Scaffolding explanations

In particular, the materials aim to demonstrate how a range of phenomena that chemists study are explained in terms of a limited set of basic chemical concepts. 

Explaining chemical phenomena (1)

Chemists use their models and theories to try and explain phenomena about chemical systems. Suggest an explanation for each of the following (you may find it useful to refer to a periodic table)

1. Lithium has a higher melting temperature (454 K) than sodium (371 K).
2. There is stronger bonding, called hydrogen bonding, between molecules of water (H₂O) than between molecules of hydrogen sulfide (H₂S).
3. The nitrogen atom is smaller than the carbon atom (ieit has a smaller covalent radius – 0.074 nm compared to 0.077 nm).
4. Chlorine is more electronegative than bromine.
5. Ammonia (NH₃) is a stronger base than phosphine (PH₃).
6. Magnesium chloride has a larger lattice energy (2489 kJmol–1) than calcium chloride (2197 kJmol–1).
7. More energy is released when sodium ions are hydrated (390 kJmol^–1) than when potassium ions are hydrated (305 kJmol^–1).
8. Nitrogen is less electronegative than oxygen.

Constructing chemical explanations

Chemists use models and theories to try and explain phenomena about chemical systems. Although chemists seems to use a wide range of different models and theories, many of them are based on the same few basic principles.

If you can learn about these basic ideas you can use them as ‘tools’ to build up chemical explanations. 

Some basic ideas used in explaining chemistry

The importance of size and charge

A large number of chemical phenomena can be explained, at least partly, in terms of simple ideas like the size and charge on ions or other particles.

Charge density

If two ions have the same charge, but are different in size, then the smaller one is said to have a greater density of charge. The ion with the greater density of charge can often have a greater effect – if it can get up close to other ions or molecules.

An ion with greater charge density can form a stronger bond with an oppositely charged ion. This can lead to a more stable ionic lattice, which therefore requires more energy to disrupt. (So the lattice energy of magnesium chloride is greater than the lattice energy of calcium chloride - as the Mg²⁺ ion, which is smaller, has a greater charge density than the Ca²⁺ ion, even though they have the same charge.)

If the ions are of similar size, but have a different charge the one with the greater charge will have the larger charge density, and may be able to form stronger bonds.

When ionic materials dissolve in water the ions are hydrated (surrounded by water molecules which bond to them). The greater the charge density of an ion the more water molecules will bond to it, and the more energy will be released when a material with that type of ion dissolves.

These ideas do not always help us predict what will happen in experiments, as sometimes there are several effects operating at once. For example, an ion with a greater charge density can be hydrated more (which would make the material more soluble) but will usually bond more strongly to oppositely charged ions (which would make the material harder to dissolve!)

In some books the terms charge:size (charge to size ratio) or charge:volume (charge to volume ratio) may be used instead of charge density.

Core charge

Many chemical processes can be – at least partly – explained in terms of the charges in the ions or molecules involved. Atoms are neutral, but separate atoms are seldom involved in chemical processes. Usually we are concerned with ions or molecules.

In what ways are ions and molecules like atoms, and how are they different?

Atoms may be thought of as a positive nucleus surrounded by several shells of electrons. (Of course, the electronic structure is more complicated than that, with different types of orbitals. However, it is often useful to think in terms of shells.) Most of the time the nuclei of the atoms do not change (and when they do this is studied by physicists). Usually only the outermost shell of electrons, the valence shell, is changed in chemical processes. The nucleus and all the inner shells are usually not significantly changed.

The term core is used to describe the nucleus of an atom, plus all the electrons that are not in the outer (valence) shell.

The charge on an atomic core is called the core charge. It is often useful to know what the core charge is.

The core charge will equal the positive charge on the nucleus plus the negative charge of all the inner-shell electrons.

Notes

The information above is a sample of the full chapter. For the full version of this chapter, see downloads below.