Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 32_17 November	© R. Paselk 2003
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Return Exam II.

The Chemistry of the Elements

The Representative Elements

Group VII, cont.

Hydrogen halides: Acidity. Strong - weak: HI > HBr > HCl >> HF. Why? Bond energies decrease HI to HF, but enthalpies of hydration increase as well, as seen in the table:

Data from Zumdahl.   Bond Energy (kJ/mol) Enthalpy of Hydration (kJ/mol) Entropy of Hydration (J/mol*K) HF 565 -510 -159 HCl 427 -366 -96 HBr 363 -334 -81 HI 295 -291 -64

Note that the bond energies and enthalpies of hydration almost cancel in their effects, so the main differences must be due to entropy. So what of HF? It is a weak acid because the small size and high charge density of fluoride ion results in strong binding of water - bound water has decreased entropy.

HF reacts with silica to give silicon fluoride. It is thus used to etch glass, dissolve silica sands for soil studies, and remove silicates from mineral samples in geology studies.

The Transition Metals

The transition elements have typical metallic properties: high reflectivity, a metallic luster, good electrical conductivity, and good thermal conductivity.

Note the electronic structures of the transition elements going across the periods (see chart below). Note the break in pattern at the 4th and 9th elements in the first and second series due to filling of d subshell (symmetry). This effect breaks down in the third series as the s and d subshells get closer due to the inner f subshells and relativistic effects.

On the other hand they exhibit a wide variation in

• hardness (e.g. Cr is about as hard as corundum (it will readily scratch glass etc.) while Au is very soft)
• melting points (e.g. from Hg = -39°C to W = 3390°C)
  • These variations in lattice strength and stability indicate that some transition metals have extensive covalent bonding interactions via their d-electrons and orbitals supplementing their metallic bonding.
• density (e.g. from Sc = 3g/cm³ to 22.6g/cm³ for Ir and Os)
  • The high densities result from the elements high atomic masses, small atomic volumes (small radii) and close-packed structures of their crystals.

The transition metals tend to be quite similar across Periods as well as within Groups in Chemical properties.

• This is a result of the fact that they all have one or two outermost (s) electrons - the differences are due to the filling of the d-electrons which are down inside the atoms (see table below).
  • Thus the chemistry of these elements will be close because of the domination of the valence electrons in bonding and the tendency to lose the s electrons more readily than the d-electrons in ionization. (overhead, Figure 20.2)
  • The second and third series of transition metals are even more alike in their group properties because the f-electrons have filled in between, reducing the atomic sizes of these elements so that the third series (5d) elements are nearly identical in size to the second series (4d) elements in the same group. (overhead, Figure 20.3)

© R A Paselk

Last modified 18 November 2003