Lesson 1: In Search of the Atom

Part a: Democritus to Dalton

Part 1a: Democritus to Dalton
Part 1b: The Inside Story of the Atom
Part 1c: Subatomic Particles

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What is Matter Made Of?

It is no surprise to most students that matter is made of atoms. But how did scientists come to believe in such a theory? What evidence is there for believing in the invisible world of atoms? The journey to understanding the atom is a demonstration of science as a verb. Science is more than a body of knowledge. Science is something that one does; it is a verb. And as a verb, science involves doing experiments and reasoning towards conclusions that can be added to the body of knowledge. As a noun – a body of knowledge – science grows in size over the course of time. And it grows in size because science is a verb. In Lesson 1, we will look at the many experiments and observations that led scientists down the pathway towards a belief in atoms.
 
 
 

A Philosopher’s View

One of the earliest theories of the atom dates back to the Greek philosopher Democritus in the 5^th century, BC. Democritus used the term “atomos” which means indivisible to describe the atom. For Democritus, the atom was an invisible and indivisible, hard, solid, and indestructible particle. The varying properties of matter were explained by the fact that atoms came in a variety of sizes and shapes. Democritus believed that atoms were in constant random motion and their collisions with one another could result in the formation of more sophisticated complexes of atoms.
 
Democritus’ atomic theory was not a product of science. He was a natural philosopher. Science as a verb was not part of the vocabulary in ancient Greek philosophy. The age of science as an empirical, evidence-based practice had not yet been born. It would be more than a couple of millennia before science would tackle the question of what is matter made of. We must fast forward through history to the early 1800s for the first evidence-based model of the atom.
   

Dalton’s Atomic Theory

John Dalton (1766 – 1844) was an English scientist who made numerous contributions to the fields of meteorology, chemistry, and physics. Dalton’s atomic theory (1803) consists of five postulates or statements regarding atoms and their interactions.
 

1. All matter is composed of extremely small particles called atoms.
2. Atoms cannot be subdivided, created, or destroyed.
3. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
4. Atoms of different elements can combine in simple whole number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.

 
Dalton did extensive studies with a variety of gases, making measurements of the amounts by which they combined, the identity of the resulting products, and the amount produced. His own studies and the findings of many 18^th century chemists formed the experimental foundation of his atomic theory. The theory can be thought of as an organized model for explaining a wide collection of observations in terms of atoms.
     

Different Types of Atoms

One of Dalton’s assertions was that there are many different types of atoms. This was foundational to explaining why there are many different types of substances. And to Dalton, mass was a relative measure of how many atoms were present in a sample of matter. For this to be true, then atoms of the same type must have the same mass and atoms of a different type must have a different mass.
   
 
 

The Law of Constant Composition

French chemist Joseph Proust is credited with discovering the Law of Constant Composition (1797). Sometimes called the Law of Definite Proportions, it states that the mass ratio of elements in a specific compound is always the same ratio for that compound. The mass ratio of oxygen to hydrogen in water is always 8:1. And the mass ratio of oxygen to carbon in carbon dioxide is always 8:3. Dalton’s atomic theory explains this finding quite easily. Compounds are made from atoms of different elements. Atoms of different elements have a different mass ratio. Therefore, it would be expected that the mass ratio of elements in a compound like water to always be the same mass ratio.
   
 
 

The Law of Multiple Proportions

Dalton claimed that atoms of two elements can combine in simple whole number ratios to form compounds. This results in a constant mass ratio of the elements in a specific compound. But Dalton also claimed that any two elements can combine to form more than one compound. So, there can be more than one ratio of atoms by which elements combine – a different mass ratio for different compounds. For example, the elements hydrogen and oxygen don’t always combine to form H₂O. Hydrogen and oxygen can also combine to form H₂O₂. The elements nitrogen and oxygen can combine to form NO. But they can also combine to form N₂O. In all such instance where there were multiple proportions by which atoms combined to form compounds, Dalton observed that the mass ratios of the elements in those compounds were simple multiples of one another. For instance, the mass ratio of hydrogen to oxygen in H₂O is 1:8. And the mass ratio of hydrogen to oxygen in H₂O₂ is 1:16. One H:O ratio is twice the other H:O ratio.
   
   

The Law of Conservation of Mass

In 1774, French chemist Antoine Lavoisier showed that a chemical reaction involves no discernible change in mass. The mass of all reactant chemicals is equal to the mass of all product chemcials. His law is often paraphrased as “Nothing is lost. Nothing is created. Everything is transformed.” This makes sense in terms of Dalton’s atomic theory. Chemical reactions involve the rearrangement of atoms. Bonds holding atoms together in the reactant chemicals are broken. Then the atoms rearrange differently to form product chemicals. All the atoms that were initially available in the reactants are present in the final state with a different arrangement. Since atoms are conserved, it makes perfect sense that mass is also conserved.
   
 
 

Moving Beyond Dalton

Dalton’s atomic model is not an atomic structure model. For Dalton, an atom was simply a rigid, solid sphere. There was no mention as to what was contained inside the sphere. There was no evidence provided as to why it had to be a sphere and not a cube or a parallelogram or even a multitude of shapes. Science is seldom stagnant. Models are modified, added to, rejected and replaced. This is the nature of science. As we shall see, the century that followed Dalton’s death led to numerous developments in Chemistry and Physics that further refined our understanding of the nature of atoms.