Atomic Theory Chemistry is the science of matter. Matter is made up of things called atoms, elements, and molecules. But have you ever wondered if atoms and molecules are real? Would you be surprised to find out that humans have never seen an atom? For this reason, atoms are still considered a theory, a very strong theory, but a theory none the less. Introducing the Atomic Theory: Atomic theory is a theory that attempts to answer the questions above. It states that all matter is made of extremely small particles called atoms. An atom is the smallest particle that still has the same properties. For example, the different tests we have been learning such as density (mass and volume), melting temperature, and evaporation temperature would all stay the same as long as the atom didn t change. So a carbon atom would still have the properties of carbon, but if you break the carbon atom apart these properties will disappear. The atomic theory, although it sounds simple, is really very complex. Although it's still only a theory, nowadays, it is widely accepted and supported by virtually everyone. And like most theories, the atomic theory has a great story behind it. What happened next? Meet Democritus Like most modern science, the atomic theory started in Ancient Greece (which explains the interesting names of things). The guy who thought it all up was known as Democritus, a philosopher, in the 5th century BC (440 B.C). Democritus stated that all matter is made up of tiny particles that move around. He believed that these tiny particles were invisible and couldn t be changed. He called them atomos, which means uncuttable in Greek. Although these atomos are all made up of the same matter, their shape and size explains all of the different types of matter on Earth. While most of Democrtius's ideas aren't acceptable today (atoms are divisible into smaller units, and they are in constant change), the revolutionary thought that matter is composed of tiny particles is accepted today as the beginning of atomic theory. For a long time, nothing. For starters, another Greek philosopher, Aristotle, did not believe Democritus. Aristotle believed that there were only 4 types of matter: earth, air, fire, and water. Aristotle was more powerful and influential than Democritus so changing beliefs didn t come easily. People also had more important things to do than think about matter (mostly fight wars and try to survive the hard life of the Middle Ages). And so, for hundreds of years, nobody cared, and atomic theory wasn't explored. People either accepted the notion that matter can be divided into smaller pieces, or that there's something that makes up matter, but since nobody could really check, it made no difference. Until one day, in the early 1800s, an English guy name John Dalton changed everything.
Dalton's theory There are a few important rules that came about in the late 1700s that were crucial to Dalton's thoughts. The most important was the Law of Conservation of mass. This law, formulated by Lavoisier, states that during a chemical reaction, the total mass of the before the reaction equals the total mass after the reaction. This suggested that matter cannot be destroyed (it s indestructible). Remember, we explored this theory when we saw how the balloon filled with gas when vinegar and baking soda mix. Dalton published his atomic theory in 1808 over 2,000 years after Democritus proposed that matter was made of tiny particles he called atomos. Here are the main points of Dalton's atomic theory: All elements are made of tiny particles, called atoms. All atoms of one element are identical, though atoms of different elements are different. Atoms of one element combine with atoms of another element to form compounds. The molecules of these compounds always have the same proportions of atoms. Atoms can't be created, destroyed, or subdivided. Lots of improvements have been made to Dalton s theory over the years. For example, we now know that atoms have smaller parts and can be divided into protons, electrons, and neutrons. We'll get to that later though. And then what happened? Thomson discovers electrons Dalton's atomic theory can be thought of as the theory of tiny spheres. Once this theory was introduced, things started to move more rapidly. Everyone seemed to be researching this or that, trying to find out something worthy and get into the science books. Some of them did. Dalton's model didn't survive long (only 90 years), and it was soon replaced by a chocolate chip. In 1897, a British scientist named J.J. Thomson found a mistake in Dalton s theory. Thomson discovered small particles inside the atom. By shining a special beam of light through a glass tube and using positive and negative charges, he could change the path of the light. The light was attracted to the positive charge and moved away from the negative charge.
