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Gallium induced structural failure of an aluminum baseball bat – Watch as the structure of aluminum is destroyed by gallium atoms that dissolve the aluminum bonds. This is a pretty good indication that matter is atomic; gallium would be unable to diffuse inside of the aluminum if it were a continuum.

What and where is antimatter? – This short article from CERN explains some basics of antimatter and brings up some of the big questions about antimatter that physicists are trying to answer.

Ptable.com – This website has a great interactive periodic table of the elements.

PhET Build an Atom Simulation

Play around with the number of protons, neutrons and electrons to see what happens to an atom.

Video Transcript

Hello there! Welcome to lecture 11: the atomic nature of matter!

How do we know what matter is made of? How can we categorize matter? Are atoms the only types of matter that exist in the universe? These questions, and more, will be answered in this lecture.

Each of the following concepts will be discussed in this video: matter, atomism, atomic theories, the periodic tale, and what the universe is made of.


It’s important for us to be able to categorize matter. From this categorization, we can learn about the various properties of different substances. We can also use this to determine what all matter has in common. Rather than being overwhelmed with the diversity of all of the different individual types of matter, we can realize that all matter can be placed into one of just a few categories.

All matter can be divided into two categories: mixtures and pure substances. In a mixture, the properties of the substance vary based on concentration. An example of this would be salt water. The boiling point, the density, the freezing point, all of these properties would vary depending on how much (and what type) of salt was used. 

Mixtures can be either homogeneous or heterogeneous. Homogeneous mixtures, such as air, water, or even lemonade, are all uniform throughout their volume. If I have a pitcher of lemonade, it doesn’t matter if I get lemonade from the top of the pitcher or the bottom of the pitcher, it will taste exactly the same. Air is a homogeneous mixture. It doesn’t matter what part of the room I breathe in: the air is essentially the same everywhere in the room.

A heterogenous mixture does not have a uniform consistency throughout. Granite, for example, is composed of different minerals. If I cut out two pieces of the same size out of this granite, each one might have different amounts of each mineral. A more delicious example of a heterogeneous mixture is a burrito. Maybe you’ve had a burrito made up where one side is mostly lettuce, and in the middle is mostly sour cream, and the meat is all the way over to the other side. Depending on where you bite, you’re going to get a different taste!

Let’s talk about pure substances for a bit. We’ll get back to mixtures in a few moments to see how they relate. In a pure substance, the properties such as density, melting point, specific heat capacity, and so on, are inherent to the material.

Pure substances can be composed of compounds, which is a substance made out of many identical molecules. Water is a compound, consisting of molecules that have two hydrogen atoms and one oxygen atom. 

Molecules consist of two or more atoms that are held together by atomic bonds. H2O is a molecule. Ozone is a molecule consisting of three oxygen atoms bonded together.

Another type of pure substance is an element. Gold, aluminum and lead are examples of solid elements. Mercury is a liquid element. Oxygen, nitrogen, and neon are all gaseous elements. An element is a substance that consists of identical atoms.

Every solid, liquid, gas, and plasma is composed of atoms. For a long time, we thought that atoms were the smallest unit of matter. However, we’ve learned that all atoms are made out of protons, neutrons and electrons. 

Protons and neutrons can be broken down into even smaller particles known as quarks. Electrons are known as leptons. Quarks and leptons are elementary particles, defined by the standard model of particle physics. The standard model is the theory we use to define all known forces and elementary particles in physics. While far from being a complete and unified theory of everything, the standard model does a fantastic job explaining what we do know, and includes the relatively recent discovery of the Higgs boson.

Let’s go back to mixtures… how do those relate to pure substances? What are the building blocks of homogeneous and heterogeneous mixtures? Mixtures can be separated into two or more chemical substances. And those substances are made of molecules, which are made of atoms, which are made of protons, neutrons, and electrons. All matter, that we know of, can be explained by the standard model. We’ll talk about some things that cannot be explained by the standard model later on in this lecture.


Throughout time, humans have attempted to answer the question: what is matter made of? If we were to cut something in half, then in half again, and again, until we could do so no more, what would we find? Do all objects of matter share common building blocks, or is everything different?

Something of particular interest was whether matter was fundamentally discrete or continuous. That is: if we conducted the experiment where we continued to cut something in half until we could do so no more, would we find that there is a discrete particle, a building block of matter? This idea is known as atomism. Or would we find that there aren’t any discrete particles, but instead a continuum, some type of fluid or substance that fills the space it occupies?

