1.3.1: Introduction to The Particulate Model of Matter

The Particulate Model of Matter (Rough Draft)

Introduction

Chemistry is the study of matter. In the modern view of matter, matter is composed of many small particles. Although seeing matter as being composed of many particles isn't always so intuitive, even ancient philosophers had some ideas about the particle nature of matter. Aristotelians, for example, believe that substances were composed of the elements: earth, air, water, and fire and each substance's properties were dependent upon the composition of these elements. Other Greek philosophers as Democritus believed matter was not made up of these elements but of smaller indivisible particles known as "atoms". Although our modern view of matter is considerably different from these views, we still consider the particulate view of matter as a fundamental way to understand the physical and chemical characteristics of substances.

Since the ancient Greek times where the idea of the atom was first described, experimental evidence has been uncovered that support a model of matter where matter is composed of tiny particles. In the particulate model of matter, matter is composed of small particles that are in constant movement. This model provides a powerful framework for understanding the physical / chemical properties of substance. Although it doesn't explain everything about matter (most scientific models don't!), it does provide a guideline for understanding some of the fundamental concepts of matter.

Key Principles (TO DO - add illustrations / simulations)

The particulate model of matter is based on several key principles:

1. All matter is made up of tiny particles - These particles may be atoms, molecules, or ions, depending on the substance.
   • Similar to building blocks, atoms may be bonded together to create a vast variety of substances
2. Particles are in constant random motion - The energy of this motion increases or decreases with temperature.
   • This is most easily experienced in gasses but even particles in the liquid and solid phase are in constant movement.
3. There are forces of attraction and repulsion between particles - The strength of these forces varies depending on the particle composition and the phase of matter.
   • The weaker these intermolecular forces are, the easier the particles are to break apart

Understanding the Particulate Model (TO DO - Clean Up Write Up)

The particulate model is most clearly illustrated in the properties of gases and in phase transitions.

Visualizing the Particulate Model in Gasses

Let's explore how the postulates above can help explain the properties of gasses. In the simulation below, go ahead and select the "Ideal" model and go ahead and add about 200 light particles to the box. This observed simulation is representative of gas particles in an "empty" container. That "empty" container is not actually empty. Rather, it is filled with particles that are in the gas phase. 

In this simulation, we can also observe how the movement of these individual particles can be observed in gas pressure. Gas pressure (i.e. - air in tires, in sports balls, when you try to crush an empty plastic bottle with the lid on) results from collisions of the gas particles on the walls of the container. The more frequent and forceful the collisions, the higher the gas pressure and vice versa. With about 200 light gas particles at a temperature of 301 K, the pressure remains at about ~23.8 atm. 

Now go ahead and adjust the temperature (the bottom center) of the simulation. You should observe that the gas particles begin to move faster or slower depending on whether you heated or cooled the gas. You should also notice that your pressure reading changes as a result of the change in particle movement with temperature. This observation is supporting evidence for the first two postulates of the particulate theory of matter!

Think of a time when you walked into a room where somebody was eating or cooking and could smell the food. This observation is also evidence supporting the particulate model! The particulate model is also supported by the observation of diffusion. When placed in a container, gas particles will expand into the entire volume of the container rather than stay in one place. In the simulation above, go ahead and the select the "Diffusion" mode. Once there, add about 100 red particles (these will add on the right side of the divided container) and 100 blue particles (these will add to the left side of the divided container). Now go ahead and press the green "Remove Divider" button. You should observe the gas particles diffusing from their original side of the container and mixing together evenly across the entire container. What you were smelling as you walked into the room were gas particles that had diffused from the food across the room and into your nose! 

Some Observations Related to Particulate Model Postulates

• Postulate 1: Notice how the gas is actually composed of many many particles. An "empty" 2 gallon soda container (~7.57 L) at sea level contain around 2.03 x 10²³ individual particles!
• Postulate 2: Increasing or decreasing the temperature affects the movement of the particles. In gasses, the movement is so rapid that gasses will fill the volume of the container and diffuse throughout the container.

