0.3 Structure of an atom

Introduction

The Atomic Molecular Theory is very powerful in helping us understand matter. We can easily understand the differences between the elements and all other compounds by thinking about the particles which make them up. Elements contain all identical atoms, and compounds contain identical molecules, each composed of definite small numbers of atoms of the elements that make up the compound. We can also do chemical algebra calculations, which allow us to make predictions about the masses of materials that are involved in chemical reactions.

Without further observations, though, this is about as far as we can go because our knowledge about the properties of atoms is very limited. We only know that atoms of different elements have different masses, and we know the relative atomic masses of the elements. But this does not give us any insight into why the atoms of different elements have such differing chemical properties. For example, carbon and nitrogen have very similar atomic masses. However, carbon in one of its forms, diamond, is a very hard, unreactive solid and in another of its solid forms, coal, carbon will burn in oxygen. Nitrogen, though, is a gas and is fairly unreactive with oxygen except at very high temperatures. Clearly, carbon atoms interact with each other very differently than nitrogen atoms interact with each other.

Without knowing anything else about atoms, we might imagine them as tiny hard spheres. But this idea will not help us understand their chemistry, such as why they form the compounds that they do. What does it mean for hydrogen and carbon to form the compound methane? We know that methane’s molecular formula is CH ₄ , but how do these five atoms hold together in a molecule? It seems reasonable to imagine that the attractions are due to magnetic forces or electrical forces between the atoms. However, atoms of hydrogen and carbon must be electrically neutral, since bulk samples of hydrogen and carbon are electrically neutral. They are also non-magnetic. How do neutral, non-magnetic atoms interact with each other? Another riddle is why they form in the particular ratios that they do. CH ₄ is a compound but there isn’t a compound with molecular formula CH ₅ .

Understanding these differing elemental and molecular properties requires us to have a deeper understanding of the properties of individual atoms. Since we cannot understand these properties by thinking of the atoms as individual hard spheres, we need to investigate the structure of the atom.

Foundation

In this study, we will assume that we know the postulates of the Atomic Molecular Theory and our measurements of relative atomic masses. We know that an element is composed of individual atoms with identical masses, and we know that the atoms of different elements have different masses, which have been measured.

We will also assume our understanding from a previous study that electricity consists of individual charged particles called electrons, which are assigned a “negative” charge. The mass of the electron is quite small relative to the mass of an atom. Atoms contain electrons, and electrons can be added to or removed from atoms by applying an electrical current.