Aristotle believed that the continuum model must be true of nature. Attributed to him is the phrase “nature abhors a vacuum.” Aristotle believed that if matter was made of particles, between those particles must exist a vacuum – nothing. He argued that a vacuum would violate physical principles. Of course, Aristotle had no evidence to back up this philosophical assertion.

What experiments could I perform to convince you that matter is made out of particles, not a continuum? 

First, I could use a convincing analogy. As I pour marbles from one container into another, we can see them move as discrete particles, and even hear their individual clinking noises as they fall into the new beaker.

Then I could switch to a smaller particle: a BB. As I pour the BBs into a new container, we can still individuate the particles through sight and sound, but they pour much smoother.

Next, I could make my particles even smaller. I’m going to pour salt into a new container. Now it becomes very difficult to tell apart the individual salt particles. They pour very smoothly and seem to move as a continuum.

Now I can pour water into a beaker. It moves very smoothly, and there’s no way to distinguish between each individual particle.

We noticed that as particle size got smaller, from marbles to BBs to salt, the substance started to act like a liquid when it was poured into a new container. Could we use this to infer that liquids are made of a particle rather than a continuum?

That was a nice analogy, and because you probably already know that matter is made of atoms, you’re probably convinced. But what if you didn’t know what we know now? What other evidence could I present to you to convince you that matter is made of atoms?

When I take 200 milliliters of BB’s and 200 milliliters of marbles and combine them together into a new container, I can see that they take up less than 400 milliliters of space. This is because the BB’s can take up space in between the individual marbles. This is how we can expect particles to configure themselves in space. Let’s see if this holds up with substances that don’t look like particles.

In this video, I have one graduated cylinder containing 50 milliliters of water, dyed blue. A second graduated cylinder contains 50 milliliters of ethanol, dyed red. Assuming both of these are continuums, rather than particles, when I pour the water in with the ethanol, I should expect to see 100 milliliters of liquid. 

I poured the contents of one cylinder into the other, making sure to get as many drops out of the cylinder as possible. I can immediately see that the two liquids have mixed together, because the color is now purple rather than being red or blue. That tells me that somehow the ethanol and water can comingle together, telling me that neither one is a continuum that cannot be permeated by another liquid. The volume of the liquid mixture is 97 milliliters: this is not 100 milliliters, as expected. What happened?! The particles that water and ethanol consist of can actually arrange themselves in between each other, making a more compact shape than they would individually.

Need more evidence?

Gallium is an elemental metal with a pretty low melting point, it becomes a liquid at 30 degrees Celsius, a comfortable summer day in the Chicago suburbs.

In this video, I have an aluminum soda can. Maybe you’ve drank soda or another liquid out of an aluminum can. While it can be crushed with a sufficient amount of force, it’s a pretty strong container given how lightweight it is.

Before pouring the gallium, I tested the strength of the can by pushing down on the top with my fingers. I poured some liquid gallium onto the top of an aluminum can, placed near a space heater to keep the gallium above its melting point. I lightly scored the top of the can with a razor blade to expose the pure aluminum. After 20 minutes, we can see the volume of gallium decrease as it diffuses into the can. 

The can is now extremely weak and breaks apart as if it were tissue paper when I press lightly with my fingers. The gallium inserted itself between the aluminum atoms and completely destroyed the bonds between each atom. If aluminum were made of a continuum, there would be no way for this to happen. Atomism is the correct interpretation of matter – all matter is made of atoms!

Atomic theories

Many atomic theories have been proposed to explain the building blocks of matter: the atom. 

Democritus, who lived around 400 BCE, said that everything was composed of atoms. Between atoms was empty space, and there were an infinite number of different types of atoms. He said that the properties of a substance related to the properties of the atoms that made it. Iron would be composed of very strong interlocking atoms. Water atoms would be smooth and slippery. Salt atoms would be sharp and pointed. Many theories such as these, made in this time period, came about due to philosophical discussions and reasoning. It would be almost two thousand years before observations and experimental evidence were used in a systematic pursuit of scientific truth. 

The scientist John Dalton, in the early 1800s, noticed that chemical compounds consisted of proportions of different elements that existed in ratios of whole numbers. That is, things combined together in such a way that very consistent units of masses were involved. We now know this to be due to atomic bonds. A single oxygen atom can combine with two hydrogen atoms to create water. Water cannot be created with half an oxygen atom. There is no such thing as a half an atom, or a quarter of an atom. This seemed like a very convincing argument in favor of atomism.

