Transcript Lecture19.f
Lecture 19: Periodic Trends • Reading: Zumdahl 12.14-12.16 • Outline – Periodic Trends • Ionization Energy, Electron Affinity, and Radii – A Case Example The Aufbau Principal (cont.) • Lithium (Z = 3) 1s22s1 1s 2s • Berillium (Z = 4) 2p 1s22s2 1s 2s 2p • Boron (Z = 5) 1s22s22p1 1s 2s 2p The Aufbau Principal (cont.) • Carbon (Z = 6) 1s22s22p2 1s 2s 2p Hund’s Rule: Lowest energy configuration is the one in which the maximum number of unpaired electrons are distributed amongst a set of degenerate orbitals. • Nitrogen (Z = 7) 1s22s22p3 1s 2s 2p The Aufbau Principal (cont.) • Oxygen (Z = 8) 1s22s22p4 1s • Fluorine (Z = 9) 2s 2p 1s22s22p5 1s 2s 2p • Neon (Z = 10) 1s22s22p6 1s 2s 2p full The Aufbau Principal (cont.) • This orbital filling scheme gives rise to the modern periodic table. The Aufbau Principal (cont.) • After Lanthanum ([Xe]6s25d1), we start filling 4f. The Aufbau Principal (cont.) • After Actinium ([Rn]7s26d1), we start filling 5f. The Aufbau Principal (cont.) • Heading on column given total number of valence electrons. Periodic Trends • The valence electron structure of atoms can be used to explain various properties of atoms. • In general, properties correlate down a group of elements. • A warning: such discussions are by nature very generalized…exceptions do occur. Periodic Trends: Ionization • If we put in enough energy, we can remove an electron from an atom. (Z-1)- ZEnergy +Z +Z e- • The electron is completely “removed” from the atom (potential energy = 0). Periodic Trends: Ionization • Generally done using photons, with energy measured in eV (1 eV = 1.6 x 10-19 J). • The greater the propensity for an atom to “hold on” to its electrons, the higher the ionization potential will be. • Koopmans’ Theorem: The ionization energy of an electron is equal to the energy of the orbital from where the electron came. Periodic Trends: Ionization • One can perform multiple ionizations: Al(g) Al+(g) + e- I1 = 580 kJ/mol first Al+(g) Al2+(g) + e- I2 = 1815 kJ/mol second Al2+(g) Al3+(g) + e- I3 = 2740 kJ/mol third Al3+(g) Al4+(g) + e- I4 = 11,600 kJ/mol fourth Periodic Trends: Ionization • First Ionization Potentials: Column 8 Column 1 Periodic Trends: Ionization • First Ionization Potentials: • Increases as one goes from left to right. • Reason: increased Z+ • Decrease as one goes down a group. • Reason: increased distance from nucleus Periodic Trends: Ionization • Removal of valence versus core electrons Na(g) Na+(g) + e- (removing “valence” electron) [Ne]3s1 [Ne] Na+(g) Na2+(g) + e- [Ne] I1 = 495 kJ/mol I2 = 4560 kJ/mol 1s22s22p5 (removing “core” electron) • Takes significantly more energy to remove a core electron….tendency for core configurations to be energetically stable. Periodic Trends: Electron Affinity • Electron Affinity: the energy change associated with the addition of an electron to a gaseous atom. eZ- (Z+1)- +Z +Z Energy Periodic Trends: Electron Affinity • We will stick with our thermodynamic definition, with energy released being a negative quantity. Wow! Periodic Trends: Electron Affinity • Elements that have high electron affinity: • Group 7 (the halogens) and Group 6 (O and S specifically). Periodic Trends: Electron Affinity • Some elements will not form ions: N? • Orbital configurations can explain both observations. Periodic Trends: Electron Affinity • Why is EA so great for the halogens? F(g) + e1s22s22p5 F-(g) 1s22s22p6 EA = -327.8 kJ/mol [Ne] • Why is EA so poor for nitrogen? N(g) + e1s22s22p3 N-(g) EA > 0 (unstable) 1s22s22p4 (e- must go into occupied orbital) Periodic Trends: Electron Affinity • How do these arguments do for O? O(g) + e1s22s22p4 O-(g) 1s22s22p5 EA = -140 kJ/mol Bigger Z+ overcomes e- repulsion. • What about the second EA for O? O-(g) + e1s22s22p5 O2-(g) EA > 0 (unstable) 1s22s22p6 [Ne] configuration, but electron repulsion is just too great. Atomic Radii • Atomic Radii are defined as the covalent radii, and are obtained by taking 1/2 the distance of a bond: r = atomic radius Atomic Radii • Decrease to right due due increase in Z+ • Increase down column due to population of orbitals of greater n. Looking Ahead • We can partition the periodic table into general types of elements. Metals: tend to give up enon-Metals: tend to gain e- Metalloids: can do either