Democritus was a Greek philosopher who lived 2,000 years ago during 400 B.C. During his studies, he hypothesized that there were very small indivisible particles (atomos = invisible) that made up matter which are called atoms today. Unfortunately, he could not test his theory because they did not have the technology 2,000 years ago. In fact, John Dalton came up with the first widely recognized atomic theory. This theory was called the Atomic Theory of Matter which stated:
- All matter is made up of atoms.
- Atoms are very small, indivisible, indestructible particles.
- All atoms of a given element are identical in mass and other properties.
- Example: ALL iron atoms are the same, or ALL carbon atoms are the same, but iron and carbon atoms are different from each other.
- Chemical compounds form when two or more kinds of atoms combine.
- Suggested that during chemical reactions, atoms rearrange, but do not change identity.
Plum Pudding Atom Simulation
After J.J. Thomson, Robert Millikan discovered the mass of an electron using an experiment called the oil drop experiment. Next, came Ernest Rutherford who is famous for the Gold Foil Experiment.
Rutherford Atom Simulation
Next, James Chadwick discovered the existence of another particle in the nucleus, the neutron. Neutrons are neutral in charge which means neutrons do not have a positive charge or a negative charge. Then Niels Bohr discovered that electrons do not just circle the nucleus randomly, but the electrons are organized into orbitals. Orbitals circle the atom at different distances, and are associated with different energy levels.
Build an Atom Simulation
All of these scientists, and MANY more helped to develop our understanding of an atom today. The Modern Theory of the Atom states that the electron "clouds" encircle a nucleus, OR electrons are in constant motion around the nucleus (protons and neutrons). An atom is the smallest unit of matter that has all of the properties of matter.
- Example: An atom of gold is the smallest piece of gold, that has all of the properties of gold. There ARE smaller parts of an atom (protons, neutrons, electrons, etc...); However, they do not have the properties of gold.
- Nucleus: Which is located at the center of an atom and is made up of protons (positively charged) and neutrons (neutral).
- Electron Shells/Energy Levels: Are the circles around the nucleus, and this is where the electrons (negatively charged) are located.
- Subatomic Particles: Protons, Neutrons and Electrons.
- Watch the following video to learn more about subatomic particles:
Although these models are very useful, there is more to the story. The Bohr's Model has limitations. For example, it is not QUITE the way atoms look in reality. Atoms have sub-orbitals within their orbitals. For example, there are different orbitals and within each orbital there are sub-orbitals.
- Orbitals: 1, 2, 3, 4, etc.
- Sub-orbitals: s, p, d, f
- So, an electron could be in orbitals called 1s, 2s, 2p, 3s, 3d, etc.