Crystal Structures and Formulae of Ionic compounds
Science Museum / Chemistry / Structures Page 2 of 2 Crystal structures and X-ray diffraction A crystal structure is defined as the particular repeating arrangement of atoms (molecules or ions) throughout a crystal. Max von Laue showed that X-rays are diffracted ( 衍射 ) in crystals and form characteristic patterns on photographic film, he proved that crystals have a lattice-like ( 晶格, 點陣 ) structure (page 3). The connection was established by the father and son partnership of William Henry Bragg and William Lawrence Bragg, for which they were awarded the Nobel Prize in Physics in 1915. The key to the Braggs' breakthrough is that crystals are made of regular, repeating patterns of atoms, like oranges packed in a box. One could mathematically predict the diffraction pattern from reflections from each successive plane of atoms within the crystal. William Henry Bragg designed the X-ray spectrometer to examine the reflections of X-rays from crystals. William Lawrence Bragg proposed a simple but powerful equation, which is known as Bragg's law, showing the connection between the wavelength of the X-rays, the distance between the planes and the angle at which the X-rays are reflected. n λ =2d sin θ The two scientists used the spectrometer to analyse the structure of several salts and small molecules, establishing fundamental mathematical relationships between the diffraction pattern and the dimensional
Science Museum / Chemistry / Structures Page 3 of 3 Lattice systems arrangement of atoms in the crystal. You can find models of some lattice systems in the Science Museum. systems with the diagrams on the right. Recognize the Other type of models
Science Museum / Chemistry / Structures Page 4 of 4 Crystal structures of some ionic compounds You can find models of the crystal structures of some ionic compounds in the Science Museum. You can predict the formulae of those compounds from their structures. Crystal structure 1 The green pieces represent cations (positive ions) and the grey pieces represent anions (negative ions). How many neighbouring green pieces does each grey How many neighbouring grey pieces does each green What is the simplest ratio of green piece to grey piece? Green : Grey = : = : Crystal structure 2 The green pieces represent cations (positive ions) and the grey pieces represent anions (negative ions). How many neighbouring green pieces does each grey How many neighbouring grey pieces does each green
Science Museum / Chemistry / Structures Page 5 of 5 What is the simplest ratio of green piece to grey piece? Green : Grey = : = : Crystal structure 3 The green pieces represent cations (positive ions) and the grey pieces represent anions (negative ions). How many neighbouring green pieces does each grey How many neighbouring grey pieces does each green What is the simplest ratio of green piece to grey piece? Green : Grey = : = : Crystal structure 4 The red pieces represent cations (positive ions) and the grey pieces represent anions (negative ions). How many neighbouring red pieces does each grey piece have? How many neighbouring grey pieces does each red piece have? What is the simplest ratio of red piece to grey piece? Red : Grey = : = :
Science Museum / Chemistry / Structures Page 6 of 6 Crystal structure 1 Solutions Green : Grey = 6 : 6 = 1 : 1 Examples: NaCl, KCl, AgCl, NaBr, KBr, AgBr, MgO, CaO, SrO, BaO, PbS and etc Crystal structure 2 Green : Grey = 8 : 8 = 1 : 1 Examples: CsCl, CsBr, CsI and etc Crystal structure 3 Green : Grey = 4 : 4 = 1 : 1 Examples: AgI, BeS, HgS, ZnS and etc Crystal structure 4 Red : Grey = 4 : 8 = 1 : 2 Examples: CaF2, BaF2, PbF2, Li2O, Na2O, K2O
Science Museum / Chemistry / Structures Page 7 of 7 Further study 1. Nobel prize in Physics 1915 Web page: http://www.nobelprize.org/nobel_prizes/physics/lau reates/1915/ 2. Crystallography Detailed explanations to crystal structures and their defection patterns. Web page: http://www.xtal.iqfr.csic.es/cristalografia/index-en. html 3. X 光的科學與運用 School library book, 可參考第七章