Lecture 7

Atoms in Combination and the Chemical Bond

Let us recall some of the basic facts we learnt about atoms :

atoms contain a central nucleus, carrying most of the mass and all of the positive charge. The nucleus is surrounded by some sort of electron cloud. The number of negative charges (electrons) surrounding the nucleus is equal to the number of positive charges (protons) inside the nucleus, so that whole atoms are electrically neutral.

Within the atom, electons are organized into different shells. For atoms of a given element (i.e. with a given number of protons), each shell has a characteristic energy. The number of electrons that can occupy any given shell is determined by the rules of quantum mechanics and is constrained by Pauli Exclusion Principle. These rules in turn provide the explanation for the regularities in the elements as represented by the Periodic Table.

The actual nature of electrons and the electron "orbits" are much more complex than a naive "solar system" model or even than the first quantization attempt suggested by Bohr's model. Even so, a simple minded picture of the atoms in terms of electron orbits will be sufficient to understand to a fair extent the basics of chemical bonds and material properties. We should nevertheless be aware that a deeper or more quantitative understanding could only be achieved by applying the laws of Quantum Mechanics.

With this pre-amble, we can now try to understand some of the basic properties of ordinary matter. A first question that might come to mind is :

given that we have learnt (remember Rutherford's experiment) that matter is mainly empty space, how can matter appear to be quite solid and why (apparently) solid bodies do not penetrate each other? The answer comes from the electron structure of matter and from the effect of the electrostatic force : bodies are prevented to penetrate each other by the repulsive electrostatic forces between like charges (i.e. between the outer layers of electrons in the approaching bodies).

As a next step, we will see that the atomic electrons also determine completely all of the chemical (and most of the physical) properties of matter. Even more surprisingly, the whole body of chemistry is determined merely by the number of electrons in the atom's outer shell !!!

The key to the understanding the chemical properties of matter is contained in two principles :

1.

given the freedom to do so, a physical system will tend to reach the condition of lowest (potential) energy.
Examples : a body brought to a given height will tend to fall back to the lower level; two opposite charges kept at a given distance will tend to move towards each other; an electron in a higher energy state in the atom will eventually jump back to the ground state, etc.

2.

for an atom, the lowest energy state corresponds to the situation where the outermost electron shell is fully occupied.

When brought together, atoms of different substances will tend to re-arrange their electrons, so as to move towards a lower energy configuration. If this does happen, the excess of energy will be released, typically in the form of heat (combustion, i.e. burning is a typical example of such a process). The lower energy status will necessarily be more stable than the higher energy one, since energy would have to be provided from the outside to go back to the higher energy condition. This "lack of energy", properly called binding energy is what gives origin to the chemical bond

Before proceeding further, let's clarify the difference between compounds and mixtures.

Oil and water don't mix (almost). If you pour oil into water, it will settle at the top. If you then run it through a blender, the oil will break up into smaller and smaller droplets, but it will still remain separated from the water, even if you make the droplets smaller and smaller. This is a mixture (actually in this case it is called an emulsion). Other examples of mixtures are sand, where the different components are free from each other, or rocks, when they are binded together, or even the atmosphere, which is a mixture of several gases.

In all of this cases there is no combination of the constituents at the atomic, but at a much larger, scale. We have instead a compound when elements combine at the level of individual atoms, to form molecules.

Chemical bonds

Remembering the statement that atoms attempt to reach the most stable configuration, which is associated with a fully occupied outer shell, the most obvious configuration for a bond is the union of two elements having respectively a single electron and a single vacancy in the outer shell. When allowed to combine, an exchange of a single electrons will take place, leaving respectively a positive and a negative ion, which will be bound together by the electrostatic attraction between them. In this way elements from the first family (Lithium, Sodium, Potassium, etc.) will readily combine with elements from the seventh (Chlorine, Bromine, Iodine, etc.). This type of bond is called the ionic bond. Ionic bonds can also involve the exchange of two electrons, either between single atoms (second and sixth family) or in the ratio 1 : 2 (one atom from the second and two from the seventh family).

A large assembly of ionically bound molecules will organize itself into regular structure (crystals), with a regular alternating pattern of the two components in a 3-dimensional grid.

A different type of bond is the metallic bond, where "surplus" electrons leave the individual atoms, but are uniformly distributed in the bulk of the materials. The attractive force between the positive ions and the "sea" of electrons is what holds the material together.

This type of bond does not require the intervention of two different elements, but is rather the one encountered in an assembly of identical atoms (i.e. an element), especially the ones identified as "metals".

The "sea" of quasi free electrons is what gives to metals their characteristic properties, e.g. their very good capability to conduct an electric current. This can also explain why the photo-electric effect operates preferentially with metals, since it is relatively easy (provided enough energy is given) to pull out an electron from the body of the substance.


When, rather than throughout the bulk of the material, electrons are shared among a cluster of a few atoms, we deal with a covalent bond. Many elements are found in nature forming molecules of covalent bonds. Oxygen and Hydrogen for instance much prefer to share a total of respectively 2 or 4 electrons among pairs of atoms, so that either atom "feels" as if it had a full shell... This is why they are found as O₂ and H₂ molecules, and in this state they are much less reactive than the individual atoms.

An element particularly versatile in the possibility of covalent bonds is Carbon (four electrons in the outer shell). Carbon can bind either with another carbon atom, and it can do so either in a single or a double bond (one or two electrons shared), or with other elements. One well known family is the hydrocarbon series , consisting of more and more complex structures of Carbon and Hydrogen covalent bonds. The first member of the family is methane, CH₄

H

H-C-H

H

followed by ethane, C₂H₆

H H

H-C-C-H

H H

propane, butane, pentane, etc... Can you guess the formula for octane ?

A C₈H₈

B C₈H₁₂₈

C C₈H₁₆

D C₈H₁₈

E C₈H₂₄

Starting from these simple rules, one can form molecules of incredible complexity, with 3-dimensional structures. Many of these are found in nature, as part of living organisms, other are produced synthetically (plastics, polimers, etc.). All these are studied in organic chemistry

Polarization of water molecules

The arrangement of the atoms, and of the shared electrons, within the molecules can give raise to other effects, due to possible asymmetries in the charge distributions. One typical example comes from water, and the polarization of its H₂O molecule. The structure of the water molecule is asymmetric, and the sharing of electrons is such that they tend to be found more probably on the side of the Oxygen, rather than of the two Hydrogen atoms. The net effect is that, although the water molecule is overall electrically neutral, the charge is distributed asymmetrically so that the Hydrogen side is preferentially positive, while the Oxygen side is negative (one then says that the molecule is polarized). As a consequence, water molecules can exert an electrostatic force onto other molecules in their vicinity, causing a rearrangement, and consequent polarization, ot their own electron cloud. The resulting force can give origin to a different type of bond, called Hydrogen Bond.

Another effect of the polarization forces is the possibility to break up an ionically bound compound into its ion components. For example, polarization of the water molecules can cause NaCl molecules to separate into their Na⁺ and Cl^- constituents, ending up with a solution of electrically charge ions. The existence of solutions explains the process of electrolysis, by means of which the elements forming a given compound can be isolated.

If a negative and a positive electrode (e.g. the two terminals of a battery) are immersed in a solution of some substance, negative and positive ions will migrate towards the electrode of opposite polarity. There they will neutralize themselves by acquiring or releasing one or more electrons, and the original element can be recovered.


















Water always contains variable amounts of dissolved materials. In some case this is good (even if you have never tasted natural mineral water, you have certainly tried carbonated water), but often this is bad (lead, pesticides, etc.). We should make a very serious attempt at maintaining the healthiness of our water supply.