s or Hz J atom J mol or -274 kj mol CHAPTER 4. Practice Exercises ΔE atom = ΔE mol =

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1 CHAPTER 4 Practice Exercises s or Hz 4.3 ΔE atom = ΔE mol = J atom J mol or -274 kj mol excitation energy = 471 kj mol kj mol 1 = 746 kj mol 1 Hg λ proton = m or nm p E 2s 1s Shorthand notation: 1s 2 2s 2 2p [He]2s 2 2p 3 2s 2 n = 2, l = 0, m l = 0, m s = +½ n = 2, l = 0, m l = 0, m s = ½ 2p 5 n = 2, l = 1, m l = +1, m s = +½ n = 2, l = 1, m l = +1, m s = ½ n = 2, l = 1, m l = 0, m s = +½ n = 2, l = 1, m l = 0, m s = ½ n = 2, l = 1, m l = 1, m s = +½ 4.15 Sc 3+, Cu +, Zn 2+

2 4.17 Metals: Ga Metalloids: Ge and As Non-metals: Se, Br and Kr Review Questions 4.1 (a) V atom [MM (g/mol)][103 kg/g] [(kg/m 3 )][N A ] d (V) 1/3 (c) Ag: Thickness = ( atoms)( m) = m Pb: Thickness = ( atoms)( m) = m 4.3 Liquids and solids have limited compressibility, indicating that their component atoms have impenetrable volumes. Gases deviate from the ideal gas equation when compressed, displaying volumes that are greater than those predicted m s (a) = m s m s 4 6 = m or cm s m s 6 9 (c) = m or mm 1 60 s m s 2 8 (d) = m or m s (a) E photon = J per photon = m v = s 1 or Hz E photon = J = m v = s 1 or Hz 4.9 In order for a phototube to generate an electric current, the energy of the photon striking the metal surface of the tube must exceed the binding energy. Infrared light has lower energy than visible

3 light and consequently cannot eject electrons from the metal surface in the phototub 4.11 E 81 (E 8 E 1 ) 2.18 x J 2.18 x 10 J x J = hc E = (6.626 x 1034 J s)(2.998 x 10 8 m/s) x x 10 8 m or nm J E 91 (E 9 E 1 ) 2.18 x J 2.18 x 10 J x J = hc E = (6.626 x 1034 J s)(2.998 x 10 8 m/s) x x 10 8 m or nm J 4.13 (a) m or 1.45 nm m or 6.38 nm (c) m or 4.41 m 4.15 n l m l m s / / / / / / (a) non-existent: l must be positive actual (c) non-existent: m l cannot be larger than l (d) actual

4 4.19 (a) 3p 2s (c) 1s (d) 2p 4.21 An electron contour surface representation gives a realistic three dimensional picture of the shape of an orbital. However, it fails to show regions inside the contour where the electron density may be zero. Consequently, the contour surfaces of 1s, 2s, 3s etc. all appear spherical A hydrogen atom contains only one electron, so there is no screening effect. In the absence of screening, orbital energy depends only on n and Z, so all n = 3 orbitals have identical energy. electron and the amount of screening decreases as l increases (a) The ionisation energy of the He 2p orbital ( J) is not much greater than that of the H 2p orbital ( J). These similar values indicate nearly equal effective nuclear charges due to nearly complete screening. In the absence of screening, the He 2p orbital has four times the ionisation energy of the H 2p orbital. The ionisation energy of the He + 2p orbital ( J) is four times greater than that of the H 2p orbital ( J). A four-fold increase when Z doubles indicates a Z 2 dependence The column location of an element is the indicator of how many valence electrons it has. Remember that s electrons as well as those in the filling block count as valence electrons: O, fourth column of p block, 4 p + 2 s = 6 valence electrons; V, third column of d block, 3 d + 2 s = 5 valence electrons; Rb, 1 valence electron; Sn, second column of p block, 2 s + 2 p = 4 valence electrons; and Cd, end of d block, 2 valence electrons (a) C, Z = 6, [He] 2ss 2p 2 Cr, Z = 24, exception, [Ar] 4s 1 3d 5 (c) Sb, Z = 51, [Kr] 5s 2 4d 10 5p 3 (d) Br, Z = 35, [Ar] 4s 2 3d 10 4p The value of E i2 is almost ten times that of E i1, indicating that the second electron removed is a core electron rather than a valence electron. The elements in Group 1 contain only one valence electron, so this is a Group 1 element. An electron affinity around 50 kj/ /mol is consistent with this assignment (see figure 4.36). The element is Cs Stable anions form from elements in Columns 16 (dianions) and 17 (monoanions). Stable cations form from metals. Ca, Cu, Cs, and Cr are all metals and are found in ionic compounds as cations. Cl is found in ionic compounds as a 1 anion. C is not found as an atomic ion.

