This chapter introduces the structure of atoms. As explained in the previous chapter, each element is made of one type of atoms. They determine both the chemical and physical properties of these elements and require a deeper understanding. First, two experimental facts necessary to understand the following chapter are introduced (2.1). This allows the introduction of Dalton's atomic theory (2.2). This rudimentary theory leads the way to more complex topics such as the mass of atoms (2.3), the structure of atoms and their isotopes (2.4 and 2.5). What follows is a discussion of the periodic table (2.6) where relations between different elements are uncovered. Finally, the organization of so called electrons within an atom is discussed (2.7). This leaves us with a basic understanding of the atom and its internal organization. The sections that follow will further enhance this understanding.
2.1 - Chemical Background
Before being able to describe the first atomic theories, two experimental facts known as the Law of Conservation of Mass and Law of Constant Composition have to be stated.
Def 2.1 - Law of Conservation of Mass
Mass is neither created nor destroyed in a chemical reaction. When substances react chemically to create new substances, the total mass of the resulting products is the same as the total mass of the substances that have reacted.
Def 2.2 - Law of Constant Composition
The elements present in a compound are present in fixed and exact proportion by mass, regardless of the compound’s source or method of preparation.
2.2 - Dalton’s Atomic Theory
John Dalton was the first to create a workable general theory of the structure of matter. While this theory is not correct on every particular matter, it provides a basis on which more complex ideas and concepts could be build.
Def 2.3 - Dalton’s Atomic Theory
Dalton’s theory consists of the following propositions:
- All matter is composed of infinitesimally small particles called atoms (believed in Dalton’s time to be indestructible but now known to be composed of even smaller parts).
- The atoms of any one element are chemically identical.
- Atoms of one element are distinguished from those of a different element by the fact that the atoms of the two elements have different masses.
- Compounds are combinations of atoms of different elements and possess properties different from those of their constituent component elements.
- In chemical reactions, atoms are exchanged between starting compounds to form new compounds. Atoms can be neither created nor destroyed.
This is illustrated by the following figure. Atoms (called elementary particles) that are combined into compounds (called compound particles) rearrange to form new compounds. Note that no mass has been lost or gained and that the composition of all compounds is in fact constant.
2.3 - Atomic Masses
In Dalton's atomic theory, the principal difference between elements is the different masses of their atoms. Since one cannot weigh individual atoms to measure their mass, a relative mass scale was proposed. Originally, Dalton assigned a relative mass of 1.0 to the lightest element known; hydrogen. He then assigned masses to other elements by comparing them with hydrogen experimentally. A relative mass scale is still in use today, although the relative masses of atoms are now determined by a technique called mass spectrometry instead of experimentally.
The relative masses of atoms are called the atomic masses of the elements. Atomic masses are now defined relative to the mass of the most common isotope of the element carbon (), whose mass is specified as exactly 12 atomic mass units (, or ).
2.4 - The Structure Of Atoms
It is assumed in Dalton's atomic theory that the atom is featureless and indestructible. This is untrue. An atom is composed of subatomic particles, some of which have an electric charge and some that do not.
Def 2.4 - Subatomic Particles
An atom consists of three types of subatomic particles:
- Protons are particles that are positively charged.
- Electrons are particles that are negatively charged.
- Neutrons are particles that are uncharged.
The charges, atomic mass and location of these subatomic particles can be found in the following table:
Subatomic Particle | Electrical Charge | Mass (amu) | Location |
proton | 1+ | 1.00728 | nucleus |
neutron | 0 | 1.00867 | nucleus |
electron | 1- | 0.005486 | outside nucleus |
The following illustrations shows a view of the atom with its subatomic particles; protons (green), neutrons (yellow) and electrons (red).
It is clear that protons and neutrons account for most of the atom's mass. They are located together in a structure that lies at the center of the atom; the nucleus.
Electrically charged particles repel one another if their charges are the same and attract one another if their charges are opposite. Even though protons and electrons have different masses, the magnitude of their respective charges are the same. That the atom is electrically neutral indicates that the oppositely charged particles are present in equal numbers; the number of protons in an atom is balanced by exactly the same number of electrons. The number of protons in the nucleus is called the atomic number and is unique for each element. Atoms of elements may acquire a charge by gaining or losing electrons (e.g. in reactions with other compounds or elements).
Def 2.5 - Ions
An atom bearing a net electrical charge is called an ion.
- A positively charged ion is called a cation
- A negatively charged ion is called an anion