Atomic Models (from Dalton to Bohr)

In Physics class, the teacher shows a billiard ball and says that this is how Dalton imagined atoms around the year $1805$, as shown in the figure.

Explain in your own words what an atom is according to Dalton's model and what two properties that "ball" had.

In an assignment, Pablo writes: "Dalton's model was wrong, so it was completely useless." You disagree. Explain two things that Dalton's model did explain well (why different elements exist and how their atoms combine) and one thing it could not explain, which forced it to be changed.

Carlos's grandmother makes a plum pudding, like the one in the figure.

Thomson imagined the atom as this dessert: explain what the pudding mass represents in his model and what the raisins represent.

In a science museum, Leo observes a cathode ray tube like the one Thomson used in his experiments, similar to the one in the figure.

Which particle did Thomson discover using this apparatus and what electrical charge does it have?

Old cathode ray tube televisions worked using cathode rays, the same ones Thomson used to discover the electron in $1897$. The electron is a particle that comes from inside the atom: explain why this discovery forced a change to Dalton's model.

To explain the models, a group of students uses a watermelon with seeds as an image of Thomson's model.

1. a) Explain what each part of the watermelon represents in that model.
2. b) Propose an everyday object or situation of your own that represents Rutherford's model and explain what each element is.
3. c) No comparison is perfect: point out a flaw in using a watermelon as a model of the atom.

Aitana watches Rutherford's gold foil experiment in a documentary: positively charged $\alpha$ particles were fired at an extremely thin sheet of gold foil, as shown in the figure.

What happened to the vast majority of the particles when they reached the foil? What conclusion did Rutherford draw about the inside of the atom?

In the gold foil experiment, three things were observed: most $\alpha$ particles passed through the foil without being deflected, a few were deflected, and very rarely, one bounced back, as shown in the figure.

Write down the conclusion Rutherford drew from each of the three observations.

In a class debate, Nora argues that the gold foil experiment disproved Thomson's model. If the atom were truly a sphere of positive charge with embedded electrons, what should have happened to the $\alpha$ particles upon reaching the foil? Which observation from the experiment does not fit this prediction? Justify your answer.

Rutherford's model is called the "planetary model" because it resembles the Solar System, as shown in the figure.

State one similarity and one difference between the planets orbiting the Sun and the electrons orbiting the nucleus (think about which force holds each system together).

When Rutherford proposed his model, he could not ignore what was already known: for example, that atoms are electrically neutral. Explain how atomic neutrality fits into Rutherford's model (positive nucleus and electrons) and why a new model must also explain the facts that the previous model already explained well.

Imagine that in the gold foil experiment, most of the $\alpha$ particles had bounced back instead of passing through the foil. What would Rutherford have concluded about the inside of the atom in that case? Which known model would that conclusion most closely resemble? Explain your reasoning.

In the gold foil experiment, for every $8000$ $\alpha$ particles fired, about $7992$ passed through the foil without being deflected, about $7$ were deflected, and only $1$ bounced back.

1. a) Calculate what percentage of the particles passed through the foil cleanly.
2. b) What fraction of the fired particles bounced back?
3. c) Using these numbers, explain why Rutherford stated that the atom is mostly empty space and that the nucleus is tiny.

$\alpha$ particles have a positive charge, just like the nucleus of the atom, as shown in the figure.

1. a) Using the forces between electrical charges, explain why a $\alpha$ particle passing near the nucleus is deflected from its path.
2. b) Why are the particles that bounce back precisely those that get closest to the nucleus?
3. c) If the foil were bombarded with uncharged particles, would you expect to see the same deflections? Justify your answer.

Some scientists repeat the experiment with a foil made of a metal other than gold and observe that twice as many $\alpha$ particles now bounce back, although they are still very few.

1. a) Is the idea that the atom is mostly empty space still valid? Why?
2. b) Propose a hypothesis explaining why more particles bounce back with this metal (think about what its nucleus might be like compared to gold).
3. c) What new experiment would you carry out to test your hypothesis? Explain your reasoning.

On a Ferris wheel at a fairground, you can only travel sitting in a carriage, never hanging in the air between two carriages. Bohr proposed something similar for electrons, as shown in the figure.

Explain what Bohr said about electron orbits and how his model differed from Rutherford's.

To explain Bohr's model, a teacher compares a staircase to a slide, as in the figure: on the staircase, you can only stand on a step, but on the slide, you can stop at any height.

Match each image with one of the two models, Rutherford's and Bohr's, and explain what each step of the staircase represents.

Rutherford's model had a problem: according to the physics of the time, an electron orbiting the nucleus should gradually lose energy and end up falling into it, as shown in the figure, meaning atoms would not be stable.

1. a) Explain what Bohr proposed to prevent this electron collapse.
2. b) Why do we say that Bohr added a "new rule" instead of simply drawing Rutherford's orbits better?

Some collectible cards of "atomic scientists" show these dates: Thomson, $1897$; Bohr, $1913$; Dalton, $1805$; Rutherford, $1911$. Order the four atomic models from oldest to most recent.

Mario's favorite game is updated frequently to fix bugs and add improvements. Something similar happens with scientific models: explain why scientists replace one model with a new one.

In the exhibition "The Detectives of the Atom" there are four misplaced panels. Match each description with its scientist: Dalton, Thomson, Rutherford, or Bohr.

1. a) "The atom is a solid, indivisible sphere."
2. b) "Electrons are embedded in a positively charged sphere, like raisins in a pudding."
3. c) "Almost the entire atom is empty space and positive charge is concentrated in a very small nucleus."
4. d) "Electrons can only occupy certain permitted orbits."

These four descriptions of atomic models are out of order:

1. a) "Electrons orbit a central nucleus, like planets around the Sun."
2. b) "The atom is a solid, indivisible ball."
3. c) "Electrons can only be in certain permitted orbits."
4. d) "The atom is a positive sphere with embedded electrons."

Order them chronologically, from the oldest model to the most recent, and write the name of the scientist who proposed each one.

Write a short encyclopaedia entry titled "The history of the atom in four models". For each scientist (Dalton, Thomson, Rutherford, and Bohr), state what their model contributed and what observation or experiment supported it.

Your eight-year-old cousin asks you why the diagrams of the atom in science books change. Write a simple explanation for them (you can compare it to updating a map when a new path is discovered), explaining how science moved from Dalton's model to Bohr's model and what role experiments played in these changes.