All Chemistry is Driven by Physics

About the author
Philippa Logan read Natural Sciences (specialising in Chemistry) at Newnham College, Cambridge. 

‘All science is either physics or stamp collecting.’ So said Ernest Rutherford, the father of nuclear physics.

And it’s true that Biology is based on Chemistry, which in turn is based on Physics – and this continuum can easily be taken to the point of absurdity. At risk of annoying pure mathematicians, we’ll look at some examples of chemical phenomena to illustrate how physics is the driving force behind them. First, however, we’ll think about how the chemical elements are organised, which itself is based on the principles of physics.

1) The Periodic Table

The Periodic Table, created by Dmitri Mendeleev in 1869, was a great step forward in the understanding of the chemical elements. It sets out the chemical elements in order of increasing atomic number, each successive element in the table having one more proton in the nucleus than the previous one.
From the Periodic Table, we can deduce many things about the chemical aspects of the elements. But it is physics that dictates the way in which the electrons are situated in the orbital shells. The orbitals are described by a mathematical equation (the Schrödinger equation); they have different quantum numbers and therefore different energies. It is an underlying principle of physics that energy must be minimised, which dictates that the lowest-energy electron shells are filled first.
The chemical properties of the elements and, importantly, their reactivity depend on the number of electrons in the outer shell. The noble gases have outer shells of valence electrons that are completely filled, which is a very stable configuration, and so they are inert. Only a few hundred compounds have been made from noble gases.
Almost all of chemistry can be explained in terms of the Periodic Table. Now let’s have a look at some specific chemical phenomena.

2) Why are some chemicals coloured?

A typical experiment that is carried out to attract students to chemistry is one that involves a colour change. The demonstrator will mix two colourless liquids together, and they will ‘miraculously’ change colour, or a coloured precipitate will appear. This is one of the most obvious illustrations of a chemical reaction – but it’s all down to physics, the structure of the atom, and the arrangement of its electrons.
For instance, let’s take these two clear liquids: potassium iodide and lead nitrate. When they are mixed, a yellow precipitate of lead iodide forms.
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Obviously, a chemical reaction has occurred, but the colour change is all down to physics. Electrons in atoms and molecules stay in quantised energy levels, so the atoms or molecules will only absorb light photons of specific energy – the energy required to take an electron from one allowed quantum state to another. That is, a specific frequency (and therefore specific wavelength) of white light will be absorbed because energy is directly proportional to frequency (and inversely proportional to the wavelength).The new molecule will take on the complementary colour to the wavelength of light most strongly absorbed.
This phenomenon of colour changing has many applications: testing solutions for their acidity or alkalinity usually involves a colour change of an indicator. Colour changes in titrations are another much-used method used in chemical analysis – but it’s all down to physics.

3) What happens at the melting point?

The melting point of a chemical element or compound is a simple concept: it is the temperature at which the element or compound changes state from a solid to a liquid. For most substances the melting and freezing points are the same. What determines the melting point is the strength of the forces between the atoms or molecules: the stronger the forces, the higher the melting point.
Sodium chloride, for instance, has a giant ionic structure: Na⁺ ions and Cl^– ions ordered in a regular three-dimensional lattice. The force in the lattice is electrostatic attraction between opposite charges, which is very strong. A great deal of thermal energy is required to overcome this force and consequently sodium chloride has a high melting point.
Conversely, simple covalent compounds have low melting points, because the only forces are intermolecular forces (Van der Waals forces), which are weak and thus easily broken.
For most substances, the melting point and the freezing points are the same. However, there are some substances for which this is not true: agar melts at 85°C but solidifies at between 32°C and 40°C. This property of melting easily but being very stable at relatively high temperatures makes agar extremely useful as a solidifying agent. With agar, as with all other compounds and elements, the exact point of freezing and melting is determined by the ease or difficulty with which the polymer can be broken and reassembled. This in turn is determined by the energy needed for the process to begin (the activation energy) which is thermodynamics, a major principle of physics. So, for agar, as for all other substances, it is the physics that explains the melting and freezing points.

4) Why does carbon exist in such different forms?

Carbon is one of the basic elements of life and of all chemistry. Organic chemistry would be nowhere without it. Yet, as an element, carbon exists in two very different forms: diamond and graphite. If you didn’t know they were made from the same element, you would hardly guess it. Diamond is extremely hard, transparent, shiny, is insulating, and is a precious material; graphite is soft, opaque, conducts electricity, and is so far from precious as to be used in pencils.
Here physics steps in once more to explain the difference, which is due to the arrangement of the atoms. In diamond, each carbon atom is strongly bonded to four others in a tetrahedral arrangement. The four valence electrons of each carbon atom create very strong covalent bonds, of the same strength in all four directions, which is why diamond is so hard. There are no free electrons, so diamond is an insulator. The very high refractive index gives diamond its treasured brilliance.
Graphite is arranged quite differently. Here, the carbon atoms are arranged in layers. Every carbon atom in each layer is bonded to three other atoms in the same plane, as if to the corners of an equilateral triangle. Three of each atom’s valence electrons are involved in these three covalent bonds; the fourth electron is delocalised, which is why graphite conducts electricity (very rare for a non-metal). The layers are held together by weak Van der Waals forces, so they can easily slide over each other, which explains graphite’s lubricating properties.