5 4.37 Non-metals are found in the upper right portion of the periodic table, metalloids along a diagonal running through the p block, and all other elements are metals. Thus, C and Cl are nonmetals, and Ca, Cu, Cs, and Cr are metals. Review Problems 4.39 (a) The photoelectric effect is a phenomenon describing electrons that are ejected from a metal surface when light of a sufficient frequency strikes the metal. Increasing the frequency of the light increases the kinetic energy of the ejected electrons. Increasing the intensity of the light increases the number of electrons ejected. (c) (d) Particle-wave duality refers to an electron or photon having properties of both a particle and a wave. Electron spin is a quantised property of electrons. Electrons can behave in only one of two ways when in a magnetic field. The Heisenberg uncertainty principle states that the position of a moving electron cannot be defined. This is due to the particle-wave duality of an electron, since waves do not have a position. Heisenberg proved that because the position and momentum of a waveparticle are linked, it is not possible to know both at the same time. The more accurately we know the momentum, the less accurately we know the position and vice versa Original 2 x frequency 4.43 (a) J 3 x 10 5 ms -1 (c) m or 2 nm 4.45 (nm) (10 14 s 1 ) E (kj mol 1 ) (a) Shielding refers to the ability of electrons to partially cancel the attraction of an incoming negative electron to the positive nucleus.

6 (c) (d) (e) All electrons are described by a set of four quantum numbers (principle quantum number, azimuthal quantum number, magnetic quantum number and spin quantum number) and the Pauli exclusion principle states that each electron in an atom must have a unique set of these quantum numbers. Thus, any orbital can contain a maximum of two electrons and these two electrons must have opposite spin quantum numbers. The aufbau principle is used to predict the ground-state electron configuration of an atom or ion. It states that each successive electron is placed in the next available lowest-energy orbital, in accordance with the Pauli exclusion principle. Hund s rule governs the way electrons are arranged in orbitals of equal energy. It states that electrons are arranged such that there is a maximum number of electrons with the same spin orientation as this is the lowest energy configuration. Valence electrons are those that are responsible for chemical activity, which is governed by the spatial and energetic accessibility of these electrons. Electrons occupying the largest orbitals of an atom are spatially accessible, and those electrons occupying the highest energy occupied orbitals of an atom are energetically accessible Periodic trends are the variations in physical and chemical properties of the elements and their compounds in moving from left to right in the periodic table. Across a period, the valence orbitals are filling so as to approach a stable noble gas configuration. First ionisation energy tends to increase across a period. This is because it requires more energy to remove an electron from an atom that has a filled, or close to filled, outer shell. This is a result of the increased attraction between the electrons and the nucleus as the valence obitals are filled. Hence E i1 is highest for those elements which have noble gas electron configurations, followed by the halogens. The lowest E i1 values across a period are for the elements of Group 1, since losing their valence electron gives them the stable noble gas electron configuration. However there are some irregularities in this trend, such as the tendency for E i1 to be lower than the trend predicts when the ionisation results from the ejection of a p-orbital electron. Electron affinity tends to become more negative across a period and tends to be negative as in most cases energy is released when adding the first electron. This is due to the increasing attraction between the electron and the nuclear charge across the periodic table. The E EA values generally become more negative due to the decreasing size of the atom across the periodic table. The most negative values are for the halogens because when an electron is added, these atoms reach noble gas electron configuration. Again this trend is not regular, as the E EA values for Group 2 elements and the noble gases are positive, as in the first case, their ns orbitals were already filled and in the latter, the atoms have filled np orbitals Size increases with increasing n value and for the same value of n, decreases with increasing Z (increasing electrical attraction). Cl, Cl, and K + all have n = 3 for the valence electrons, whereas Br has n = 4, so Br is the largest of this group. Among the others, the extra electron in Cl makes it larger than Cl and K + has higher Z than Cl, so the order of decreasing radius is: Br > Cl > Cl > K +

