Development of Periodic Table

Transcription

1 Development of Table in the same group generally have similar chemical properties. are not identical, however.

2 Development of Table Dmitri Mendeleev and Lothar Meyer independently came to the same conclusion about how elements should be grouped.

3 Development of Table Mendeleev, for instance, predicted the discovery of germanium (which he called ekasilicon) as an element with an atomic weight between that of zinc and arsenic, but with chemical properties similar to those of silicon.

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6 The atomic number depends on the number of protons in the nucleus, while the atomic weight depends (mainly) on the number of both protons and neutrons in the nucleus.

7 Trends In this chapter, we will rationalize observed trends in Sizes of atoms and ions. Ionization energy. Electron affinity.

8 Effective Nuclear Charge 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.

9 The attraction of an electron to the nucleus depends on The charge nucleus The distance the electron is from the nucleus. In large atoms, the electrons in outer shells do not experience the full charge nucleus because inner shells of electrons shield them from the nucleus.

10 Effective Nuclear Charge The effective nuclear charge, Zeff, is found this way: Zeff = Z S where Z is the atomic number and S is a screening constant, usually close to the number of inner electrons.

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13 The 2p electron of a Ne atom.

14 In an germanium atom, which electron will experience the greatest effective nuclear charge? An electron in a 1. 2s orbital 2. 2p orbital 3. 3s orbital 4. 3p orbital 5. 3d orbital

15 In an germanium atom, which electron will experience the greatest effective nuclear charge? An electron in a 1. 2s orbital 2. 2p orbital 3. 3s orbital 4. 3p orbital 5. 3d orbital

16 The effective nuclear charge experienced by 3p electrons in phosphorus is

17 Correct Answer: The effective nuclear charge is given by the equation: Zeff = Z S where Z represents number of protons in the nucleus and S represents the average number of electrons between the nucleus and the electron in question. Here: Zeff = = +5

18 Sizes of Atoms It is difficult to define atomic radii for single atoms because the outside electron cloud is ambiguous.

19 Sizes of Atoms The non-bonding atomic radius is defined as onehalf distance between two atoms when they collide.

20 Sizes of Atoms The bonding atomic radius is defined as one-half distance between covalently bonded nuclei.

21 Sizes of Atoms Adding two bonding radii together gives a useful estimate of bond length.

22 Sizes of Atoms Bonding atomic radius tends to decrease from left to right across a row due to increasing Zeff. increase from top to bottom of column due to increasing value of n

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24 No

25 Order the following according to increasing atomic radius. Ge Si Se Cl Ge < Si < Se < Cl Se < Si < Ge < Cl Si < Cl < Ge < Se Cl < Si < Se < Ge Si < Ge < Se < Cl

26 Order the following according to increasing atomic radius. Ge Si Se Cl Ge < Si < Se < Cl Se < Si < Ge < Cl Si < Cl < Ge < Se Cl < Si < Se < Ge Si < Ge < Se < Cl

27 Based on atomic radii, which following bonds would be expected to be the shortest? H H H F F F Cl Cl

28 Correct Answer: H H H F F F Cl Cl The shortest bonding atomic radius belongs to the H atom, so an H2 molecule will have the shortest H X (where X is any atom) bond distance.

29 Sizes of Ions Ionic size depends upon: Nuclear charge. Number of electrons. Orbitals in which electrons reside.

30 Sizes of Ions Cations are smaller than their parent atoms. The outermost electron is removed and repulsions are reduced.

31 Sizes of Ions Anions are larger than their parent atoms. Electrons are added and repulsions are increased.

32 Sizes of Ions Ions increase in size as you go down a column. Due to increasing value of n.

33 Sizes of Ions In an isoelectronic series, ions have the same number of electrons. Ionic size decreases with an increasing nuclear charge.

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35 Order the following according to increasing atomic/ionic radius. N3- Li+ C O C < Li+ < O2- < N3N3- < O2- < C < Li+ Li+ < C < N3- < O2Li+ < C < N3- < O2Li+ < C < O2- < N3-

36 Order the following according to increasing atomic/ionic radius. N3- Li+ C O C < Li+ < O2- < N3N3- < O2- < C < Li+ Li+ < C < N3- < O2Li+ < C < N3- < O2Li+ < C < O2- < N3-

37 Which is larger: Na+ or Na? Na+ Na

38 Correct Answer: Na+ Na Both have the same number of protons, but Na with one more electron will be larger.

39 Which is larger: Cl or Cl? Cl Cl

40 Correct Answer: Cl Cl Both have the same number of protons, but Cl with one more electron will be larger.

41 Lecture 7b- Sections

42 Ionization Energy Amount of energy required to remove an electron from the ground state of a gaseous atom or ion. First ionization energy is that energy required to remove first electron. Second ionization energy is that energy required to remove second electron, etc.

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44 the photoelectric effect

45 Ionization Energy It requires more energy to remove each successive electron. When all valence electrons have been removed, the ionization energy shows a HUGE increase.

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48 I2 for the carbon atom is greater.

49 Trends in First Ionization Energies As one goes down a column, less energy is required to remove the first electron. For atoms in the same group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.

50 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, Zeff increases. And the electron s distance from the nucleus decreases.

