02 Structure of Materials

What is the atomic weight?

• The weighted average of the atomic masses of the atom’s naturally occurring isotopes

What is the heaviest natural occuring element?

• Uranium (92)

What represents the atomic number?

• The number of protons in nucleus

What are isotopes?

• Species with the same number of protons but differing number of neutrons

• Same name, different mass

What happens with the energy of electrons?

• Is quantified and defines the electron’s position in an energy state (Bohr) or in a shell (Schrödinger)

What is the implication of energy absorption of an electron?

• Jump to a higher energy level (more outer orbitals)

What is the implication of energy emission of an electron?

• Jump to a lower energy level (more inner orbitals)

What is the main difference between the Bohr and the wave-mechanical atomic models?

• Bohr: discrete energy levels (and electrons)

• Wave-mechanical: probabilistic distribution of energy levels (and electrons)

What are quantum numbers used for?

• To specify size, shape and spatial orientation of an electron’s probability density

What do shells describe?

• Distance of an electron from the nucleus (position)

How are shells specified?

• By quantum number n = 1, 2, 3, 4, 5 …

• Letters K, L, M, N, O related to each numbers (respectively).

What do subshells describe?

• Shape of the electron subshell

How are subshells specified?

• s, p, d, or f (lowercase)

What do the other two quantum numbers describe?

• Spatial orientation of the electron orbital angular momentum

• Magnetic spin of an electron

What is the electron configuration?

• How electrons are distributed within the orbitals of an atom

Pauli Exclusion Principle

• Each electron state can hold no more than two electrons, which must have opposite spins

How many electrons can each subshell acommodate?

• s: 2

• p: 6

• d: 10

• f: 14

What are valence electrons?

• Electrons on the outermost filled shell

• Participants of bonding

What is the electronegativity? What is its trend behavior?

• The attraction of the atomic nucleus to an additional electron

• Increses moving from left to right and from bottom to top in the periodic table

• The higher the electronegativity, the proner the element is to accept electrons (the more reactive)

What is the most electronegative element? What is the less?

• Most: Fluorine

• Less: Francium

Which kinds of bonds are there? (4)

• Ionic

• Covalent

• Metallic

• Van der Waals (secondary, i.e. between atoms of different molecules)

Describe the interaction between two atoms in the sphere-spring model

• Spring between atoms

• Magnitude of force as a function of the interatomic distance

• Attraction forces with a + sign (F_A) due to atomic bond

• Repulsive forces with a - sign (F_R) due to electromagnetic charges

• Net force F_N = F_A + F_R

• Both attractive and repulsive force decay with the distance

How is the energy of a bond calculated?

• Integral of F_N wrt. interatomic distance r

• Positive for repulsion

• Negative for attraction

• Equilibrium achieved for F_N = 0 and minimum energy

How is the bonding energy related to the melting temperature?

• The larger the bonding energy, the larger the melting temperature

How is the elastic modulus E related to the bonding energy?

• The larger the bonding energy, the larger the E modulus

Describe how ionic bonds are formed

• More electronegative atom attracts electrostatically the valence electron(s) of the less electronegative atom

• For differences in electronegativity > 1.8

• Ex: ionic salts, metallic-non metallic bonds

What are the main characteristics of ionic compounds?

• Create regular lattice structures (crystalline structures)

• Hard and brittle

• Electrically and thermally insulative

Describe how are covalent bonds are formed

• Each of the two atoms share at least one valence electron to the bond

• Stable electron configuration assumed

• Directional bonds, belonging only to sharing atoms

• For an atom with N’ valence electrons, it can bond with at most 8-N’ atoms

• For differences in electronegativity < 1.7, both with electronegativities larger than 1.5

What are the main characteristics of ionic compounds?

• Usually stable

• Orientation and distance of atoms fix defined for covalent bonds

• Ex: crystalline Ge, Si, C; methan; water; polymers

Describe how are covalent bonds are formed

• Sea/cloud of valence electrons (electron gas)

• Positive charged atom cores forming lattice structures (cubic space centered, cubic face centered, hexagonal) depending on the type of atoms

What are the main characteristics of ionic compounds?

• Good electrical and thermal conductivities

• Good deformability between lattice planes

• Chemical reactivity

• (Generally) weaker than covalent and ionic bond (lower hardness compared to ceramics)

How are Van der Waals bonds formed? What are their characteristics?

• Result from coulombic attraction between positive end of a dipole and the negative end of another (dipole usually formed by covalent bonds)

• Could be formed by symmetrical or induced dipoles

• Keep together already formed molecules or chains

SQ: Name the two atomic models cited, and note the difference between them

• Bohr atomic model and wave-mechanical atomic model

• Difference:

  • Bohr: asummes electrons located at well defined orbits

  • Wave-mechanical: assumes electrons whose position is not well-defined in atomic orbitals. Position as a probability distribution.

SQ: Describe the important quantum-mechanical principle that relates to electron energies

• Energy of electrons is quantized in discrete values

• An energy change (absorption or emission) leads to a change of orbital (outer for absorption, lower for emission)

SQ: Schematically plot attractive, repulsive, and net energies versus interatomic separation for two atoms or ions

• Everything goes to 0 with an increase of r

• Net energy is a sum of the attractive energy and the repulsive energy

• Attractive energy with - sign

• Repulsive energy with + sign

SQ: Briefly describe ionic, covalent, metallic, and secondary (van der Waals or hydrogen) bonds

• Ionic: stolen atom for ΔEn (difference of electronegativity) > 1.8. Leads to strong, brittle, insulative, crystalline materials

• Covalent: shared electrons for ΔEn < 1.7. High stability (hardness) and fixed spatial position

• Metallic: sea/cloud of electrons with positive charged cores forming lattice structures. Form conductive, ductile, reactive compounds

• Van der Waals: coulombic attraction between symmetric or induced dipoles of already formed molecules/ chains (secondary bond)

SQ: Note what materials exhibit each of these bonding types

• Ionic: NaCl, MgO, etc.

• Covalent: Ge, Si, C

• Metallic: iron, mercury, wolfram, some alloys

• Van der Waals: polymers