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Three images are shown. The first image shows a cube with black dots at each corner and a red dot in the center. This cube is stacked with seven others that are not colored to form a larger cube. The second image is composed of eight spheres that are grouped together to form a cube with one smaller sphere in the center. The name under this image reads “Body-centered simple cubic structure.” The third image shows five horizontal layers of purple spheres with layers of smaller green spheres in between.
Ionic compounds with similar-sized cations and anions, such as CsCl, usually form a simple cubic structure. They can be described by unit cells with either cations at the corners or anions at the corners.

We have said that the location of lattice points is arbitrary. This is illustrated by an alternate description of the CsCl structure in which the lattice points are located in the centers of the cesium ions. In this description, the cesium ions are located on the lattice points at the corners of the cell, and the chloride ion is located at the center of the cell. The two unit cells are different, but they describe identical structures.

When an ionic compound is composed of a 1:1 ratio of cations and anions that differ significantly in size, it typically crystallizes with an FCC unit cell, like that shown in [link] . Sodium chloride, NaCl, is an example of this, with Na + and Cl having radii of 102 pm and 181 pm, respectively. We can think of this as chloride ions forming an FCC cell, with sodium ions located in the octahedral holes in the middle of the cell edges and in the center of the cell. The sodium and chloride ions touch each other along the cell edges. The unit cell contains four sodium ions and four chloride ions, giving the 1:1 stoichiometry required by the formula, NaCl.

Three images are shown. The first image shows a cube with black dots at each corner and a red dot in the center. This cube is stacked with seven others that are not colored to form a larger cube. The second image is composed of eight spheres that are grouped together to form a cube with one much larger sphere in the center. The name under this image reads “Body-centered simple cubic structure.” The third image shows seven horizontal layers of alternating purple and green spheres that are slightly offset with one another and form a large cube.
Ionic compounds with anions that are much larger than cations, such as NaCl, usually form an FCC structure. They can be described by FCC unit cells with cations in the octahedral holes.

The cubic form of zinc sulfide, zinc blende, also crystallizes in an FCC unit cell, as illustrated in [link] . This structure contains sulfide ions on the lattice points of an FCC lattice. (The arrangement of sulfide ions is identical to the arrangement of chloride ions in sodium chloride.) The radius of a zinc ion is only about 40% of the radius of a sulfide ion, so these small Zn 2+ ions are located in alternating tetrahedral holes, that is, in one half of the tetrahedral holes. There are four zinc ions and four sulfide ions in the unit cell, giving the empirical formula ZnS.

Two images are shown. The first image shows a cube with black dots at each corner and a red dot in the center of each face of the cube. This cube is stacked with seven others that are not colored to form a larger cube. The second image is composed of eight spheres that form the corners of a cube with six other spheres located in the face of the cube. The spheres are connected to one another by lines. The name under this image reads “Z n S, face-centered unit cell.”
ZnS, zinc sulfide (or zinc blende) forms an FCC unit cell with sulfide ions at the lattice points and much smaller zinc ions occupying half of the tetrahedral holes in the structure.

A calcium fluoride unit cell, like that shown in [link] , is also an FCC unit cell, but in this case, the cations are located on the lattice points; equivalent calcium ions are located on the lattice points of an FCC lattice. All of the tetrahedral sites in the FCC array of calcium ions are occupied by fluoride ions. There are four calcium ions and eight fluoride ions in a unit cell, giving a calcium:fluorine ratio of 1:2, as required by the chemical formula, CaF 2 . Close examination of [link] will reveal a simple cubic array of fluoride ions with calcium ions in one half of the cubic holes. The structure cannot be described in terms of a space lattice    of points on the fluoride ions because the fluoride ions do not all have identical environments. The orientation of the four calcium ions about the fluoride ions differs.

Questions & Answers

how can someone understand chemistry vividly
Mercy Reply
Maybe by reading proofs or practical work and application in modern world.
Abdelkarim
what is isotopes
Samuel Reply
whats de shape of water
Amara Reply
water has no shape because it's liquid
Wil
water is a shapeless, odourless, colourless and tasteless substance that only takes the shape of its container.
mikefred
i think they're referring to the molecular shape?
It has no shape but takes the shape of the container
kpadonu
what isthe maening pkw
Wilson Reply
introduction to chemistry for beginner
Lansana Reply
hi
noble
through out human history
marwan Reply
Yes?
Esther
hello
noble
yes
Bol
what
noble
is euglena a unicellular organ
Agio Reply
is euglena a unicellular organism
Agio
how is hydrogen can be heated
Buwembo Reply
what is difference between atom and molecule
Aqeela Reply
Atom is the smallest part of matter; it consists of equal number of protons and electrons. It may have neutrons. A molecule is a compound made of atoms covalently bonded.
Abdelkarim
does amoeba has structure
Mercy Reply
what is the effect of green house on the earth 🌎
kolawole Reply
what is the effect of green house on the earth 🌎
kolawole
what's an atom?
Davy Reply
it's the smallest unit of Matter
Orsine
atom
Eden
smallest part of an element
lydia
also, depending on its (atom's) structure, that is the amount of protons and neutrons and electrons, is the determining factors of what element it is.
Richard
is a smallast particals of an element
Buwembo
it is the smallest part of an element that can take part in a chemical reaction
Ayub
is the smallest part of an element
Jonathan
An atom is the smallest indivisible part of a matter
kpadonu
Oy kl konsa test hay or kitna hay?
Faisal Reply
differences between solid liquid and gaseous state
Ochei Reply
modification of John dalton atomic theory
Ochei
the differences between soliq liquid and gas is that in solid the particle are strongly bonded together by forces of cohesion and the particle are not able to move about but only vibrate in a fixed position but in liquid the particle are loosely bond together and the particle are able to move about
kolawole
2.4g of magnesium reacts with 0.3mol of hydrochloric acid write a balanced chemical equation for the reaction. (b)Determine the limiting reactant
Sheldon Reply

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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