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Topic 3: Periodic Trends and Atomic Spectroscopy Topic 3: Periodic Trends and Atomic Spectroscopy Introduction Valence Electrons are those in the outer most shell of an element and are responsible for the bonding characteristics of that element. Core Electrons are the other electrons of an element and generally play no part in the reactivity and bonding of the element. Example and question: Write the electron configuration of the following elements and ions and identify which electrons are core electrons and which are valence electrons. Electron configuration Core electrons Valence electrons C 1s2 2s2 2p2 1s2 2s2 2p2 O O2– 1s2 2s2 2p6 1s2 2s2 2p6 Na Ca Ca2+ Fe3+ Se2– Periodic Trends The First Ionisation Energy increases across a period and decreases down a group. The first ionisation energy is: M (g) M+ (g) + e‐ The trend reflects the effective nuclear charge experienced by the electron being removed. Blackman Figure 4.49 19 The detail can often provide valuable information about orbital energies. 2500 2000 1500 1000 I.E. (kJ/mol) 500 0 F P H B C N Li O Si Si Cl Al Ar Ne Na He Be Mg Electron Affinity is the energy associated with X (g) + e‐ X‐ (g). It is usually exothermic but the trends are complex. What can be said is that the electron affinity generally becomes more negative across a period. Blackman Figure 4.51 The Atomic Radius decreases across a period and increases down a group. It is difficult to determine the exact size of an isolated atom, so the atomic radius is defined as half the distance between two atoms of the same element. Cations are always smaller, and anions larger, than the neutral atoms from which they are formed. Blackman Figure 4.47 Radius in nanometres: Li+ 0.060 C4+ 0.015 Li 0.123 C 0.077 F 0.072 C4– 0.260 F – 0.136 Question: Complete the following table: Element N3– O2– F– Ne Na+ Mg2+ Al3+ Atomic No No of electrons Relative size 20 Electronegativity increases across a period and decreases down a group. Electronegativity is an empirical scale that represents the ability of an atom, when in a compound, to attract the electrons of a chemical bond towards itself. It was introduced by Linus Pauling who assigned values to the elements on an arbitrary scale from 0 ‐ 4. Blackman Figure 5.5 Questions: Order these elements in terms of increasing atomic radius: Bi, Ca, F, S, Se Order these elements in terms of increasing electronegativity: Co, F, Ge, Rb, S Order these elements in terms of increasing first ionisation energy: As, Cs, N, Ne, Pb Explain why Cl has a higher electron affinity than Al. Why are there no values of electronegativity assigned to the Noble gases? Why are anions always larger than the neutral atom from which they are derived? 21 Spectroscopy and Electronic Transitions When an electron changes from one energy level to another light is either absorbed (electron moves from a low energy to higher energy orbital) or emitted. The energies of the orbitals are unique to each element (it depends on the number of electrons present and the number of protons in the nucleus). Studying these transitions is called spectroscopy. Visible versus X-ray Spectrometry The visible and UV wavelength range corresponds to changes in outer electron configurations for most atoms. The energies involved are similar to or less than the ionization energy of the element. When an atom bonds to form a compound, the valence electrons are involved and this may change the energy of the electrons involved. Consequently the samples to be analysed must be decomposed into their constituent atoms by breaking chemical bonds so that the electronic configurations are of atoms and not molecules. Emission spectroscopy records the energy change when an electron of an excited state atom ‘falls’ to a lower energy orbital. This gives rise to the colours seen in fireworks or the ‘flame tests’ for certain elements. Absorption spectroscopy records the absorption of energy by Blackman Figures 41.5 and 4.19 an atom when an electron is promoted to a higher energy orbital. Atomic absorption spectroscopy (AAS) is more sensitive than emission spectroscopy as there are a vastly greater number of atoms with electrons in the ground state (the starting point for AAS) than in the excited state (the starting point for emission spectroscopy). X‐rays probe much higher energy changes in core electron configurations. These are insensitive to bonding (which mainly effects outer shell electrons), so elaborate preparations are not required. Other restrictions arise when working with x‐ray and higher energies. Generation of X-rays by electron bombardment X‐rays are generated in a “cathode ray tube” by accelerating electrons from a cathode into a metal target anode. When the electrons strike the anode they collide and emit Bremsstrahlung or “braking” radiation in the x‐ray wavelength range. Braking may occur by one or more collisions, leading to a broad spectrum of emitted x‐rays which have a well defined maximum energy (or minimum wavelength) corresponding to stopping by one collision. 22 Example: What is the minimum X‐ray wavelength obtained when 30 keV electrons impact on a Cu target? The material used for the target is irrelevant to the bremsstrahlung minimum wavelength. 30 keV electron have been accelerated by a 30,000 eV potential difference E = 30,000 eV x 1.602 x 10‐19 J eV–1 = 4.81 x 10‐15 J The maximum X‐ray energy or minimum wavelength correspond to complete stopping in one collision, 6.626 x 10 J s 3.00 x 10 m s 4.13 10 m 41.3 pm 4.81 10 J X-Ray Fluorescence In addition to the broad spectrum Bremsstrahlung, target anodes may exhibit sharp lines characteristic of the atom(s) in the anode. These arise when incident electrons have enough energy to ionise the atom by ejecting an electron from a core (e.g. 1s) atomic orbital. X‐ray photons are emitted when electrons drop from higher energy orbitals to fill the vacancy. Because these wavelengths are characteristic of core orbital energies, they are relatively unaffected by any changes in outer (valence) electron energies associated with bonding. X‐ray wavelengths are characteristic of the element being bombarded, and an X‐ray fluorescence spectrum can be used to identify elements in a sample. X‐ray emission spectrum of a rhodium target showing the X‐ray fluorescence (‘K lines’) superimposed on top of the continuous spectrum. (http://en.wikipedia.org/wiki/X‐ray_fluorescence ) 23.
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