Atomic models (from Dalton to Bohr)
If you kept breaking a piece of iron in two, again and again, would you eventually reach a piece so small that it could not be broken any further? That question is more than two thousand years old. The answer we give today did not occur to anyone all at once: it was built up gradually, through four different models of the atom, each one better than the last.
Dalton: the atom as a solid ball
When $1{,}0\text{ g}$ of hydrogen reacts with $8{,}0\text{ g}$ of oxygen, exactly $9{,}0\text{ g}$ of water is always produced, and if you add more oxygen it does not react partially, the extra simply remains: why do chemical elements always combine in fixed, exact proportions by mass?
In 1805, John Dalton proposed that all matter is made of atoms: tiny, solid, indivisible little balls.
With this very simple idea, Dalton could already explain why elements are different from one another and why they always combine in the same proportions to form compounds.
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…
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.
Thomson: the plum pudding
When you rub a plastic pen with a woollen jumper and find that it attracts little pieces of paper floating in the air, matter is showing electrical properties: if atoms were solid, neutral spheres as Dalton said, where does that electric charge come from?
In 1897, J. J. Thomson was carrying out experiments with cathode ray tubes and discovered a tiny, negatively charged particle coming from inside the atom: the electron. This meant something enormous: Dalton's atom, which was supposed to be indivisible, actually had smaller pieces inside it.
Since the whole atom is neutral, Thomson imagined that the electrons (negative charge) were embedded in a mass of positive charge, like plums in a pudding.
Iker'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…
Iker'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…
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?
Rutherford: the almost empty atom
In 1911, Ernest Rutherford fired $\alpha$ particles (with positive charge) at a very thin gold foil, in order to "feel" the inside of Thomson's atom.
If the atom were a plum pudding, with the positive charge spread throughout its volume, no particle should bounce back. The fact that some did can only be explained if all the positive charge is concentrated in a tiny, very heavy point.
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…
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,…
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 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…
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.
- a) Calculate what percentage of the particles passed through the foil cleanly.
- b) What fraction of the fired particles bounced back?
- c) Using these numbers, explain why Rutherford stated that the atom is mostly empty space and that the nucleus is tiny.
Bohr: electrons in allowed orbits
Rutherford's model had a serious problem: according to the laws of physics known at the time, an orbiting electron should gradually lose energy and end up falling into the nucleus. If that happened, no atom would be stable, and yet atoms are stable.
In 1913, Niels Bohr proposed a new rule: electrons cannot orbit at just any distance from the nucleus; they can only occupy certain allowed orbits, like the rungs of a ladder. As long as an electron is on one of those "rungs", it does not lose energy and does not fall.
Move the sliders in the interactive and notice how the shells fill up, one by one, as you add electrons.
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…
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…
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…
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.
- a) Explain what Bohr proposed to prevent this electron collapse.
- b) Why do we say that Bohr added a "new rule" instead of simply drawing Rutherford's orbits better?
Why models change
The four models are not four unrelated ideas: each one arises from an experiment that the previous model could not explain.
Some collectible cards of "atomic scientists" show these dates: Thomson, $1897$; Bohr, $1913$; Dalton, $1805$; Rutherford, $1911$. Order the four atomic models from…
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.
In the exhibition "The Detectives of the Atom" there are four misplaced panels. Match each description with its scientist: Dalton, Thomson, Rutherford, or Bohr. a) "The…
In the exhibition "The Detectives of the Atom" there are four misplaced panels. Match each description with its scientist: Dalton, Thomson, Rutherford, or Bohr.
- a) "The atom is a solid, indivisible sphere."
- b) "Electrons are embedded in a positively charged sphere, like raisins in a pudding."
- c) "Almost the entire atom is empty space and positive charge is concentrated in a very small nucleus."
- d) "Electrons can only occupy certain permitted orbits."
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…
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.
Back to the beginning: Yes, you would reach a limit: the iron atom is the smallest portion that is still iron; if you try to break it up further, it stops being iron and you get loose subatomic particles (protons, neutrons and electrons).