Elements of Materials Science and Engineering ChE 210 Chapter 2

Chapter Outline Individual atoms and ions Molecules Macromolecules Three dimensional bonding Interatomic distances Generalization based on atomic bonding Understanding of interatomic bonding is the first step towards understanding/explaining materials properties

Individual atoms and ions Atoms = nucleus (protons and neutrons) + electrons Charges: Electrons and protons have negative and positive charges of the same magnitude, 1.6 × 10-19 Coulombs. Neutrons are electrically neutral. Masses: Protons and Neutrons have the same mass, 1.67 × 10-27 kg. Mass of an electron is much smaller, 9.11 × 10-31 kg and can be neglected in calculation of atomic mass. # protons gives chemical identification of the element # protons = atomic number (Z) # neutrons defines isotope number The atomic mass (A) = mass of protons + mass of neutrons

Atomic mass units. Atomic weight. The atomic mass unit (amu) is often used to express atomic weight. 1 amu is defined as 1/12 of the atomic mass of the most common isotope of carbon atom that has 6 protons (Z=6) and six neutrons (N=6). Mproton ≈ Mneutron = 1.66 x 10-24 g = 1 amu. The atomic mass of the 12C atom is 12 amu. The atomic weight of an element = weighted average of the atomic masses of the atoms naturally occurring isotopes. Atomic weight of carbon is 12.011 amu. The atomic weight is often specified in mass/mole.

Atomic mass units. Atomic weight. A mole is the amount of matter that has a mass in grams equal to the atomic mass in amu of the atoms (A mole of carbon has a mass of 12 grams). The number of atoms in a mole is called the Avogadro number, Nav = 6.023 × 1023. Nav = 1 gram/1 amu. Example: Atomic weight of iron = 55.85 amu/atom = 55.85 g/mol

Calculations Diamond (carbon):  = 3.5 g/cm3, M = 12 g/mol The number of atoms per cm3, n, for material of density  (g/cm3) and atomic mass M (g/mol): n = Nav x  / M Graphite (carbon):  = 2.3 g/cm3, M = 12 g/mol n = 6×1023 atoms/mol × 2.3 g/cm3/12 g/mol = 11.5 x 1022 atoms/cm3 Diamond (carbon):  = 3.5 g/cm3, M = 12 g/mol n = 6×1023 atoms/mol x 3.5 g/cm3/12 g/mol = 17.5 x1022 atoms/cm3 Water (H2O) d = 1 g/cm3, M = 18 g/mol n = 6×1023 molecules/mol x 1 g/cm3/18 g/mol = 3.3 x 1022 molecules/cm3

Calculations For material with n = 6 × 1022 atoms/cm3 we can calculate the mean distance between atoms L = (1/n)1/3 = 0.25 nm. ! the scale of atomic structures in solids – a fraction of 1 nm or a few A.

The Periodic Table Elements in the same column (Elemental Group) share similar properties. Group number indicates the number of electrons available for bonding. 0: Inert gases (He, Ne, Ar...) have filled subshells: chem. Inactive IA: Alkali metals (Li, Na, K…) have one electron in outermost occupied s subshell - eager to give up electron – chem. Active VIIA: Halogens (F, Br, Cl...) missing one electron in outermost occupied p shell - want to gain electron - chem. active

Periodic Table - Electronegativity Electronegativity - a measure of how willing atoms are to accept electrons Subshells with one electron - low electronegativity. Subshells with one missing electron –high electronegativity. Electronegativity increases from left to right Metals are electropositive – they can give up their few valence electrons to become positively charged ions

Bonding Energies and Forces

Bonding Energies and Forces The repulsion between atoms, when they are brought close to each other, is related to the Pauli principle: when the electronic clouds surrounding the atoms starts to overlap, the energy of the system increases abruptly. The origin of the attractive part, dominating at large distances, depends on the particular type of bonding.

Bond Angles Hybridized orbitals are formed by linear combination of pure atomic orbitals The type of hybridization determines the bond angle in the molecule: s + 3p 4 Sp3 BA = 109.28o s + 2p  3 sp2 BA = 120o s + p  2 sp2 BA = 180o

In symmetrical molecules, e.g. CH4 Atoms are equally spaced around the central atom. ~ placing the carbon at the cube centre and pointing the orbitals towards four of the 8 corners. Distortion occurs if some of electrons are present as lone pair, e.g. H2O and NH3

Delocalized electrons  bond is a single covalent bond and is formed by sharing of two electrons    Not all the valence electrons are localized e.g. benzene

Example 2-2.1, p 30 How much energy is required or released if 2.6 Kg of acetylene C2H2 react with hydrogen to form C2H4? One triple and one H—H bond are eliminated Two C—H and one double bonds are formed H—H 435 KJ/mol C—H 370 KJ/mol C=C 680 KJ/mol C‬≡C 890 KJ/mol

Example 2-2.2 p 31 How many delocalized electrons in naphthalene molecule How many wave patterns (electron states Formula: C10H8 Total valence electrons = (4/C)(10 C) + (1/H)(8 H) = 48 Electrons in bonds: C—H 8x2 = 16 C—C 11x2 = 22 Reminder = 48-(16+22)= 10 delocalized electrons Electron states = number of carbon atoms

Macromolecules (polymers) Large number of small repeated units called mers High molecular weight compounds Natural and synthetic Linear or branched Thermoplastic or thermosetting Melting range and not melting points No boiling points

Linear Molecules Linear polymeric chains such as polyvinyl chloride (PVC) n = degree of polymerization: the number of mers per molecule