The Effects of Microscopic Forces on Macroscopic Properties

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Microscopic forces between atoms explain why contact forces exist between solid objects and why materials can change states of matter. Covalent, ionic and hydrogen bonds also have effects on macroscopic properties. Read on about forces and bonds.

What are Microscopic Forces?

Are you sitting in a chair right now? What is keeping you from falling through the chair straight to the floor? We take for granted that two solid objects (like you and the chair) will not pass through each other, but even solids are made up of mostly empty space, so what actually prevents this from happening?

Electrostatic contact forces keep these people from falling through their chairs.
electrostatic contact forces keep these people from falling through their chairs

It turns out that forces exerted by the tiny particles that make up all matter, atoms, are all it takes to keep you in your chair. Atoms repel each other because of microscopic electrostatic forces that are exerted by even smaller particles inside atoms, positively charged protons and negatively charged electrons.

Like charges repel and unlike charges attract.
like charges repel and unlike charges attract

Two particles with the same type of charge (either both positive or both negative) will repel each other, while two particles with opposite charges (one positive and one negative) will attract each other.

So, when two atoms get close to one another, three things happen:

  1. The protons in the nucleus of each atom repel each other
  2. The electrons in each atom also repel each other
  3. Meanwhile, the protons in one atom attract the negatively charged electrons in the other atom

The effect of all this attraction and repulsion is that atoms will attract each other when they are not very close together, but when they get very close to each other, the repulsive forces will be greater than the attractive forces and the atoms will push each other away. It is just this 'pushing away' by neighboring atoms that keeps you from falling through the chair. The atoms in you are pushing against the atoms in the chair and the atoms in the chair are pushing back on you at the same time.

Liquids, Solids, and Gases

In addition to creating contact forces between surfaces, microscopic electrostatic forces between atoms also determine whether a substance will be a solid, liquid, or gas. Even though you can't see them, atoms are always in motion. As the temperature of the object increases, the atoms vibrate more and more. As they move, they push against neighboring atoms. This makes solid objects expand as they heat up.

Eventually, if the vibrations of the atoms get strong enough, the atoms will push each other far enough apart that the solid loses its structure and becomes a liquid. In a liquid, atoms are farther apart from each other than they are in a solid.

If the liquid continues to heat up, eventually the atoms will move even farther apart and then the liquid will become a gas. Atoms in a gas are very far apart from each other, especially compared to atoms in a solid.

Increasing temperature causes atoms to move and push each other, causing transformations from solid to liquid to gas.
solid liquid and gas

Atomic Bonding and its Effect on Macroscopic Properties

Covalent Bonds

In addition to general repulsive forces between neighboring atoms and molecules, there are other types of intermolecular bonding that can affect the macroscopic properties of materials. Covalent bonding between atoms occurs when neighboring atoms share electrons. This creates a very strong bond between the atoms, and materials that contain a lot of covalent bonding are typically solid at room temperature. Why do you think that is?

Covalent bonds create solid structures, like this silicon.
covalent bonding

When the atoms share their electrons, they cannot move very far apart from each other. To become a liquid or gas, the atoms must move apart from each other, and covalent bonding makes this much more difficult. This is why materials that have a large amount of covalent bonding, like plastics and ceramics, are typically solids at room temperature (and even much higher temperatures)!

Ionic Bonds

A weaker type of bonding that can exist between neighboring atoms is ionic bonding. Ions form when atoms gain or lose electrons and become charged. Then, the positive and negatively charged ions attract each other and create an ionic bond. Ionic bonding is weaker than covalent bonding because the atoms do not actually share electrons.

Salts, including the table salt (sodium chloride) you probably used at lunch today, are made of ions that are ionically bonded to each other. Even though salt is a solid at room temperature, if you add a little water, it will quickly dissolve because the water molecules can easily separate the sodium and chloride ions in the salt. This won't work for atoms that are covalently bonded together. Just try to dissolve a piece of plastic in a glass of water! It won't work!

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