6.3C: Solid Metallic Elements
- Page ID
- 2578
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A metal (from Greek μέταλλον métallon, "mine, quarry, metal") is a material (an element, compound, or alloy) that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable - that is, they can be hammered or pressed permanently out of shape without breaking or cracking - as well as fusible (able to be fused or melted) and ductile (able to be drawn out into a thin wire). About 91 of the 118 elements in the periodic table are metals (some elements appear in both metallic and non-metallic forms).
Atoms of metals readily lose their outer shell electrons, resulting in a free flowing cloud of electrons within their otherwise solid arrangement. This provides the ability of metallic substances to easily transmit heat and electricity. While this flow of electrons occurs, the solid characteristic of the metal is produced by electrostatic interactions between each atom and the electron cloud. This type of bond is called a metallic bond.
Cubic and hexagonal close packing.
Crystalline solids consist of repeating patterns of its components in three dimensions (a crystal lattice) and can be represented by drawing the structure of the smallest identical units that, when stacked together, form the crystal. This basic repeating unit is called a unit cell.
Many metals adopt close packed structures i.e. cubic close packed (face centred cubic) and hexagonal close packed structures. A simple model for both of these is to assume that the metal atoms are spherical and are packed together in the most efficient way (close packing or closest packing). For closest packing, every atom has 12 equidistant nearest neighbours, and therefore a coordination number of 12. If the close packed structures are considered as being built of layers of spheres then the difference between hexagonal close packing and cubic close packed is how each layer is positioned relative to others. It can be envisaged that for a regular buildup of layers:
- hexagonal close packing has alternate layers positioned directly above/below each other, A,B,A,B,...
- cubic close packed (face centered cubic) has every third layer directly above/below each other, A,B,C,A,B,C,...
Body centred cubic
This is not a close packed structure. Here each metal atom is at the centre of a cube with 8 nearest neighbors, however the 6 atoms at the centres of the adjacent cubes are only approximately 15% further away so the coordination number can therefore be considered to be 14 when these are included. Note that if the body centered cubic unit cell is compressed along one 4 fold axis the structure becomes cubic close packed (face centred cubic).
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cubic close packing (ccp) packing efficiency =74% CN=12 |
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hexagonal close packing (hcp) packing efficiency =74% CN=12 |
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body-centred cubic packing (bcc) packing efficiency =68% CN=8 |
Trends in melting point
Melting points are chosen as a simple measure of the stability or strength of the metallic lattice. Some simple trends can be noted. The transition metals have generally higher melting points than the others. In the alkali metals (Group 1) and alkaline earth metals (Group 2) the melting point decreases as atomic number increases, but in transition metal groups with incomplete d-orbital subshells, the heavier elements have higher melting points. For a given period, the melting points reach a maximum at around Group 6 and then fall with increasing atomic number.
Mercury, caesium and gallium have melting points below 30 °C whereas all the other metals have sufficiently high melting points to be solids at "room temperature". The structures of the metals can be summarised by the table below which shows that most metals crystallise in roughly equal amounts of bcc, hcp and ccp lattices.
Crystal structure of metallic elements in the periodic table | ||||||||||||||||||
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1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |
H |
He |
|||||||||||||||||
453.69 Li bcc |
1560 Be hcp |
MP (K) At. Symbol Lattice type |
B | C | N | O | F | Ne | ||||||||||
370.87 Na bcc |
923 Mg hcp |
933.47 Al ccp |
Si | P | S | Cl | Ar | |||||||||||
336.53 K bcc |
1115 Ca ccp |
1814 Sc hcp |
1941 Ti hcp |
2183 V bcc |
2180 Cr bcc |
1519 Mn |
1811 Fe bcc |
1768 Co hcp |
1728 Ni ccp |
1357.8 Cu ccp |
692.68 Zn hcp |
302.91 Ga |
Ge | As | Se | Br | Kr | |
312.46 Rb bcc |
1050 Sr ccp |
1799 Y hcp |
2128 Zr hcp |
2750 Nb bcc |
2896 Mo bcc |
2430 Tc hcp |
2607 Ru hcp |
2237 Rh ccp |
1828 Pd ccp |
1235 Ag ccp |
594 Cd |
430 In |
505 Sn |
904 Sb |
Te | I | Xe | |
301.59 Cs bcc |
1000 Ba bcc |
2506 Hf hcp |
3290 Ta bcc |
3695 W bcc |
3459 Re hcp |
3306 Os hcp |
2719 Ir ccp |
2041.4 Pt ccp |
1337.33 Au ccp |
234.32 Hg |
577 Tl hcp |
600.61 Pb ccp |
544.7 Bi |
527 Po |
At | Rn |
Group 1: Alkali metals
The alkali metals have their outermost electron in an s-orbital and this electronic configuration results in their characteristic properties. The alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterized homologous behaviour.
The alkali metals have very similar properties: they are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. They can all be cut easily with a knife due to their softness, exposing a shiny surface that tarnishes rapidly in air due to oxidation by atmospheric moisture and oxygen. Because of their high reactivity, they must be stored under oil to prevent reaction with air, and are found naturally only in salts and never as the free element. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, excluding hydrogen (H), which is only nominally considered a group 1 element.
Group 2: Alkali Earth Metals
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Lithium -bcc |
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Magnesium -hcp |
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Calcium -ccp |
Return to the course outline or move on to Lecture 5: Structure of the elements Boron, Carbon and Phosphorus, Sulfur.
References
Much of the information in these course notes has been sourced from Wikipedia under the Creative Commons License.
'Inorganic Chemistry' - C. Housecroft and A.G. Sharpe, Prentice Hall, 4th Ed., 2012, ISBN13: 978-0273742753, pps 24-27, 43-50, 172-176, 552-558, 299-301, 207-212
'Basic Inorganic Chemistry' - F.A. Cotton, G. Wilkinson and P.L. Gaus, John Wiley and Sons, Inc. 3rd Ed., 1994.
'Introduction to Modern Inorganic Chemistry' - K.M. Mackay, R.A. Mackay and W. Henderson, International Textbook Company, 5th Ed., 1996.