Periodic Relationships Among the Elements

Transcription

1 Periodic Relationships Among the Elements Chapter 8 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 When the Elements Were Discovered 2

3 ns 1 Ground State Electron Configurations of the Elements d 1 d 5 ns 2 ns 2 np 1 ns 2 np 2 ns 2 np 3 ns 2 np 4 ns 2 np 5 ns 2 np 6 d 10 4f 5f 3

4 Classification of the Elements 4

5 Electron Configurations of Cations and Anions Of Representative Elements Na [Ne]3s 1 Ca [Ar]4s 2 Al [Ne]3s 2 3p 1 Na + [Ne] Ca 2+ [Ar] Al 3+ [Ne] Atoms lose electrons so that cation has a noble-gas outer electron configuration. Atoms gain electrons so that anion has a noble-gas outer electron configuration. H 1s 1 H - 1s 2 or [He] F 1s 2 2s 2 2p 5 F - 1s 2 2s 2 2p 6 or [Ne] O 1s 2 2s 2 2p 4 O 2-1s 2 2s 2 2p 6 or [Ne] N 1s 2 2s 2 2p 3 N 3-1s 2 2s 2 2p 6 or [Ne] 5

6 +1 +2 Cations and Anions Of Representative Elements

7 Isoelectronic: have the same number of electrons, and hence the same ground-state electron configuration Na + : [Ne] Al 3+ : [Ne] F - : 1s 2 2s 2 2p 6 or [Ne] O 2- : 1s 2 2s 2 2p 6 or [Ne] N 3- : 1s 2 2s 2 2p 6 or [Ne] Na +, Al 3+, F -, O 2-, and N 3- are all isoelectronic with Ne What neutral atom is isoelectronic with H -? H - : 1s 2 same electron configuration as He 7

8 Electron Configurations of Cations of Transition Metals When a cation is formed from an atom of a transition metal, electrons are always removed first from the ns orbital and then from the (n 1)d orbitals. Fe: [Ar]4s 2 3d 6 Fe 2+ : [Ar]4s 0 3d 6 or [Ar]3d 6 Mn: [Ar]4s 2 3d 5 Mn 2+ : [Ar]4s 0 3d 5 or [Ar]3d 5 Fe 3+ : [Ar]4s 0 3d 5 or [Ar]3d 5 8

9 EXAMPLE 8.1 An atom of a certain element has 15 electrons. Without consulting a periodic table, answer the following questions: (a) What is the ground-state electron configuration of the element? (b) How should the element be classified? (c) Is the element diamagnetic or paramagnetic? Strategy (a) We refer to the building-up principle discussed in Section 7.9 and start writing the electron configuration with principal quantum number n 5 1 and continuing upward until all the electrons are accounted for. (b) What are the electron configuration characteristics of representative elements? transition elements? noble gases? (c) Examine the pairing scheme of the electrons in the outermost shell. What determines whether an element is diamagnetic or paramagnetic? Solution (a) We know that for n 5 1 we have a 1 s orbital (2 electrons); for n 5 2 we have a 2 s orbital (2 electrons) and three 2 p orbitals (6 electrons); for n 5 3 we have a 3 s orbital (2 electrons). The number of electrons left is and these three electrons are placed in the 3 p orbitals. The electron configuration is 1 s 2 2 s 2 2 p 6 3 s 2 3 p 3. (b) Because the 3 p subshell is not completely filled, this is a representative element. Based on the information given, we cannot say whether it is a metal, a nonmetal, or a metalloid. (c) According to Hund s rule, the three electrons in the 3 p orbitals have parallel spins (three unpaired electrons). Therefore, the element is paramagnetic. Check For (b), note that a transition metal possesses an incompletely filled d subshell and a noble gas has a completely filled outer shell. For (c), recall that if the atoms of an element contain an odd number of electrons, then the element must be paramagnetic. Practice Exercise An atom of a certain element has 20 electrons. (a) Write the ground-state electron configuration of the element, (b) classify the element, (c) determine whether the element is diamagnetic or paramagnetic. 9

10 The shielding Effect and Effective Nuclear Charge تأثير الحجب في الذرات متعددة اإللكترونات وشحنة النواة المؤثرة In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons. The nuclear charge that an electron experiences depends on both factors. 2009, Prentice-Hall, Inc.