51 Trends in First Ionization Energies However, there are two apparent discontinuities in this trend.

52 Trends in First Ionization Energies The first occurs between Groups IIA and IIIA. Electron removed from p-orbital rather than sorbital Electron farther from nucleus Small amount of repulsion by s electrons.

53 Trends in First Ionization Energies The second occurs between Groups VA and VIA. Electron removed comes from doubly occupied orbital. Repulsion from other electron in orbital helps in its removal.

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55 Which will have the highest ionization energy? C N O Al Si

56 Which will have the highest ionization energy? C N O Al Si

57 Which will be the largest? I = ionization energy I1 of Na I2 of Na I1 of Mg I2 of Mg I3 of Mg

58 Which will be the largest? I = ionization energy I1 of Na I2 of Na I1 of Mg I2 of Mg I3 of Mg

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60 They have the same electron configuration: 3 [Ar]3d.

61 Electron Affinity Energy change accompanying addition of electron to gaseous atom: Cl + e Cl

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64 They are equal in magnitude and opposite in sign.

65 Trends in Electron Affinity In general, electron affinity becomes more exothermic as you go from left to right across a row.

66 Trends in Electron Affinity There are again, however, two discontinuities in this trend.

67 Trends in Electron Affinity The first occurs between Groups IA and IIA. Added electron must go in p-orbital, not sorbital. Electron is farther from nucleus and feels repulsion from s-electrons.

68 Trends in Electron Affinity The second occurs between Groups 4A and 5A. Added electron must go in an occupied p-orbital.

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70 Based on periodic trends, which following elements is expected to have the largest (i.e., most negative) electron affinity? 1. K 3. Na 5. Si 7. S

71 Correct Answer: K Na Si S S has most negative electron affinity in this list.

72 Lecture 7c- Sections

73 of Metal, Nonmetals, and Metalloids

74 Metals versus Nonmetals Table 7.3 on page 277 Differences between metals and nonmetals tend to revolve around these properties.

75 Metals versus Nonmetals Metals tend to form cations. (Low I.E.) Nonmetals tend to form anions. ( exo E.A.)

76 Metals Tend to be lustrous, malleable, ductile, and good conductors of heat and electricity.

77 Metals Compounds formed between metals and nonmetals tend to be ionic. Metal oxides tend to be basic.

78 Metal oxide (a.k.a. base anhydride) reactions Metal oxide + water metal hydroxide MgO(s) + H2O(l) Mg(OH)2(aq) but only if it dissolves a bit in water Metal oxide + acid water + salt MgO(s) + HCl(aq) MgCl2(aq) + H2O(l)

79 Nonmetals Dull, brittle substances that are poor conductors of heat and electricity. Tend to gain electrons in reactions with metals to acquire noble gas configuration.

80 Nonmetals Substances containing only nonmetals are molecular compounds. Most nonmetal oxides are acidic.

81 nonmetal oxide (acid anhydride) reactions nonmetal oxide + water acid SO3(g) + H2O(l) H2SO4(aq) nonmetal oxide + base water + salt CO2(g) + 2NaOH(aq) Na2CO3(aq) + H2O(l)

82 Metalloids Have some characteristics of metals, some of nonmetals. For instance, silicon looks shiny, but is brittle and fairly poor conductor.

83 Group Trends

84 Alkali Metals Soft, metallic solids. Name comes from Arabic word for ashes.

85 Alkali Metals Found only as compounds in nature. Have low densities and melting points. Also have low ionization energies.

86 Alkali Metals Their reactions with water are famously exothermic. 2M(s) + H2O(l) 2MOH(aq) + H2(g)

87 Alkali Metals Alkali metals (except Li) react with oxygen to form peroxides. K, Rb, and Cs also form superoxides: K + O2 KO2 Produce bright colors when placed in flame.

88 Alkaline Earth Metals Have higher densities and melting points than alkali metals. Have low ionization energies, but not as low as alkali metals.

89 Alkaline Earth Metals Be does not react with water, Mg reacts only with steam, but others react readily with water. Reactivity tends to increase down the group. Mg(s) + 2H2O(g) Mg(OH)2(aq) + H2(g)

90 Hydrogen Hydrogen is a nonmetal. It has a high ionization energy because its single electron experiences no nuclear shielding When it does lose it s electron it isn t really an atom anymore. It is a proton.

91 Group 6A Oxygen, sulfur, and selenium are nonmetals. Tellurium is a metalloid. The radioactive polonium is a metal.

92 Oxygen Two allotropes: O2 O3, ozone Three anions: O2, oxide O22, peroxide O21, superoxide Tends to take electrons from other elements (oxidation)

93 Sulfur Weaker oxidizing agent than oxygen. Most stable allotrope is S8, a ringed molecule.

94 Group VIIA: Halogens Prototypical nonmetals Name comes from the Greek halos and gennao: salt formers

95 Group VIIA: Halogens Large, negative electron affinities Therefore, tend to oxidize other elements easily React directly with metals to form metal halides Chlorine added to water supplies to serve as disinfectant

96 Group VIIIA: Noble Gases Astronomical ionization energies Positive electron affinities Therefore, relatively unreactive Monatomic gases

97 Group VIIIA: Noble Gases Xe forms three compounds: XeF2 XeF4 (at right) XeF6 Kr forms only one stable compound: KrF2 The unstable HArF was synthesized in 2000.