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Essential Science for Teachers: Physical Science

Extending the Particle Model of Matter A Closer Look

A Closer Look

Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.

Electron Structure of Water

Dark Matter in the Universe

The Electron Structure of Water and its “Shape”

Our best understanding of electrons: Quantum mechanics

In order to understand what it means for a particle to have a shape and how this affects its macroscopic properties, we must first refine our idea of what an electron is.

When subatomic particles, including the electron, were discovered, they were assumed to be tiny spheres — in fact, they were assumed to be so tiny that they didn’t have a diameter at all. In the early 20th century, however, more refinements were made in the theory of the behavior of very small things, and it was discovered that the world at the microscopic level is not as clearly defined as it seems to be at the macroscopic level.

Rather than calculating “paths” of electrons within atoms or molecules, we calculate the probability of finding the electrons within various parts of the space around the nuclei. Scientists describe the area around a nucleus where they expect to find electrons a “probability cloud.”

It turns out that when two atoms share or exchange electrons, the shape of the “cloud” where you would expect to find an electron changes shape. Although a single hydrogen atom has an electron cloud which looks like a sphere, [AP, Mark says “SHOW THIS] when it shares its electron with an oxygen atom, the overall shape of the cloud changes, forming a “V.”

How are these “electron clouds” related to the forces between particles?

Recall what was said in this session’s video about the electrostatic force: positive charges and negative charges attract one another. The electron cloud in a molecule or atom is negatively charged. The nucleus of an atom or the nuclei of a molecule are positively charged because they contain positively charged protons. The electron cloud stays fairly close to the nucleus because there is an attractive force between them. However, when there are many atoms or molecules of the same substance, there is also an attractive force between the electron cloud of one particle and the nucleus (or nuclei) of its neighboring particles. This electrostatic attractive force is the “force between particles” referred to throughout this series.

It is the shape of the particle, i.e., the shape of its electron cloud, which determines the strength of the force between particles. To clarify further, let’s look at two examples of the shape of molecules, water and carbon dioxide.


The oxygen atom at the “corner” of the water molecule (H20) has a particularly strong pull on the electrons of the hydrogen atoms. As a result, the probability of finding the electron closer to the hydrogen atom is reduced. Because of this, the two “prongs” of the water molecule are slightly more positively charged and the “corner” is more negatively charged. We call this kind of molecule a “polar” molecule because the electrons are shared unevenly, resulting in an uneven shape:

The uneven charge distribution, i.e., the shape of the electron cloud, also results in a strong force between water molecules, which explains why water is a liquid at room temperature and has a relatively high boiling point.

Carbon Dioxide

The shape of the carbon dioxide molecule, C02, is linear. The carbon atom at the center does not have a particularly strong pull on the electrons from the two oxygen atoms on either side. In this case, there is not a negatively charged side and a positively charged side, as there is in a water molecule. As a result, the forces between the molecules of carbon dioxide are weaker than those between water molecules. Thus, when particles of carbon dioxide, at room temperature, are moving about and colliding, they cannot “hold on” to each other, explaining why carbon dioxide is a gas at room temperature.

Dark Matter

How can we determine the mass of far-away galaxies?

In this session’s video, we mentioned that a new kind of matter has been proposed to explain the motions of galaxies. In 1933, astronomer Fritz Zwicky made careful observations of a gravitationally bound cluster of galaxies called the Coma cluster, which is estimated at over 1000 galaxies, located 350 million light years away in the northern constellation Coma Berenices.

Zwicky determined how fast the galaxies are moving relative to each other and, using Newton’s theory of gravity, calculated their mass: the faster they are moving, the greater the mass of the cluster. Zwicky found the mass of the cluster to be 400 times what one would expect by inferring the mass from the brightness of the cluster. He termed this missing matter “dark matter.”

Even though dark matter can be indirectly observed in many places in the universe including our own galaxy (by measuring the speed of rotation of stars around the galactic center), no one has yet detected it directly using any kind of telescope. Current estimates are that dark matter comprises 90% or more of all the mass in the universe.

Series Directory

Essential Science for Teachers: Physical Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2004.
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  • ISBN: 1-57680-749-5