ATOMIC Structure and MOLECULAR Structure: Atoms, Molecules, and Crystals in Cultural Heritage

Tangible heritage is an essential aspect of our world’s cultural heritage. It consists of physical items varying from small artifacts to large buildings and archaeological sites. But what are these heritage objects, such as Leonardo da Vinci’s Mona Lisa, for example, made of? Atoms, molecules, and crystals!

The atom and the atomic structure

The atom is composed of a central region, called the nucleus, and of electrons, which surround the nucleus. The nucleus is made up of protons, positively charged particles, and neutrons, neutral particles. Because of the positively charged protons in the nucleus, the nucleus itself has a positive charge.

The electrons, which are outside the nucleus, are negatively charged particles that circle around the nucleus. They are attracted by the positive charge of the nucleus, thus keeping the atom together.

The electrons are organized on different shells surrounding the nucleus, represented here by the blue circle and the orange circle. Each of these shells corresponds to a different energy level. So the shells closer to the nucleus (the blue circle here) have lower energy than the ones that are further away (orange circle). The lower energy shells get populated first, and when they are filled with the maximum number of electrons allowed in that shell, then the outer shells get populated.

The number of electrons around the nucleus is equal to the number of protons inside the nucleus. And this gives us the atomic number (Z), which is the number of protons in the nucleus. This is a number that is specific to each element in the periodic table. The elements in the periodic table are organized function of their increasing atomic number. In the periodic table, we see that the further we go in periods and down the groups, the higher the atomic number. So we’ll have more and more protons in the nucleus and more electrons surrounding it. That means that we need to add more shells in order to accommodate all the electrons of the heavier elements.

These shells and electrons and how the electrons can move from one shell to another one are very important when discussing different scientific techniques, especially X-ray fluorescence (XRF), my favorite scientific technique that is used to study tangible heritage. Different scientific methods can be used to study heritage objects made up of all the different elements. Some techniques are better suited than others to study certain chemical elements.

From atoms to molecules

Atoms can bond to one another to form molecules. There are different types of bonds that the different elements can form with one another, but what’s important to know here is that they can associate to form molecules with varying degrees of complexity.

A simple example is the water molecule. It is made up of only three atoms—one oxygen atom and two hydrogen atoms. The water molecule is formed by binding the two hydrogen atoms to the oxygen atom.

Why, you may wonder, do we care about water in cultural heritage?! We care a great deal about water in cultural heritage. That’s because humidity, that is, water, can lead to the aging and, therefore, the deterioration of art objects.

Similarly to creating a simple molecule, like water, by putting three atoms together, we can do the same with a larger number of atoms and a larger variety of elements. This way, we can create much more complex molecules, each with its own special properties.

From molecules to crystal structures

Atoms and molecules can further associate to form crystals. Starting from one molecule, we can get different types of crystal structures depending on how the molecules are organized with respect to one another in the unit cell, how many molecules of the same kind there are in the crystal unit, how big the unit cell is, etc. These are called polymorphs, and that’s another one of my favorite research topics, besides cultural heritage.

In the unit cell, each atom has an exact position, and by translating the unit cell in all the directions of space, we create the material which is based on the composition and structure of that unit cell. An example of crystal structure relevant to cultural heritage is titanium dioxide, the chemical name for the titanium white pigment. This pigment led to the discovery of a series of painting forgeries and to the arrest of the art forger. This forger is Wolfgang Beltracchi, and he was caught because science detected the presence of titanium white in a certain painting where it shouldn’t have been.

Knowledge of the atomic, molecular, and crystal structure of tangible heritage materials can help us learn more about the materials which the artists used in their work. That will help us better conserve and maybe even restore them, and it can also help to identify fake paintings. Therefore, understanding the chemical structure of tangible heritage is very important for the preservation of cultural heritage.