Properties of Solids

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the strongest material in the world? CHEMISTRY & YOU It’s not steel or any synthetic plastic, but a form of pure carbon known as fullerene nanotubes.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A Model for Solids In most solids, the atoms, ions, or molecules are packed tightly together. Solids are dense and not easy to compress. Because the particles in solids tend to vibrate about fixed points, solids do not flow. The general properties of solids reflect the orderly arrangement of their particles and the fixed locations of their particles.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A Model for Solids When you heat a solid, its particles vibrate more rapidly as their kinetic energy increases. The melting point (mp) is the temperature at which a solid changes into a liquid. –At this temperature, the disruptive vibrations of the particles are strong enough to overcome the attractions that hold them in fixed positions.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A Model for Solids The freezing point (fp) is the temperature at which a liquid changes into a solid. The melting and freezing points of a substance are at the same temperature. At that temperature, the liquid and solid phases are in equilibrium. Solid Liquid melting freezing

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A Model for Solids In general, ionic solids have high melting points because relatively strong forces hold them together. –Sodium chloride, an ionic compound, has a rather high melting point of 801°C. Molecular solids have relatively low melting points. –Hydrogen chloride, a molecular compound, melts at –112°C.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Explain why solids do not flow, even though their particles are constantly moving. In a solid, the particles are packed tightly together and vibrate around fixed points. Even though the particles vibrate, they are limited in their movement and cannot flow.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells In a crystal, the particles are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice. What determines the shape of a crystal?

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells The shape of a crystal reflects the arrangement of the particles within the solid. In sodium chloride, sodium ions and chloride ions are closely packed in a regular array. The ions vibrate about fixed points in the crystal.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems Crystals are classified into seven groups, or crystal systems. a = b = c  =  =  = 90 o Cubic a = b ≠ c  =  =  = 90 o Tetragonal a ≠ b ≠ c  =  =  = 90 o Orthorhombic a ≠ b ≠ c  =  = 90 o ≠  Monoclinic a ≠ b ≠ c  ≠  ≠  ≠ 90 o Triclinic aaaaaaabbb b bb c c c c c c c b a = b ≠ c  =  = 90 o,  = 120 o Hexagonal a = b = c  =  =  ≠ 90 o Rhombohedral

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems The edges are labeled a, b, and c. The angles are labeled α, β, and γ. a = b = c  =  =  = 90 o Cubic a = b ≠ c  =  =  = 90 o Tetragonal a ≠ b ≠ c  =  =  = 90 o Orthorhombic a ≠ b ≠ c  =  = 90 o ≠  Monoclinic a ≠ b ≠ c  ≠  ≠  ≠ 90 o Triclinic aaaaaaabbb b bb c c c c c c c b a = b ≠ c  =  = 90 o,  = 120 o Hexagonal a = b = c  =  =  ≠ 90 o Rhombohedral

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems The seven crystal systems differ in terms of the angles between the faces and in the number of edges of equal length on each face. a = b = c  =  =  = 90 o Cubic a = b ≠ c  =  =  = 90 o Tetragonal a ≠ b ≠ c  =  =  = 90 o Orthorhombic a ≠ b ≠ c  =  = 90 o ≠  Monoclinic a ≠ b ≠ c  ≠  ≠  ≠ 90 o Triclinic aaaaaaabbb b bb c c c c c c c b a = b ≠ c  =  = 90 o,  = 120 o Hexagonal a = b = c  =  =  ≠ 90 o Rhombohedral

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems The shape of a crystal depends on the arrangement of particles within it. The smallest group of particles within a crystal that retains the geometric shape of the crystal is known as a unit cell.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems A crystal lattice is a repeating array of any one of fourteen kinds of unit cells. Each crystal system can be composed of from one to four types of unit cells.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Crystal Systems The figure below shows the three kinds of unit cells that can make up a cubic crystal system. Simple CubicBody-CenteredFace-Centered In a simple cubic unit cell, atoms or ions are arranged at the corners of an imaginary cube. In a body-centered cubic unit cell, the atoms or ions are at the corners and in the center of an imaginary cube. In a face-centered cubic unit cell, there are atoms or ions at the corners and in the center of each face of an imaginary cube.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes Some substances can exist in more than one form.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes Some substances can exist in more than one form. Diamond is one crystalline form of carbon. A different form of carbon is graphite. In 1985, a third crystalline form of carbon was discovered. This form is called buckminsterfullerene.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes In diamond, each carbon atom in the interior of the diamond is strongly bonded to four others. The array is rigid and compact. In graphite, the carbon atoms are linked in widely spaced layers of hexagonal arrays. In buckminster- fullerene, 60 carbon atoms form a hollow sphere. The carbons are arranged in penta- gons and hexagons.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes The physical properties of diamond, graphite, and fullerenes are quite different. Diamond has a high density and is very hard. Graphite has a relatively low density and is soft and slippery. The hollow cages in fullerenes give them strength and rigidity.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes Diamond, graphite, and fullerenes are crystalline allotropes of carbon. Allotropes are two or more different molecular forms of the same element in the same physical state. Although allotropes are composed of atoms of the same element, they have different properties because their structures are different.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Allotropes Only a few elements have allotropes. In addition to carbon, these include phosphorus, sulfur, oxygen (O 2 and O 3 ), boron, and antimony.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What structural properties make fullerene nanotubes the strongest material in the world? CHEMISTRY & YOU

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What structural properties make fullerene nanotubes the strongest material in the world? Each carbon atom is covalently bonded to three other carbon atoms. The structure creates a spherical cage or cylindrical tube. This shape allows force to be distributed evenly across the surface so that the entire structure can withstand great force and is extremely strong. CHEMISTRY & YOU

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Not all solids are crystalline in form; some solids are amorphous. An amorphous solid lacks an ordered internal structure. Rubber, plastic, and asphalt are amorphous solids. Their atoms are randomly arranged. Non-Crystalline Solids

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Crystal Structure and Unit Cells Non-Crystalline Solids Other examples of amorphous solids are glasses. A glass is a transparent fusion product of inorganic substances that have cooled to a rigid state without crystallizing. Glasses are sometimes called supercooled liquids. The irregular internal structures are intermediate between those of a crystalline solid and those of a free-flowing liquid.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the difference between an amorphous solid and a crystalline solid? Particles in a crystalline solid are arranged in an orderly, repeating pattern or lattice. Particles in an amorphous solid are arranged randomly.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts The general properties of solids reflect the orderly arrangement and the fixed locations of their particles. The shape of a crystal reflects the arrangement of the particles within the solid.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms melting point: the temperature at which a substance changes from a solid to a liquid; the melting point of water is 0°C freezing point: the temperature at which a liquid changes into a solid crystal: a solid in which the atoms, ions, or molecules are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms unit cell: the smallest group of particles within a crystal that retains the geometric shape of the crystal allotrope: one of two or more different molecular forms of an element in the same physical state; oxygen (O 2 ) and ozone (O 3 ) are allotropes of the element oxygen

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms amorphous solid: describes a solid that lacks an ordered internal structure; denotes a random arrangement of atoms glass: a transparent fusion product of inorganic substances that have cooled to a rigid state without crystallizing