Periodic Relationships

Transcription

1 Periodic Relationships 1

2 Tabulation of Elements Mendeleev (1869) Arranged by mass Tabulation by chem.& physical properties Predicted missing elements and properties 2

3 Modern Periodic Table Argon vs. potassium problem. Now ordered by atomic number, not mass. 3

4 Periodic Table Representative elements (1,2,13-17) -incomplete s or p subshells Noble gases (18) -filled p subshells (except He) -ns 2 np 6 4

5 Periodic Table Transition elements (groups 3-11) -incompletely filled d subshell -d block Group 12 Lanthanides & actinides -incompletely filled f subshells -f block 5

6 Groups Similar properties of a group result from similar electron configuration. e.g. Group 1: ns 1 Valence e - Group 2 : ns 2 Group 17: ns 2 np 5 (e - in highest n ) Valence e - : key to bonding; determines chemical properties. 6

7 Element Song 7

8 Writing Free Elements Metals: use empirical formula (Fe) H 2, N 2, O 2, F 2, Cl 2, I 2 : diatomic Phosphorus : use P 4 Carbon: use empirical formula, C Sulfur (S 8 ) use S Metalloids: use empirical formula 8

9 e - Configuration of Ions Representative Elements: ions have noble gas e - configuration Na: [Ne]3s 1 Na + [Ne] Ca: [Ar]4s 2 Ca 2+ [Ar] F: 1s 2 2s 2 2p 5 F - [Ne] Note: F - and Na + are isoelectronic 9

10 Transition Element Cations Aufbau: e - fill ns prior to (n-1)d but Ion: ns removed before (n-1)d Mn: [Ar]4s 2 3d 5 Mn 2+ : [Ar]3d 5. WEIRD. valence e - removed first 10

11 Transition Metal Ions ns removed before (n- 1)d is one reason that transition elements are similar. Fe 2+, Mn 2+, Co 2+ 11

12 Periodic Trends 1. Atomic radius 2. Ionic radius 3. Ionization energy 4. Electronegativity Caution: Atoms are not anthropomorphic. 12

13 Atomic Radius: Trends 13

14 Atomic Radius Down a group: electrons are placed in higher energy levels (farther from the nucleus). Radius increases down a group. 14

15 Shielding Li atom: 1s 2 2s shield outer e - from protons 15

16 Effective Nuclear Charge Due to shielding, nuclear charge (Z) felt by valence e - is reduced. Z eff = Z s where s is the shielding constant, roughly the number of inner shell e - For Li: Z eff = Z s = 3 ~2 = ~1 16

17 Shielding: Coulomb s Law e - Z eff F = q 1 q 2 r 2 Since Z eff (q 2 ) is less than Z, the force attracting the e - is less. 17

18 Shielding by Inner e - Li atom has Z eff less than +3 So Li is larger than expected. 18

19 Atomic Radius: Trends Across row: decreasing size Z increases & number of shielding e - constant (only inner shell e - shield) Li - vs Be (>Z eff )

20 Atomic Radius Model 20

21 Atomic Radius Try It: Arrange these atoms in order of increasing size. N, O, P, S 21

22 Ionic Radius: Trends Cation is much smaller than its atom. (loss of e - with same Z eff ) Anion is much larger than its atom. (gain of e - with same Z eff ) Li + F Li + F - 22

23 Ionic Radii (model) Compare isoelectronic ions Na + & F - 23

24 Ionic Radii: Comparisons For isoelectronic ions: Al 3+ Mg 2+ Na + = [Ne] Larger (WHY?) N 3- O 2- F - Smaller WHY? = [Ne] 24

25 Ionic Radii Place in order of increasing size? Fe, Fe 2+, and Fe 3+ 25

26 Ionic Radii Try It: Arrange these ions in order of increasing size. O -2, F -, Na +, Mg

27 Ionization Energy Chemical properties are determine by the valence electrons. Ionization energy: energy (kj/mol) to remove e - from gaseous atom in ground state. If IE high, e - held tightly. 27

28 Ionization Energy I 1 energy + X(g) X + (g) + e - endo- or exothermic? I 2 energy + X + (g) X 2+ (g) + e - I 3 energy + X 2+ (g) X 3+ (g) + e - 28

29 Ionization Energy IE always endothermic (+) (energy put in to remove e - ) I 1 < I 2 < I 3 Why? Coulomb s law I explains Na +, Mg 2+, etc. (after Na loses 1 e -, the number of shielding e - drops from 8 to 2) 29

30 Ionization Energies explain the electron shell model of the atom 30

31 Measuring Ionization Energy (photoelectric effect (text section 7.2) X-Ray Photoelectron Spectroscopy (PES) X-ray ejected e - photon Energy of photon (hn) measures the energy needed to eject e - (ionization energy) 31

32 PES PES electron values reveal the atomic shell and subshell energies. Li shows one e - with low and two with higher PES values 32

33 Number of Electrons PES of Magnesium Energy of X-ray photon [Mg] = 1s 2 2s 2 2p 6 3s 2 33

34 I 1 vs. Z I 1 I 1 across Period down Group I 1 34

35 Ionization Energy Trends Across a row (I 1 increases): Explain I in terms of shielding and Z. 1 eff Na vs. Mg Down a group (I 1 decreases): Explain I in terms of energy levels (n). 1 Na vs. K Explain both using Coulomb s law. 35

36 I 1 Irregularities: Be vs. B Be: 899 kj/mol 2s 2 B: 801 kj/mol 2s 2 2p 1 Partial Shielding: 2s (lower energy) slightly shields 2p. 36

37 I 1 Irregularities: N vs. O N: 1400 kj/mol 2s 2 2p 3 O: 1314 kj/mol 2s 2 2p 4 e - repulsion: O is lower than N due to e - repulsion in same orbital. 37

38 Ionization Energy Which has smaller I 1 and why? O or S Al or Si P or S Which has smaller I 2 and why? Li + or Be + 38

39 Electronegativity A relative scale to measure the ability of an atom to attract bonding electrons. Trend parallels ionization energy. 39

40 Electronegativity High EN: F (4.0) O (3.5) Low EN: Na (0.9) Ba (0.9) 40

41 Metallic Character Metals lose e - to become cations. Which element is the most metallic? (smallest ionization energy) Nonmetals gain e - to become anions. Which element is the least metallic? (largest ionization energy) 41

42 Especially note: trends and why shielding characteristic reactions Section