Coordinated Science- Chemistry
“Pudding and Clouds”
Name:
Date:
Period:
Purpose: In this lesson you will analyze why models of the atom have changed in the past two hundred years.
Part I: Using Evidence to Evaluate Models
In 1803, John Dalton proposed that atoms were simply solid spheres. In the decades that followed, chemists
collected a lot of evidence suggesting there was more to the atom. This evidence meant the scientists’ models
had to change. Read descriptions of the atomic models on the other handout and answer the questions below.
In 1897, J.J. Thomson showed it is possible to remove a negatively charged particle from atoms using
electrical forces. He suggested that atoms were a positively charged body full of smaller negatively charged
particles. Atoms were neutral because the positive and negative charges canceled each other out.
1. Explain why Thomson’s evidence required a change from the solid sphere to the plum pudding model.
In 1911, Ernest Rutherford did his famous gold foil* experiment. He showed that if tiny positively charged
alpha particles** are shot at metal atoms, some of the alpha particles hit something dense and bounce back
but most of the alpha particles go right through. This was evidence that atoms are mostly empty space.
2. Explain how Rutherford’s evidence required a change from the plum pudding model to a nuclear model.
(bonus: Explain why this model is called “nuclear”.)
* In addition to gold, Rutherford also tried Al, Fe, Pb and other metals
** alpha particles are radioactive particles released from uranium atoms
In 1913, Niehs Bohr discovered that the farther away from the center of an atom the negatively charged
particles (electrons) are, the easier they are to remove. He also discovered that electrons within an atom
have different levels of energy.
3. Explain how Bohr’s evidence required a change from the nuclear model to the solar system model (now
called the Bohr model).
Part II: Interpreting the Evidence
1. Which evidence (or discovery) supports the existence of negatively charged particles (electrons)?
2. Which evidence (or discovery) supports the existence of a nucleus (the dense center part of an atom)?
3. In 1918, Ernest Rutherford discovered that the nucleus was composed of positively charged particles,
which were named protons. In 1932, British physicist James Chadwick discovered that the nucleus was
also composed of neutral (uncharged) particles called neutrons.
How do the three small particles (electrons, protons, neutrons) of an atom differ from one another?
4. A helium atom has two electrons, two protons, and two neutrons.
a. How many positive charges does helium have? ________
b. How many negative charges does helium have? ________
c. What is the charge on the helium atom? ________ Explain your reasoning.
Making Sense:
1. Why have there been so many different models for the atom?
2. Construct the best model of the atom you can. It does
not have to be an exact copy of any of the previous models,
but use all of the evidence given.
Draw your model here.
Explain why you think this is the best model.
3. Atoms are not flat. How can you revise the model you constructed to account for this?
Reading:
Atomic Model
Through Time
In the early part of the 19th century, John Dalton revived the ancient
notion that all matter is made up of indivisible particles called atoms.
He pictured the atom as a solid sphere that could not be divided into
anything smaller. Over the next two hundred years scientific evidence
led to a more and more sophisticated model of the atom.
First came evidence of a tiny atomic structure with a negative charge. J.J. Thomson named
this structure a corpuscle; they are now called electrons. Thomson pictured these electrons
scattered throughout an atom as raisins might be in a pudding – hence the Plum Pudding Model.
Thompson recognized that overall, an atom is neutral (not negatively charged). He concluded that
there must also be an equal positive charge in the atom to offset the negative charge of electrons.
In Thomson’s mind the “pudding” portion of the plum pudding consisted of positively charged stuff.
(We might think of this as a Chocolate Chip Cookie Dough Model, with negative chocolate chips in a
positive dough.)
A physicist named Ernest Rutherford was the next to identify more complex structures within
the atom itself. His ingenious experiment used a very, very thin piece of gold foil, just a few atoms
in thickness. He shot tiny alpha particles at the gold foil. Most of the alpha particles went through
the foil but were deflected at a wide angle. But the most interesting finding to Rutherford was that
a small percentage of the particles (about 0.01%) actually bounced off the gold foil right back in the
direction they came from. Rutherford concluded that there was a small, very dense structure in the
middle of the atom. He called this structure the nucleus. He also said that the nucleus had a
positive charge on it and that the alpha particles were bouncing off the nucleus whenever they
happened to strike it directly. Rutherford pictured
the negatively charged electrons circling around the
nucleus, leaving a lot of space between them for alpha
particles to pass through.
The scientific evidence collected by Neils Bohr led
him to conclude that the particles called electrons
could only be found around the nucleus at specific
distances. These areas around the nucleus where
electrons are found are also referred to as electron
shells. These shells are sometimes represented as
concentric spheres. According to Bohr, electrons
could be located at specific distances from the
nucleus, but not in the spaces between the shells.
(The word shell implies that the electrons occupy only
the surface of each sphere.)
The last model in our series is called the Electron
Cloud Model. The work of many scientists led to this
final model, which, you may notice, includes a second
heavy particle in the nucleus (called a neutron). The
other notable feature of this model is that the electron
is shown in continuous motion and is depicted as a
probability cloud. This “cloud” represents the place
where an electron would probably be found at any
given moment. It is thought that a German physicist
named Erwin Schrödinger came up with the idea of
the electron cloud.
There have been further revisions to the electron cloud model which predict different shaped
clouds, and a de Broglie model that explains electron levels in terms of wave-particle duality and
integral values for orbital wavelengths. While it is outdated and incomplete, though, we use the
Simple Atomic Model (often called a Bohr Model) because it shows in a simple way the chemistry of
how electrons in atoms interact. We just need to think of the “shells” as energy levels and probable
distances rather than fixed orbits.