0.2 The structure of an atom

Foundation

We begin as a starting point with the atomic molecular theory. We thus assume that most of the common elementshave been identified, and that each element is characterized as consisting of identical, indestructible atoms. We also assume thatthe atomic weights of the elements are all known, and that, as a consequence, it is possible via mass composition measurements todetermine the molecular formula for any compound of interest. In addition, we will assume that it has been shown by electrochemicalexperiments that atoms contain equal numbers of positively and negatively charged particles, called protons and electronsrespectively. Finally, we assume an understanding of the Periodic Table. In particular, we assume that the elements can be groupedaccording to their common chemical and physical properties, and that these chemical and physical properties are periodic functionsof the atomic number.

Goals

The atomic molecular theory is extremely useful in explaining what it means to form a compound from itscomponent elements. That is, a compound consists of identical molecules, each comprised of the atoms of the component elements ina simple whole number ratio. However, our knowledge of these atoms is very limited. The only property we know at this point is therelative mass of each atom. Consequently, we cannot answer a wide range of new questions. We need a model which accounts for theperiodicity of chemical and physical properties as expressed in the Periodic Table. Why are elements which are very dissimilar inatomic mass nevertheless very similar in properties? Why do these common properties recur periodically?

We would like to understand what determines the number of atoms of each type which combine to form stablecompounds. Why are some combinations found and other combinations not observed? Why do some elements with very dissimilar atomicmasses (for example, iodine and chlorine) form very similar chemical compounds? Why do other elements with very similar atomicmasses (for example, oxygen and nitrogen) form very dissimilar compounds? In general, what forces hold atoms together in forming amolecule?

Answering these questions requires knowledge of the structure of the atom, including how the structures of atomsof different elements are different. Our model should tell us how these structural differences result in the different bondingproperties of the different atoms.

Observation 1: scattering of α particles by atoms

We have assumed that atoms contain positive and negative charges and the number of these charges is equal inany given atom. However, we do not know what that number is, nor do we know how those charges are arranged inside the atom. Todetermine the location of the charges in the atom, we perform a "scattering" experiment. The idea is straightforward:since we cannot "see" the atomic structure, then we instead "throw" things at the atom and watch the way inwhich these objects are deflected by the atom. Working backwards, we can then deduce what the structure of the atom must be.