Thomson concluded that the special light must be negatively charged. His experiments soon found that every type of atom had these negative charges. These negatively charged particles that Thomson discovered are now called electrons. In the end, Thomson theorized that the electrons were mixed throughout an atom kind of like chocolate chips in chocolate chip ice cream. He compared the atom to being a soft blob (kind of like ice cream) with chips floating throughout. Atoms are Mostly Empty Space In 1911, a former student of Thomson s named Ernest Rutherford decided to test Thomson s Chocolate Chip Ice Cream Theory (it isn t really called that but it sounds tastier ). Rutherford shot extremely small particles of light at a sheet of gold foil. He surrounded the gold foil with a special material that would light up (kind of like a fire fly lighting up in the night sky) if it was hit by the light particles. Rutherford theorized that if atoms were soft blobs, the light particles would pass through the gold foil in a perfect straight line. Most of the particles did pass straight through leading him to believe that atoms were mostly empty space. What surprised him though was that some light particles were deflected with some even bouncing straight back. Based on this observation, he had a new addition to Thomson s atomic theory that atoms must have both empty space and a small part made up of matter. This led him to modify Thomson s atomic theory. Rutherford s addition added a new term, nucleus. This is the small part of matter that makes up the center of an atom. I ll avoid the long explanation, but Rutherford believed this nucleus was positively charged. Rutherford believed that the negatively charged electrons that Thomson discovered were moving around in the empty space of an atom and the nucleus was in the center. Empty Space Through his experiments, Rutherford was able to calculate that the nucleus was 100,000 times smaller than the atom. That would be the same size of a needle sitting in the middle of Genoa Middle School. That s really small! Where are the Electrons? But there appeared something terribly wrong with Rutherford's model of the atom. The theory of magnetism predicted that opposite charges attract (the positive side of a magnet is always attracted to the negative side of a magnet). If this was the case, then why didn t the negative electrons move towards the positive nucleus? In 1913 a Danish scientist working with Rutherford, Niels Bohr, came up with a theory that said the electrons do not spiral into the nucleus and came up with a rule for what does happen.
Bohr discovered that electrons are spinning around in orbits around the nucleus very similar to how the planets spin around the sun. The one difference that Bohr found is that unlike the planets, electrons do change their orbits around the nucleus where the planets keep their orbit. Interestingly, he asked other scientists to pretend the rule is correct and use it. The picture to the right shows how Bohr thought electrons moved around the nucleus. The ovals wrapped around the middle blob (the nucleus) are the paths that electrons travel inside the atom. If you ve ever watched Jimmy Neutron, you have seen this picture or one similar to it. What has changed since 1913? Even though we have a better idea of what an atom looks like, lots of scientists continue to work on the atomic theory. Most of the atomic theory work that has happened since Bohr s discovery has focused on how the electrons move around inside an atom. For example, we no longer say an electron is moving in an orbit. Instead we say they are inside a cloud that surrounds the nucleus. So Bohr was not totally correct. This may seem like a small change in the atomic theory, but to the discoverers (Erwin Schrodinger and Werner Heisenberg), it was big enough to win a Nobel Prize in science. Still a Theory? The atomic theory is still just a theory because we still have never seen an atom. Unfortunately atoms are much smaller than the wavelength of light, so we cannot take pictures of them. If we use a special type of light such as X rays, the energy of the X ray destroys the atom! The best we can do is draw a picture of what we THINK atoms look like. That s why it is still a theory. Right now, scientists believe atoms would look like round balls. There is some hope though. Since molecules are bigger than atoms, they should give a good idea of what atoms look like. In August of 2009 (only 2 years ago) scientists were able to take a picture of their first molecule. The molecule to the right looks kind of like the honeycombs of a beehive. To give you an idea of how big the molecule in the picture is, it is one million times smaller than a grain of sand. Scientists continue trying to snap the first image of an atom. Until that happens though, the atomic theory will remain just that, a theory.
1. What does the atomic theory state? Atomic Theory Guided Notes 2. Which country is most often credited with the discovery of modern science? 3. Which philosopher originally thought up the atomic theory? 4. What revolutionary thought did this philosopher have? 5. What important law was crucial to helping Dalton? 6. What does this law state? 7. What are Thomson s negatively charged particles now called? 8. What part of the atom did Rutherford discover? 9. Where did Bohr believe electrons were located? 10. Bohr s discovery of how electrons spin in an atom is similar to what in our solar system? 11. Scientists no longer say electrons are moving in orbit. Where are electrons now? 12. Why is the atomic theory still just a theory?