Crystal Structures and Formulae of
Ionic compounds
Science Museum / Chemistry / Structures
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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
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arrangement of atoms in the crystal.
Lattice systems
You can find models of some lattice systems in the Science Museum. Recognize the
systems with the diagrams on the right.
Other type of models
Science Museum / Chemistry / Structures
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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
piece have?
How many neighbouring grey pieces does each green
piece have?
What is the simplest ratio of green piece to grey piece?
Green : Grey = _____ : _____ = _____ : _____
Find examples of ionic compounds having this type of crystal structure on page 6.
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
piece have?
How many neighbouring grey pieces does each green
piece have?
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piece have?
What is the simplest ratio of green piece to grey piece?
Green : Grey = _____ : _____ = _____ : _____
Find examples of ionic compounds having this type of crystal structure on page 6.
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
piece have?
How many neighbouring grey pieces does each green
piece have?
What is the simplest ratio of green piece to grey piece?
Green : Grey = _____ : _____ = _____ : _____
Find examples of ionic compounds having this type of crystal structure on page 6.
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 = _____ : _____ = _____ : _____
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Find examples of ionic compounds having this type of crystal structure on page 6.
Solutions
Crystal structure 1
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
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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
光的科學與運用
School library book，可參考第七章