In the late 1800s, JJ Thomson discovered that the particles emanating from cathode ray tubes were much smaller than hydrogen, the lightest atom that we knew of. These particles are known to us today as electrons. Electrons were known to create electric current. This discovery of electrons by themselves was evidence that atoms are not indivisible, but can be broken down into smaller particles. 

JJ Thomson’s model, known as the plum pudding model, attempted to explain how atoms worked based on this knowledge. On the whole, it was known that atoms were electrically neutral. If they weren’t neutral, then all atoms would be attracted together due to electric forces. It was also known from the cathode ray tube experiments that electrons could move around by themselves. The plum pudding model hypothesized that an atom was made of a positively charged substance (pudding) that contained mobile electrons (plums).

This was a good hypothesis at the time, but was soon shown to be incorrect. In the early 1900s, Ernest Rutherford and his colleagues conducted an experiment where they shot alpha particles at a thin piece of gold foil. They found that many of the alpha particles passed through the thin piece of foil undeflected. This implied that most of the volume of gold foil is composed of empty space. 

On the other hand, some particles bounced backward completely. To this, Rutherford stated: “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” This was a remarkable result because there must be another force that caused the alpha particles to bounce backward like this. Alpha particles, we now know, are composed of two protons and two neutrons. When they came to interact with the nuclei in the gold foil, they bounced back due to the force of electrical repulsion between the protons in the gold nucleus and the protons in the alpha particles. 

This experiment led to the discovery of the atomic nucleus. Now our atomic model shows a positively charged nucleus, surrounded by mostly empty space in which electrons move around.

Niels Bohr, in 1913, proposed that electrons orbit the nucleus of an atom much in the same way that planets orbit a star. These orbits were well-defined, and only a finite number of orbits could exist. (In other words, only certain orbits were possible, an electron could not move around in any arbitrary orbit.) These orbits were defined by the amount of energy that electrons contained, which were measured by experiment as existing in very finite, discrete quantities.

What do we know about the atom today? Does the Bohr model stand the test of time? Based on the advances of quantum physics, we know that atomic models are not so straightforward. Electrons do not simply orbit the nucleus of an atom in circular paths. Instead, they exist probabilistically. That means that we cannot know the exact location of an electron, but can model the probability of where we expect an electron to be, given its energy level. In this way, we can state that electrons, and all subatomic particles, act like waves rather than as single particles existing at a single point in space.

Eventually, the neutron was discovered, completing our atomic picture. Neutrons are neutral, meaning they do not influence the charge of an atom, but do contribute to its mass. Different isotopes of a substance were shown to have different masses. Because each atom is defined by the number of protons in its nucleus, atoms that had the same number of protons but different masses must contain different numbers of neutrons.

The periodic table

The periodic table of the elements is a table in which all elements are placed. The periodic table was developed over a span of many years, and was proposed when scientist Dmitri Mendeleev noticed periodic repetitions of chemical properties of atoms.

The columns on the periodic table represent elements that have similar chemical properties. Today we know that the columns correspond to the configuration of electrons in each element. For example, the column all the way to the right consists of noble gases. These gases do not readily react with other substances, and today we know this is because each of these elements contains a full shell of valence electrons. 

The periodic table provides us with a lot of information about each element. Inside each box is a symbol that is used to define the element in a chemical formula. Frequently the name of the element will be spelled out as well. Oxygen has the symbol O. Lead has the symbol Pb.

The number at the top of each box tells us the atomic number of each element. The atomic number uniquely defines the element, and is equal to the number of protons inside each atom’s atomic nucleus. We can therefore say that the number of protons is what defines each atom. Any atom that contains two protons is, by definition, a helium atom, regardless of how many neutrons or electrons are present.

Before we talk about the next property given on the periodic table, let’s discuss isotopes. As mentioned, an element is defined by the number of protons in the nucleus. Let’s say I have an atom with six protons. That element is carbon! Maybe I have a handful of carbon atoms. Every one of them has six protons. Most of those carbon atoms also have six neutrons, but some have seven neutrons, and a few may even have eight neutrons. Elements that have different numbers of neutrons are known as isotopes.

Many elements have isotopes that occur naturally. Some are stable, and some are radioactive. Carbon 12 is the most abundant isotope of carbon, it is a stable isotope and is present in all organic matter. Carbon 14 is less abundant and is radioactive. This isotope of carbon is used in carbon dating. The ratio of carbon 12 and carbon 14 in an organic substance can tell us how old that object is.

