Unit 7: Historical Models of the Atom

Unit Overview:

In the last unit, you explored how chemists study matter through its interactions by examining changes in matter, which are classified as either chemical or physical. Recall that in unit 4, you learned that an element is a pure substance that is made up of a single type of atom. Elements are often referred to as the building blocks of matter because they are the smallest type of matter than can exist on their own. The smallest particle of an element that can exist on its own is an atom.

In this unit, you will begin to dig more deeply into these particles, which make up elements. You will explore the historical development of the model of the atom.

What are the origins of the concept of the atom?

The concept of the atom began in philosophy. A philosopher explains the world around them through thinking and logic. The concept of the atom was first considered in the 5th century BC. This concept is credited to the Greek philosopher, Democritus, and the word atom is based on the Greek word “atomos,” which literally translated means “not cut” and is interpreted to mean indivisible. Democritus proposed that the world was made up of the smallest piece of matter, that combines to form other forms of matter. How did Democritus develop this idea? Although we cannot know exactly how he came up with this idea, you can imagine that perhaps one day, he was lying under a tree, and he may have thought to himself: “I can cut this tree down to gather wood for a fire. I can cut the tree into smaller logs. I can cut the small branches into kindling. I wonder if I ever get to the point that I can no longer cut it into smaller pieces?” Then, he imagined the answer to his question was, “Yes. Eventually, I will cut this into the smallest pieces of the tree that can exist, and imagined the world was made up of the smallest particle, some piece that he could ‘not cut,’ thereby theorizing the existence of the atom.”

During this time period, Democritus’ concept of the atom was shared by another Roman philosopher, Leucippus. However, the philosopher, Aristotle, shared a competing philosophy, in which he proposed the existence of 5 elements -- earth, water, air, fire, and aether. The Aristoelian perspective was more widely accepted.

What are the reaction laws?

The concept of the atom is revisited in the realm of science much later in time. A scientist explains the world around them through thinking and logic about experimental evidence. Two very important laws that describe chemical reactions helped to ground the concept of the atom in scientific theory.

1. The law of conservation of matter states that no matter can be created or destroyed during any ordinary chemical reaction.

● Scientist: Antoine Lavoisier, 1789

● Summary of the experiment: Metals were burned in a closed system. The total mass of the system before and after the burning process was recorded.

● Experimental evidence: The beginning mass of the system was the same as the ending mass of the system.

● Explanation: The total mass of the system remains constant throughout a change. Therefore, the same amount of matter that was present before the reaction is present after the reaction. This explanation became the law of conservation of matter after many other scientists replicated the results.

2. The law of definite proportions states that the mass of one element that combines with a fixed mass of another element to form a compound is always the same. In other words, elements combine in the same small whole-number ratios to form a compound.

● Scientist: Joseph Proust. 1804

● Summary of the experiment: Proust mathematically compared the mass of elements that were used to synthesize carbon dioxide and copper carbonate.

● Experimental evidence: The ratio of the masses of elements that combined remained constant. That is, when a smaller amount of 1 element was used, it also required a smaller amount of the other element; when comparing the ratio of the masses that combined, the ratio was always the same.

● Explanation: The constant ratio of elements to form compounds indicates that elements always combine in the same way to form a different substance. This explanation became the law of definite proportions after many other scientists replicated the results.

3. The law of multiple proportions states that when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.

● Scientist: John Dalton

● Summary: The existence of this third reaction law was predicted by the 5 postulates of the atomic theory. [outlined below] After proposing the postulates to explain the above 2 reaction laws, Dalton predicted that if his postulates were correct, then the statement of the law of multiple proportions should also be able to explain chemical reactions.

● Experimental evidence: After Dalton predicted its existence, many scientists gathered experimental evidence that compared the masses of elements that combine to form more than 1 compound. This evidence showed that the mass ratios of elements between compounds were also a whole number.

