crystals. In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. Since the electrons are free to move, they lead to good thermal and electrical conductivity. The metallic bonding does not have the strongly directional character of covalent bonds.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Metallic Bonding electron Ion cores Metallic bonds Bonding energies: range from 68 kJ/mol (0.7 eV/atom) for Hg to 850 kJ/mol (8.8 eV/atom) for W. Melting temperatures: -39 C for Hg and 3410 C for W. Stronger bonds lead to higher melting temperature: atomic scale property  macroscale property.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Ionic Bonding Atoms like to have a filled outer shell of electrons. Sometimes, by transferring electrons from one atom to another, electron shells are filled. The donor atom will take a positive charge, and the acceptor will have a negative charge. The charged atoms or ions will be attracted to each other, and form bonds. The most common examples of ionic crystals are found in the structures of the alkalindhalides, of which the most familiar is ordinary salt, NaCl. This structure in which each ion is surrounded by six nearest neighbors of the other kind is called Face-Centered-Cubic (FCC). The distance between like ions in NaCl crystals is 5.63 Ao

ATOMIC AND MOLECULAR STRUCTURE & BONDING Ionic Bonding Structure Bonding Ionic bond for NaCl Occurs for electronegative (positive) elements that can form close-shell configurations. Electronegativity plays a role here, e.g., H-F. Structure is determined by electrostatic attraction EA = –A/r (Coulomb’s Law) and collapse is prevented by electronic and nuclear repulsion ER = +B/rn. For closed-shell gases (Ar, Kr,…) n~12. Generally need quantum mechanics.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Covalent Bonding The cohesive forces in covalent crystals arise from the sharing of an electron-pair between each two atoms. Some atoms like to share electrons to complete their outer shells. Each pair of shared atoms is called a covalent bond. Covalent bonds are called directional because the atoms tend to remain in fixed positions with respect to each other. As a result, covalent bonds result in poor electrical and thermal conductivity. Examples include diamond, and the O-O and N-N bonds in oxygen and nitrogen gases. Diamond is an example of a crystal whose atoms are linked by covalent bonds. Each carbon atom has four nearest neighbors and shares an electron pair with each of them. Silicon, germanium, and silicon carbide are among the crystals having the same structure as that of diamond.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Covalent Bonding Tetrahedral structure of silica (SiO2)

ATOMIC AND MOLECULAR STRUCTURE & BONDING Hydrogen bond (H2O Dipole) Van der Waals Bonding Example of Permanent Dipole: H-Cl Gas  and H2O   The Cl has a large electronegativity (3.0) compared to H (2.1); hence, Cl grabs the electron away from H. (H-Fl and H20 are other examples of this.) Notably, such "Hydrogen bonds" with permanent dipoles lead to higher melting points and boiling temperatures, more than can be expected from their low molecular weights. Such bonds are crucial for polymer chemistry. Hydrogen bond (H2O Dipole)

ATOMIC AND MOLECULAR STRUCTURE & BONDING Linear Thermal Strain L/L0 =  (T - T0) Larger E Smaller  Smaller E Larger  r0 r E   asymmetry at r0 No asymmetry at r0 No affect on r(T) or V(T)   as E0  (less negative) Parabolic E vs. r shape Volume Thermal Strain V/V0 = V (T - T0) Symmetric well: No expansion possible Atoms just vibrate back and forth!

ATOMIC AND MOLECULAR STRUCTURE & BONDING • Non dense, random packing • Dense, regular packing Dense, regular-packed structures tend to have lower energy.

ATOMIC AND MOLECULAR STRUCTURE & BONDING Atomic Packing Crystalline materials... • atoms pack in periodic, 3D arrays • typical of: -metals -many ceramics -some polymers crystalline SiO2 Noncrystalline materials... • atoms have no periodic packing • occurs for: -complex structures -rapid cooling "Amorphous" = Noncrystalline noncrystalline SiO2

Summary: Bonding, Structure, Properties ATOMIC AND MOLECULAR STRUCTURE & BONDING Summary: Bonding, Structure, Properties Ceramics Large bond energies Ionic and Covalent bonds large Tm, E Small  Metals Varying bond energy Metallic bonding intermediate Tm, E,  Polymers directional properties Covalent and Secondary secondary dominates outcome small Tm, E large 