The standard atomic weight of an element is also given on the periodic table. The standard atomic weight is the weighted average of the mass of all known isotopes of the element. This just means that the standard atomic weight takes into consideration the different isotopes of an element that exist, and the probability of finding each of those isotopes. The standard atomic weight is equal to the sum, for all isotopes, of the abundance of that isotope times the mass of that isotope. This explains why the standard atomic weight given on the periodic table is usually not a whole number. Averaged over all of the isotopes, we’re not going to wind up with an integer number. 

Sometimes the standard atomic weight is provided in parenthesis, or not at all. This is when the number or abundance of isotopes is not yet known.

Other properties of an element are not going to be provided on the periodic table, but can be given to define an element. Perhaps you hear about helium 3. Written out, it looks like 3 and then the symbol of the element. This 3 is the atomic mass number of the isotope. The atomic mass number is equal to the number of protons and the number of neutrons in the element. Because the element is helium, we know that it has two protons. Two protons + the number of neutrons is equal to 3. Therefore, we can conclude that helium 3 has one neutron.

The atomic mass number can therefore be used to determine the number of neutrons in an isotope using the equation Np + Nn equals atomic mass number. Remember that the number of protons is given by the atomic number.

Let’s look at another example. 136-Xe is an isotope of xenon. Xenon has 54 protons. 54 protons plus the number of neutrons in this isotope equals 136. This particular isotope of xenon therefore has 82 neutrons.

Finally, let’s talk about ions. Atoms, on average, are electrically neutral. This means that the number of protons in the nucleus is completely balanced by an equal number of electrons orbiting that nucleus. When the number of protons and the number of electrons is no longer equal, due to an excess or absence of electrons, then the atom is charged. We call a charged atom an ion. The number of protons minus the number of electrons tells us the charge. A hydrogen atom has one proton. If a particular hydrogen atom has two electrons, then we know the charge of that atom is one minus two, or negative one. The charge is represented as a superscript after the atomic symbol. This particular ion is represented as H minus.

Let’s look at another example and use it to determine the number of protons, number of neutrons, and number of electrons. Say we have an atom of 40-Ca 2 plus. This element is calcium, so the number of protons is defined by the atomic number: 20. The atomic mass number is 40. 20 protons plus the number of neutrons equals 40, so there are 20 neutrons. Finally, the charge is positive two. 20 minus the number of electrons equals 2. Therefore, there are 18 electrons in this ion of calcium.

What is the universe made of?

Is that all there is to the universe? Protons, neutrons, and electrons? Not exactly.

First, let’s talk about antimatter. Antimatter particles have the same mass as normal matter, but the opposite charge. Protons have an antiparticle known as the antiproton, which are negatively charged particles with the same mass as a proton. Electrons have an antiparticle known as the positron, which is a positively charged particle with the same mass as an electron. Neutrons also have an antiparticle known as the anti-neutron. When a particle and its antimatter partner come into contact, they annihilate each other and create energy. The amount of energy created can be calculated using the famous equation e equals mc-squared. E is the energy generated, m is the mass of the particles, and c is the speed of light.

Some antimatter is naturally produced, as the result of radioactive decay, for example. Antimatter is also artificially produced in particle colliders. These antiparticles take a lot of energy to create and only exist for a short time before getting annihilated by ordinary matter.

It appears pretty clear to scientists that our universe is composed almost entirely of what we consider to be normal matter, and not antimatter. Why is this? We don’t know. This asymmetry of matter and antimatter is one of the biggest unsolved questions in physics. 

Many observations of the motions of galaxies indicate that they cannot be comprised entirely of particles represented in the standard model of particle physics. In addition, observations about the rate of expansion of the universe cannot account for the amount of energy known to exist in the universe. In fact, the standard model probably only accounts for about 5% of the matter and energy that exist in the universe. Dark matter and dark energy are hypothesized concepts that enable the mathematics of the universe to work out correctly based on our observations. These concepts are called “dark” due to the fact that the matter and energy do not interact with electromagnetism. In other words: we cannot see dark matter or dark energy because they do not interact with light. It is hypothesized that 27% of our universe is composed of dark matter, and the remaining 68% is dark energy. This is another area of intense research in physics today.

Thanks for taking the time to learn about the atomic nature of matter. Until next time, stay well.