Lecture 5 - Periodic properties of the elements Flashcards
(22 cards)
Learning objectives - slide 8
5.1 Be able to describe the structures of 118 elements
5.2 Explain and exemplify the trend from metallic to non-metallic behaviour across the periodic table
5.3 Explain and exemplify the trend from non-metallic to metallic behaviour down the periodic table
5.4 Explain the influence of single vs multiple bonding on the structures of elements
5.5 Describe the dependence of chemical reactivity on structure
5.6 Discuss allotropes as examples of structure-property relationships in chemistry
5.7 Describe trends in binding energy through periodic table
Periodic trend caused from multiple bonding down the group
Multiple bonds occur at the top of the group (ie C=C, N=N, O=O etc which often means trend from simple molecules to giant or infinite covelant down the group
Structures of metals
- it can be understood in terms of simple packing of spheres
- can think of spherical metal ions with electrons delocalised in crystal orbitals giving conductivity
How can the “spheres” (spherical metal ions) be packed?
There are 4 ways, list their efficiency
Primitive cubic - 52%
Body centred cubic (bcc) - 68%
Hexagonally close packed (hcp) - 74%
Cubic close packed (ccp) / face centred cubic (fcc) - 74%
Brief description of primitive cubing
Spheres in each corner and are all in contact along cell edge
Coordination no of 6
- alpha-Po is an example
- solid Hg has related tetragonal structure
Brief description of bcc
Same as pc but with an atom in the center of the unit cell
8 neighbours at 0.87a
6 neighbours at 1a
To get unit cell length, a cut diagonal through top right, middle, bottom left (contact along body diagonal)
M at (0,0,0) and (1/2,1/2,1/2)
Coordination no of 8
- found for alkali metals
Brief description of hcp
Packing follows ABABABAB pattern (diagram)
It forms an anticubeoctahedron (diagram)
Coordination no of 12
Layers in unit cell are perpendicular to the body diagonal of the cube
Brief description of ccp
Packing follows ABCABCABC pattern (diagram)
It forms a cubeoctahedron (diagram)
Coordination no of 12
Layers in unit cell are perpendicular to the body diagonal of the cube
What type of structures do group 1 have? Why?
- fewer electrons than orbitals
- low EN
- favours metallic
- bcc structures
What type of structures do group 2 have? Why?
- fewer electrons than orbitals
- low EN
- favours metallic
Be - hcp
Mg- hcp
Ca - ccp
Sr - ccp
Ba - bcc
More electrons than Gp1 therefore stronger bonding so can pull atoms closer together leading to more efficient packing structures
What type of structures do group 13 have? Why?
B: giant covelant (boron clusters are non-trivial so beyond classic 2 atoms sharing 2 e-s)
- 3 valence electrons with 4 valence orbitals
- therefore fewer e- than valence orbitals - favour metallic
- EN is high at top of group - favours covelant
Al - metallic
Ga - metallic
Molten at 30-1000 degrees C (large melting range due to high EN)
In - metallic
EN low at bottom of group favouring metallic
Tl - metallic
What are the Metalloids
H, B, Si, Ge, As, Sb, Te, Po
Look at periodic table - kind of staircases down
What type of structures do group 14 have? Why?
Top of group have high EN and are small - favouring covalent and multiple bonds. As you go down the group you move to single bonds and then finally at the bottom of the group they have low EN and are small - metallic
C: allotropes- diamond made of s.b. graphite and C60 made of d.b.
Si and Ge have diamond structures
Sn above 13 *C is a white non- metal and brittle
Sn below 13 *C is a grey metal
Pb - metallic
More info on carbon allotropes
Graphite
- fused planar benzene rings
- vdw between layers
Diamond
- tetrahedral
- much higher mp than other 2 structures due to breaking covelant bonds instead of IMF
C60
- molecules held together by vdw
Properties of allotropes
- compounds with molecular structures are generally more volatile and have low mp/bp. This is caused by weak vdw interactions between the molecules
- molecular compounds are generally solvable in organic solvents. This is due to weak vdw interactions in the solid state
- molecular forms are more reactive than infinite. Eg when reacting with a gas (eg O2 or F2) the rate limiting step may be breaking off a small surface component. The activation energy of this will be lower for molecules.
-infinite structures are generally denser and harder. Small molecules pack less efficiently and the structures are less rigid than infinite structures where all atoms have chemical bonds (not vdw) to neighbours.
What type of structures do group 15 have? Why?
N - forms N≡N
P - allotropes (all single bonds)
As, Sb and Bi - all metallic (there are allotropes for As and Sb but v.v. Low abundance so not important)
More info on phosphorus’ allotropes
White
P4 molecules (shaped like Egyptian pyramids)
Reacts with O2 in air and have low bp/mp
High solubility and reactivity
Red
Polymeric amorphous (a type of polymer where the long chains of molecules are arranged randomly and without a defined, repeating structure)
Black
Infinite 3D
Can get orthorhombic x2, rhombohedral and cubic (diagrams on slide 32)
Violet
What type of structures do group 16 have? Why?
O: + -
O=O, also can have O=O—O
S:
forms S₈ rings made of s.b.
Se and Te:
Infinite covelant solids
Seₓ forms chains (kind of triangular check diagram on slide 36)
Po:
Metallic with primitive cubic structure
What type of structures do group 17 have? Why?
Down the group, there is:
increasing vdwf and increasing bond length.
All have X₂ structures
F: gas
Cl: gas
Br: liquid
I:solid, when pressure is increased it can become metallic
At: highly radioactive metal (not so fussed about this)
Inter and intramolecular bonds in the halide X₂ structure
Down the group X－X bond length increases
Down the group shortest X－X …. X－X distances increases
Therefore ratio of inter/ intra ( (X－X …. X－X) / X－X ) gets smaller down the group
What type of structures do group 18 have? Why?
They are all:
Noble gases
Filled orbitals therefore no chemical bonds
When cooled, solidify into ccp or hcp
Vdwf
Ne and Kr are used in lighting
Atomisation (binding) enthalpy
Follows trend in number of electrons used in bonding by each element
Eg Li is low because only 1 and C and Si are the highest as have 4 e- used. Note: group 18 have a BE of 0
1/2 X₂ (s) —> X(g) or 1/n Xn (s) —> X(g)
DIAGRAM ON SLIDE 40