● Explanation: The whole number ratio of elements between compounds indicates that elements must consist of the smallest unit that combines differently with other atoms of different elements.

What is the atomic theory?

Based on the above 2 reaction laws, John Dalton proposed 5 postulates to explain why these reaction laws described chemical reactions. These 5 postulates are now referred to as the atomic theory.

Postulates of Dalton’s Atomic Theory:

1. All matter consists of tiny particles called atoms.

2. Atoms of the same element have identical properties; atoms of different elements have differing properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms can combine in small whole-number ratios to form compounds.

5. During chemical reactions, atoms can be combined, separated, or rearranged.

This theory both predicted the existence of the third reaction law and provided scientific evidence of the existence of the atom.

Watch the Atomic Theory video that is shown below to further consider the postulates of Dalton’s Atomic Theory:

Practice 1: Take this online quiz.

What are the historical models of the atom?

The experimental evidence was used to develop the model of the atom. Based on the reaction laws and Dalton’s atomic theory, the first model of the atom is referred to as the spherical model of the atom.

What it looks like:

What it's based on: ● Law of conservation of matter ● Law of definite proportions ● Law of multiple proportions

Why it explains the evidence: If matter cannot be created nor destroyed and can only combine in small, whole-number ratios, then whole atoms must react.

However, further experimental evidence was used to develop the model of the atom over time. Many scientists and many experiments have contributed to our understanding of the atom. Two of those historical experiments are described below:

1. The Cathode Ray Tube Experiment

● Scientist:JJ Thomson, 1897

● Summary of the experiment: Cathode rays, light given off by elements when an electrical current is passed through them, were placed in a magnetic field.

● Experimental evidence: The cathode ray was attracted to the positive end of the magnetic field.

● Explanation: The particles that make up the cathode rays are conducting electricity and must be negatively charged.

● Discovery: The electron is a negatively charged particle that exists within the atom.

● Model of the atom: This evidence is described by Thomson’s plum pudding model of the atom.

What it looks like:

What it's based on: ● Thomson’s Cathode Ray Tube Experiment ● The discovery of the electron

Why it explains the evidence: There is evidence of a negatively-charged particle within the atom, but matter is also neutral, so the negative particles are placed into a sea of a positive charge.

In order to help you visualize the experiment, please watch the following video:

2. The Gold Foil Experiment

● Scientist: Ernest Rutherford, 1909

● Summary of the experiment: Alpha particles, highly energetic, positively-charged particles were shot through gold foil, detecting what happened to them.

● Experimental evidence: The majority of the alpha particles went straight through the foil; a small percentage of them were deflected at angles; a very few of them bounced back from the foil, never making it through.

● Explanation: The majority of the alpha particles that went straight through the foil did not encounter any other charged particle within the foil. However, the small percentage that was deflected at angles came close to a positive charge and the very few that did not make it through the foil directly encountered a positive charge within the foil.

● Discovery: The nucleus is a dense center of positive charge that exists within the atom.

● Model of the atom: This evidence is described by Rutherford’s nuclear model of the atom.

What it looks like:

What it is based on: ● Rutherford’s Gold Foil Experiment ● The discovery of the nucleus

Why it explains the evidence: Because so few alpha particles encountered a positive charge within the foil, it cannot be a sea of positive charge, but rather a dense center of a positive charge.

In order to help you visualize the experiment, please watch the following video:

Practice 2: Take this online quiz. (But you can ignore the last 3 questions - we’ll get to them in the next unit.)

ChemLab: Rutherford Scattering Phet Simulation

Overview:

The results of Rutherford’s Gold Foil Experiment caused changes to the model of the atom. In this lab, you will explore his experiment, considering the variables that were tested.

Directions:

1. Download the Student Exploration Sheet.

2. Practice using the Phet simulation.

3. Follow the instructions in the Exploration Sheet to explore the relationships between the amount of solute and solvent. Be sure to record your answers, so that you can upload your completed lab sheet.