CHEMISTRY 1000 Topic #2: The Chemical Alphabet Summer 2007 Dr. Susan Lait Gallium, Ga Sodium, Na Forms of Carbon

2 The Periodic Table: A Chemical Index In 1869, Dmitri Mendeleev ( ) noticed that certain elements exhibited similar behaviour – most notably, the ratios with which they formed molecules with hydrogen and with oxygen. By arranging the elements in order of increasing mass and such that similar elements formed columns, he developed the first periodic table:

3 The Periodic Table: A Chemical Index Mendeleev’s periodic table was incomplete – all of the _______ ______ were missing, but it was remarkably accurate in other respects. If there appeared to be a ‘missing’ element, he left a blank space, assuming that it would be discovered at a later date. He was proven correct with the discoveries of ________ (69.7 u) in 1875 and ____________ (72.6 u) in In 1913, H.G.J. Moseley ( ) noted that the periodic table would be more descriptive if the elements were listed in order of increasing ____________ rather than increasing mass. This led to the modern periodic table and law of periodicity:

4 The Periodic Table: A Chemical Index

5 Terminology used to describe regions of the periodic table: Periods Groups s-block (“alkali metals” and “alkaline earth metals”) p-block (group 13, group 14, “pnictogens”, “chalcogens”, “halogens” and “noble gases”) d-block (“transition metals”) f-block (“lanthanides” and “actinides”) Metals (conductors) Nonmetals (insulators) Metalloids (intrinsic semiconductors) You are expected to memorize the names and symbols for the first 36 elements on the periodic table (i.e. hydrogen to krypton)

6 What is a Metal? Most of the elements in the periodic table are metals. How can we recognize if an element is a metal? It’s opaque and its smooth surfaces reflect light (“metallic luster”). It’s malleable (can be hammered into sheets without breaking). It’s ductile (can be stretched into wires without breaking). It has a high boiling point. (The melting points of metals vary widely – though most have high melting points too.) It conducts heat and electricity. These properties arise because of the structure of metals. The simplest metals can be considered to behave as an organized arrangement of ‘cations’ surrounded by a ‘sea of electrons’:

7 Structure of Metals Metals usually form crystal lattices in which the atoms are closely packed. These lattices are held together by electrostatic attractions between the cations and the electrons. These crystal lattices are made up of repeating units called unit cells. All of the unit cells in a crystal lattice are identical and have the same symmetry as the overall lattice. There can be no “gaps” between unit cells. LATTICE: NOT:OR:

8 Symmetry Forms of symmetry that must be present in the unit cell if they are present in the overall lattice:

9 Lattices and Unit Cells The unit cell must also be the smallest unit that meets these conditions. Find the “unit cell” in each of the following pictures: Note that these are two-dimensional pictures while metals are three-dimensional!

10 Lattices and Unit Cells There are seven three-dimensional crystal systems: In CHEM 1000, we will focus only on the cubic and the hexagonal crystal systems as they describe the vast majority of metals.

11 Lattices and Unit Cells

12 Lattices and Closest Packing How do these structures arise? Consider what would happen if you filled the bottom of a box with marbles. How would they naturally arrange themselves? Why? If you were to add a second layer of marbles, where would they go? OR

13 Lattices and Closest Packing The marbles on the previous page adopted a “closest packing” arrangement that is observed in the structures of many metals. There are two kinds of “closest packing” lattices: cubic closest packed and hexagonal closest packed. The difference between these two lattices arises when the third row of atoms is added:

14 Lattices and Closest Packing Where’s the hexagon in hexagonal closest packing (hcp)? Rotate the image from the previous page so that we can see the lattice in three-dimensions: Note that the layer sequence is red-blue-red-blue (more generally referred to as ABAB) Find the unit cell in both pictures. =

15 Lattices and Closest Packing Where’s the cube in cubic closest packing (ccp)? Rotate the image from the previous page so that we can see the lattice in three-dimensions: Note that the layer sequence is red-blue-yellow-red-blue-yellow (more generally referred to as ABCABC) A unit cell contains atoms from four of the layers from the picture on the left. On the unit cell at the left, label which layer each atom comes from (A, B or C). Note that, in addition to the atom at each corner of the cube, there is also an atom in the center of each face of the cube. For this reason, cubic closest packing (ccp) is also called face centered cubic (fcc). ==

16 Cubic Lattices Face-centered cubic (fcc or cpp) is one of three types of cubic unit cells. The other two are body-centered cubic (bcc) and simple cubic: Note that these pictures are showing parts of the atoms that are not contained by the unit cell.

17 Cubic Lattices To see what fraction of each atom is actually inside the unit cell, we look at “sliced” views: Every atom completely inside a cell is only in 1 cell. Every atom along a cell face is in 2 cells. (½ in each) Every atom along an edge is in 4 cells. (¼ in each) Every atom in the corner is in 8 cells. (1/8 in each) How many atoms are inside each of the three cubic cells?

18 Lattices and Co-ordination Number Every atom in a lattice is in contact with the same number of other atoms. This is referred to as its co-ordination number. What is the co-ordination number of an atom in each kind of lattice we’ve seen? Be sure to consider all unit cells in which the atom resides. Simple cubic (e.g. Po)Face-centered cubic (e.g. Cu) Body-centered cubic (e.g. Na)Hexagonal closest packed (e.g. Mg)

19 How can we Determine a Lattice’s Structure Crystalline solids (including metals) can be analyzed by x-ray crystallography, in which an x-ray is passed through a crystal. The crystal acts as a diffraction grating (the x-rays can pass through gaps in the crystal structure but not through the atoms themselves), and analysis of the resulting diffraction pattern allows a chemist to determine the structure of the crystal (elements as well as arrangement of atoms).

20 Lattices, Density and Metallic Radii As you might expect from looking at the unit cells, lattice type is often related to density. As a general rule, the trend is that _____ lattices are the least dense, _____ have middling densities and _____ and _____ lattices are the most dense. One way to experimentally determine a metallic radius is via x-ray crystallography. Another is to measure the density of a metal for which you know the lattice type. How would you go about measuring the density of a sample of metal (assuming that it is safe to handle and relatively unreactive)?

21 Lattices, Density and Metallic Radii Aluminum has a density of g/cm 3, and the atoms are packed into a face-centered cubic unit cell. Calculate the metallic radius of an aluminum atom.

22 Lattices, Density and Metallic Radii Lithium has a metallic radius of 152 pm and the atoms are packed into a body-centered cubic unit cell. Calculate the density of lithium.