11 The shielding Effect and Effective Nuclear Charge The effective nuclear charge, Z eff, is found this way: Z eff = Z S where Z is the atomic number and S is a )الجزء من شحنة screening constant,النواة المحجوب عن جذب الكترونات التكافؤ ) usually close to the number of inner electrons. 2009, Prentice-Hall, Inc.

12 Effective nuclear charge (Z eff ) is the positive charge felt by an electron. Z eff = Z - s 0 < s < Z (s = shielding constant) Z eff Z number of inner or core electrons Z Core Z eff Radius (pm) Na Mg Al Si

13 Effective Nuclear Charge (Z eff ) 13

14 What Is the Size of an Atom? The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

15 Atomic Radii metallic radius covalent radius 15

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17 Sizes of Atoms Bonding atomic radius tends to decrease from left to right across a row (due to increasing Z eff ). increase from top to bottom of a column (due to increasing value of n). Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

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19 Trends in Atomic Radii 19

20 Comparison of Atomic Radii with Ionic Radii 20

21 Cation is always smaller than atom from which it is formed. Anion is always larger than atom from which it is formed. 21

22 The Radii (in pm) of Ions of Familiar Elements 22

23 Sizes of Ions Ionic size depends upon: The nuclear charge. The number of electrons. The orbitals in which electrons reside. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

24 Sizes of Ions Cations are smaller than their parent atoms. The outermost electron is removed and repulsions between electrons are reduced. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

25 Sizes of Ions Anions are larger than their parent atoms. Electrons are added and repulsions between electrons are increased. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

26 Sizes of Ions Ions increase in size as you go down a column. This is due to increasing value of n. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

27 Sizes of Ions In an isoelectronic series, ions have the same number of electrons. Ionic size decreases with an increasing nuclear charge. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

28 EXAMPLE 8.2 Referring to a periodic table, arrange the following atoms in order of increasing atomic radius: P, Si, N. Strategy What are the trends in atomic radii in a periodic group and in a particular period? Which of the preceding elements are in the same group? in the same period? Solution From Figure 8.1 we see that N and P are in the same group (Group 5A). Therefore, the radius of N is smaller than that of P (atomic radius increases as we go down a group). Both Si and P are in the third period, and Si is to the left of P. Therefore, the radius of P is smaller than that of Si (atomic radius decreases as we move from left to right across a period). Thus, the order of increasing radius is N, P, Si. Practice Exercise Arrange the following atoms in order of decreasing radius: C, Li, Be. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

29 Chemistry in Action: The 3 rd Liquid Element? 117 elements, 2 are liquids at 25 0 C Br 2 and Hg 223 Fr, t 1/2 = 21 minutes Liquid? 29

30 Ionization energy is the minimum energy (kj/mol) required to remove an electron from a gaseous atom in its ground state. I 1 + X (g) X + (g) + e - I 2 + X + (g) X 2 + (g) + e - I 3 + X 2+ (g) X 3 + (g) + e - I 1 first ionization energy I 2 second ionization energy I 3 third ionization energy I 1 < I 2 < I 3 30

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32 Trends in First Ionization Energies Generally, as one goes across a row, it gets harder to remove an electron. As you go from left to right, Z eff increases. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

33 Trends in First Ionization Energies However, there are two apparent discontinuities in this trend. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

34 Trends in First Ionization Energies The first occurs between Groups IIA and IIIA. In this case the electron is removed from a p-orbital rather than an s-orbital. The electron removed is farther from nucleus. There is also a small amount of repulsion by the s electrons. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

35 Trends in First Ionization Energies The second occurs between Groups VA and VIA. The electron removed comes from doubly occupied orbital. Repulsion from the other electron in the orbital aids in its removal. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