But what about postulate 3? Are there forces of attraction or repulsion between the particles observed in the above simulation? Technically the answer is yes but for particles in the gas phase, intermolecular forces are relatively weak (more on that in later chapters) compared to the particle movement as the particles are relatively spread out. In liquids and solids, where the temperature and particle movement are not quite as rapidly and particles are much closer together, intermolecular forces play a much larger role. 

Visualizing the Particulate Model in States of Matter

Let's explore a new simulation and visualize how intermolecular forces may attract particles together in the liquid and solid phase. In the simulation below, go ahead and select the "Phases" mode. The default view is of neon atoms in the solid phase. Notice how the particles are much closer together but are vibrating in a set location. If you select "Liquid" on the right hand menu, you will see what the atoms look like in the liquid phase. The particles are still close together but there is much more sliding movement and you may even see particles that have escaped from the main liquid phase into the gas phase (the floating particles). If you select "Gas", the particles should now be energetic enough to freely move around in the container as seen in the previous simulation.

Let's now explore how the strength of attraction (Interaction Strength) relates to the phase. Select the "Phase Changes" mode on the simulation. Change the atoms and molecules to "Adjustable Attraction" and keep the Interaction Strength at "strong". You should now be observing purple particles that are in the solid phase. Slowly add heat to the sample. As you add heat you should observe particles moving faster and eventually begin to "escape" from the solid phase. However, if you observe the particle collisions carefully, at lower temperatures this collision may result in the particles sticking back to the particles. This observation is a result of the strong attractive force between the particles. 

Now go ahead and "reset" the model by pressing the circular arrow button on the bottom right corner of the simulation. Switch back to the "Adjustable Attraction" but now change the "Interaction Strength" to weak. Even without changing the temperature, you should observe the solid melting to more of a liquid phase. Now go ahead and start adding heat. Notice it doesn't take much for the particles to "escape" and when the particles collide together, they're unlikely to stick back to each other. This observation is a result of weak attractive force between the particles. 

Particles that contain weak attractive forces aren't likely to stick to each other and thus are more likely to form gasses (though at low temperatures, you can still get liquids and solids). Particles that contain strong attractive are more likely to stick to each other and are thus more likely to be in the liquid or solid phase (though, once again, you can add a lot of heat to overcome those forces). These observations illustrate postulate 3 of the particulate model. 

Observations Related to Particulate Model Postulates

• Postulate 2: Even in the solid and liquid phase, the particles are constantly moving but the particles remain somewhat locked in place compared to gasses.
• Postulate 3: Matter becomes a solid or liquid when the attractive forces are strong relative to the kinetic energy. Particles with high attractive forces are more likely to be liquids and solids.

Drawing Particulate Diagrams (TO DO - Add diagrams / instructions? Perhaps leave for Classification of Matter)

One way to visualize the particles in matter is via the use of particle diagrams. Particle diagrams are visual representations of the particles in matter where the particles are represented as dots. Realistically, there are an overwhelming number of particles found even in the smallest amount of substances but in a particle diagram, you just draw a manageable countable number of particles. Although particle diagrams may be a simple representation, it is important to pay attention to specific details that differentiate substances. Here are a few items to consider for particle diagrams:

• Pay close attention to whether your particles should be atoms or molecules
  • Draw individual atoms with sufficient space where it's clear they are individual particles
  • Draw molecules with the individual atoms touching each other to make clear the particles are molecules
• Be purposeful about the space you leave between your particles
  • The amount of space between the particles can help differentiate what phase of matter the substance you're representing is in
• For mixtures, ensure that the ratio of particles drawn is indicative of the mixture's particle ratio

Limitations

While extremely useful, the particulate model has limitations:

• It provides a simplified view of matter
• It doesn't account for quantum effects that occur at very small scales
• It doesn't fully explain some properties of plasma (the fourth state of matter)

Attributions

• Gas Properties by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC-BY-4.0 (https://phet.colorado.edu).
• States of Matter by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC-BY-4.0 (https://phet.colorado.edu).