5) Why does ice float?

It’s winter, and you’ll probably have plenty of chances to see ice floating on puddles – not to say ice floating in your glass of water. You probably don’t pay a second thought to it, but actually it’s unusual for a substance in solid form to float on the same substance in liquid form. This means that the substance must be less dense than itself in liquid form – uncommon, to say the least. Most solids are more dense than their liquids, and therefore sink to the bottom.
This time, the reason concerns the physics of hydrogen bonds. Although hydrogen bonds have only about 10% the strength of covalent bonds, they have a significant influence, especially when there are a lot of them.
Hydrogen bonding occurs when a hydrogen atom is located between two very electronegative atoms, such as oxygen atoms. The water molecule, H₂O, is polar, with dipole-dipole attractions between molecules. But in fact, the bonding between molecules is much stronger than these dipole-dipole attractions, as hydrogen bonding comes into play.

When water is in the liquid state, the molecules have higher kinetic energy; as they move around, the hydrogen bonds break and reform easily. However, when water freezes and becomes ice, the water molecules have much lower kinetic energy and the hydrogen bonds can now hold the water molecules in position. The resulting structure is similar to the three-dimensional lattice of diamond, and is held together by covalent bonds and hydrogen bonds; the water molecules are further apart in the solid state than in the liquid form.
This is why water expands when it turns to ice and is why ice is less dense, and so floats on water. All because of the physics of those hydrogen bonds.

6) Why do ionic compounds conduct electricity only when molten?

Ionic compounds conduct electricity when they are molten or in solution, but typically not when they are in the solid form. Why should this be? Well, the answer lies in the physics of the chemical bonding.
Bonding is all about electrons. With ionic bonding, electrons are swapped from one atom to another in such a way that each atom has a full outer shell, either by gaining or losing electrons. Electrostatic attraction between the anions and cations formed typically results in a giant crystalline lattice of closely stacked ions, in which individual molecules cannot be distinguished. In the solid form, the ions are not free to move, and the electrons are also held tightly, so the compound cannot conduct electricity.
However, when molten or in solution, the ions are free to move and can transport electrons between the cathode and the anode – so they can conduct electricity.
As a counter-example, a few ionic compounds (e.g. rubidium silver iodide) can conduct electricity in the solid state. Here, the silver ion is relatively mobile within the crystal lattice due to vacant ion-spaces in the lattice, and thus is an electrical conductor.

7) What makes a reaction suddenly happen?

The study of chemistry involves the study of chemical reactions. But what makes a reaction happen?
The Maxwell-Boltzmann distribution helps explain how a reaction can suddenly reach the required activation energy, and how temperature also affects the likelihood of a reaction taking place. It refers specifically to the speed of particles in a gas: the particles are all moving at different speeds, a few moving slowly, some very fast, but the majority of them at intermediate speeds. As the energy of a particle depends on its speed, the particles also have different energies; the peak on the Maxwell-Boltzmann curve represents the most probable energy.

The graph, derived from statistical mechanics, illustrates how particles must have a certain activation energy before they are able to react (dark blue curve). If the temperature is increased from T₁ toT₂, the shape of the curve changes (light blue curve): its peak becomes lower, and moves to the right. The activation energy remains the same, but more particles have this required activation energy, and therefore the reaction proceeds at a faster rate.
All reactions depend on the particles having sufficient activation energy, and therefore all chemical reactions depend on physics.

8) Why is nitrogen triiodide so unstable?

Nitrogen triiodide, NI₃, is a contact-explosive. At the lightest movement – the touch of a feather or a slight puff of air – small quantities explode with a loud, sharp bang, releasing nitrogen and a purple cloud of iodine vapour.
2 NI₃ (s) → N₂ (g) + 3 I₂ (g)
It is far more explosive than nitroglycerin, but, unlike nitroglycerin, its shock sensitivity cannot be reduced with stabilising agents and therefore it has no practical use as an explosive.
The instability of NI₃ results from the structure of the molecule. Three large iodine atoms (atomic number 53) are held in close confinement near a relatively small nitrogen atom (atomic number 7). The bonds are under great strain, and the activation energy for the decomposition is therefore extremely low – and the energy released in the reaction is great as one of the products of the explosive reaction, nitrogen, is very stable.
Just another example of how physics – in this case, the structure of the molecule – affects the chemistry of the compound. 

Philately and physics

Chemists and biologists are, not surprisingly, defensive about their subjects, and would strongly defend them against being compared to stamp-collecting. And in truth, there is no hard and fast dividing line between the three major sciences. Nevertheless, once you delve into biological processes and chemical phenomena, it’s interesting to see how so much of what goes on depends on the laws of physics. You could say that anyone studying Chemistry or Biology is studying Physics as well, whether they are aware of it or not.