36 Increasing First Ionization Energy General Trends in First Ionization Energies Increasing First Ionization Energy 36

37 Electron affinity is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion. X (g) + e - X - (g) F (g) + e - F - (g) DH = -328 kj/mol EA = +328 kj/mol O (g) + e - O - (g) DH = -141 kj/mol EA = +141 kj/mol 37

38 Trends in Electron Affinity In general, electron affinity becomes more exothermic as you go from left to right across a row. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

39 Trends in Electron Affinity There are again, however, two discontinuities in this trend. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

40 Trends in Electron Affinity The first occurs between Groups IA and IIA. The added electron must go in a p-orbital, not an s-orbital. The electron is farther from nucleus and feels repulsion from the s-electrons. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

41 Trends in Electron Affinity The second occurs between Groups IVA and VA. Group VA has no empty orbitals. The extra electron must go into an already occupied orbital, creating repulsion. Periodic Properties of the Elements 2009, Prentice-Hall, Inc.

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43 Variation of Electron Affinity With Atomic Number (H Ba) 43

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45 Diagonal Relationships on the Periodic Table 45

46 Increasing reactivity Group 1A Elements (ns 1, n 2) M M e - 2M (s) + 2H 2 O (l) 4M (s) + O 2(g) 2MOH (aq) + H 2(g) 2M 2 O (s) 46

47 Group 1A Elements (ns 1, n 2) 47

48 Increasing reactivity Group 2A Elements (ns 2, n 2) M M e - Be (s) + 2H 2 O (l) Mg (s) + 2H 2 O (g) M (s) + 2H 2 O (l) No Reaction Mg(OH) 2(aq) + H 2(g) M(OH) 2(aq) + H 2(g) M = Ca, Sr, or Ba 48

49 Group 2A Elements (ns 2, n 2) 49

50 Group 3A Elements (ns 2 np 1, n 2) 4Al (s) + 3O 2(g) 2Al (s) + 6H + (aq) 2Al 2 O 3(s) 2Al 3+ (aq) + 3H 2(g) 50

51 Group 3A Elements (ns 2 np 1, n 2) 51

52 Group 4A Elements (ns 2 np 2, n 2) Sn (s) + 2H + (aq) Pb (s) + 2H + (aq) Sn 2+ (aq) + H 2 (g) Pb 2+ (aq) + H 2 (g) 52

53 Group 4A Elements (ns 2 np 2, n 2) 53

54 Group 5A Elements (ns 2 np 3, n 2) N 2 O 5(s) + H 2 O (l) 2HNO 3(aq) P 4 O 10(s) + 6H 2 O (l) 4H 3 PO 4(aq) 54

55 Group 5A Elements (ns 2 np 3, n 2) 55

56 Group 6A Elements (ns 2 np 4, n 2) SO 3(g) + H 2 O (l) H 2 SO 4(aq) 56

57 Group 6A Elements (ns 2 np 4, n 2) 57

58 Increasing reactivity Group 7A Elements (ns 2 np 5, n 2) X + 1e - X - 1 X 2(g) + H 2(g) 2HX (g) 58

59 Group 7A Elements (ns 2 np 5, n 2) 59

60 Group 8A Elements (ns 2 np 6, n 2) Completely filled ns and np subshells. Highest ionization energy of all elements. No tendency to accept extra electrons. 60

61 Compounds of the Noble Gases A number of xenon compounds XeF 4, XeO 3, XeO 4, XeOF 4 exist. A few krypton compounds (KrF 2, for example) have been prepared. 61

62 Comparison of Group 1A and 1B The metals in these two groups have similar outer electron configurations, with one electron in the outermost s orbital. Chemical properties are quite different due to difference in the ionization energy. Lower I 1, more reactive 62

63 Properties of Oxides Across a Period basic acidic 63

64 Chemistry in Action: Discovery of the Noble Gases Sir William Ramsay 64