Trends down the Groups

In the simplest theory of metallic bonding, the pool of electrons (blue) holds the metal together through interaction with the positive cores. 

All these trends reveal the importance of the increase in distance of the outer electrons from the nucleus as the successive electron shells build up. This increasing separation weakens the hold that the nucleus has on the outer electrons. 

Ionization energy is also the principal factor determining the metallic and non-metallic properties of the elements. It is instructive to explore how these properties of the elemental forms of Groups III to VI follow the trend, down the Group, as illustrated in Sections 9 to 12. 

Finally, bond formation between like elements depends on a number of factors that are dependent on the size of atoms. In particular, the ability of elements to engage in jt—jt bonding with each other results in the formation of multiple bonds. Examples of the tendency to form jt bonds occur in Groups III to VI, and you should note how this is affected by the covalent radii, which increase as we descend each Group. 

1 Substances containing multiple bonds are more easily made or preserved when they contain second-row rather than third-row typical elements. 

---

Historically, the theoretical background to bonding of Group 15 ligands to metals has been developed for phosphines, and treatment of the heavier analogs is in terms of the trends down the group and any differences relative to the PR3. In recent years the metal-phosphine bonding model... 

Describe the solid state structures of (a) the alkali metals and (b) the alkali metal chlorides, and comment on trends down the group. 

Every element (except He, Ne, and Ar) forms at least one oxide, many by direct combination. For this reason, a useful way to classify elements is by the acid-base properties of their oxides. The oxides of Group 6A(16) exhibit expected trends down the group, with SO3 the most acidic and P0O2 the most basic. 

The irregular trend down the group is because of the different ways in which the RAM and vary. The RAM values increase faster than the ionic radii, and the interactions between the two parameters cause the minimum density to be at calcium [although density is not treated in this book, the densities of the elements do show a periodicity that is worthy of study]. 

The melting points show a pattern, or trend, down the group. It is therefore possible to predict that the melting point of astatine is approximately 380 °C. The same can be done with other physical properties, such as the melting points, and atomic properties, such as first ionization energy. 

The halides of arsenic, antimony and bismuth illustrate the following trends down the Group (a) increasing metallic character of the elements (b) the inert pair effect—that is, the tendency towards an increased stability of the Group number minus two oxidation number (+3) (c) a tendency to higher coordination numbers. 

H->0 < H->S < H->Se < H-,Te. Because bond strengths decrease and acid strength increases down the group, it appears that bond strength dominates the trend in acid strength in these two sets of binary acids. 

Explain the trend of decreasing lattice enthalpies of the chlorides of the Group 2 metals down the group. 

The halogens show smooth trends in chemical properties down the group fluorine has some anomalous properties, such as its strength as an oxidizing agent and the lower solubilities of most fluorides. 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

The elements become more reactive down groups 1 and 2. The atoms of the elements lower down the groups lose electrons more easily than those higher up, as illustrated by the trends in ionisation energies of the elements. 

---