7 4.53 (a) In a one-electron atom or ion, orbital energy depends only on n and Z, both of which are the same for the hydrogen atom 2s and 2p orbitals. In multi-electron atoms, orbitals with the same n value but different l values are screened to different extents. The orbital with the lower l value is less screened, and hence is more stable S +, with three unpaired electrons, has more unpaired electrons than S or S (a) Mg has the second smallest radius among alkaline earths and S has an anion that is isoelectronic with Ar, the Row 3 noble gas: MgS. K, beginning of Row 4, has a cation isoelectronic with Ar at the end of Row 3, and the Row 2 element with the highest electron affinity is F: KF. (c) Be is the alkaline earth element with the highest second ionisation energy and it combines in 1:2 ratio with elements from Row 17: BeCl 2. Additional Exercises 4.59 Minimum kinetic energy: J 4.61 u = 4.63 Wavelength: m h m x J s ( x kg)(7.5 x m) = m/s E kinetic = 1/2 mu 2 E kinetic = 1/2 ( kg)( m/s) 2 = J Fraction lost = 7.5 x J 6.15 x J = 0.12

8 4.65 n 1 = 4 and n 2 = photons emitted per minute st Line = n = 4 to n = 3 2 nd Line = n = 5 to n = 3 3 rd Line = n = 6 to n = 3 4 th Line = n = 7 to n = 3 5 th Line = n = 8 to n = (a) Binding energy is the energy required to eject an electron from an atom. For metal A, electrons are not ejected until a frequency of s 1 is reached, therefore the binding energy is the energy corresponding to this frequency: E = hv = ( J s)( s 1 ) = J Similarly for metal B at a frequency of s 1 E= hv = ( J s)( s 1 ) = J Therefore, metal B has the higher binding energy because higher frequency photons are required to generate photoelectrons. Kinetic energy = photon energy binding energy E photon hc (6.626 x 1034 J s)(2.998 x 10 8 m/s) 1.25 x x J m Hence, using this photon energy and the binding energy calculated in part (a), the kinetic energies are:

9 (c) Metal A: J J = J Metal B: J J = J The wavelength range over which electrons can be ejected from one metal but not from the other corresponds to the frequency range of s 1 to s 1 : At s 1 = c v x 108 m/s 4.0 x s -1 = m or 750 nm At s 1 = c v x 108 m/s 6.5 x s -1 = m or 460 nm Thus, the wavelength range over which photons can eject electrons from one metal but not the other is 460 nm to 750 nm To determine configurations of ions, start with the ground state configuration of the neutral atom and then remove electrons: Ce has valence configuration 6s 2 5d 1 4f 1 ; remove the two s electrons: Ce 2+, 5d 1 4f 1 La has valence configuration 6s 2 5d 1 ; remove one s electron: La +, 6s 1 5d 1 Ba has the normal configuration, 6s 2 These differ because of the nearly-equal energies of the 6s, 5d, and 4f orbitals Cl is larger than K + because although theses ions have the same number of electrons, K + and Ar have a larger Z. By similar arguments Ar > K +. Thus, a = Cl, b = K +, and c = Ar The data in appendix C indicate that noble gas elements have unfavourable electron affinities (> 0) and relatively large first ionisation energies (> 1350 kj mol 1 ). Consequently, these are unlikely to either gain or lose electrons to form stable chemical compounds. Nevertheless, Xe and Kr (to a lesser extent) do form compounds The filling pattern after 7p mirrors the pattern after 6p: 8s, then 5g (or perhaps 6f) (a) Successive ionisation energies for any element increase because there is less effective screening as electrons are removed. The fourth ionisation energy is much greater than any of the other three because the electron removed comes from the 2p rather than an n = 3 orbital. (c) As electrons are removed, the reduction in screening results in tighter binding and a smaller ion; thus Al 4+ < Al 3+ < Al 2+ < Al +. Electron affinity is the energy released when an electron is added, so it is the reverse of ionisation energy and thus Al 4+ has the largest (most negative) electron affinity (a) Be is 1s 2 2s 2 and B is 1s 2 2s 2 2p 1. Although B has a higher Z value, the combination of a less stable highest occupied orbital in B and electron electron repulsion in Be accounts for B having a lower E i1 than Be.

10 S is [Ne] 3s 2 3p 4 and P is [Ne] 3s 2 3p 3. Here, increased electron electron repulsion in S, for which one of the p orbitals is doubly occupied, destabilises the electron more than the stabilisation provided by an increased Z value.