 The atom is the smallest unit of matter that has the properties of a chemical element. If you break up an atom, it no longer has the properties of that element. Atoms are very, very tiny. We can't see them with a regular microscope, but we can see them using the most powerful ones, though these are not found in a classroom. To give you a sense of the size of an atom, a stack of one million atoms would be the same thickness as a sheet of paper. A typical adult human has seven billion billion billion atoms in them. In this simulation, you can see us zooming in on an atom, and this will give you an idea of the size of these things. The take-home point is that they are very, very, very small. These are constructed out of three main subatomic particles, and subatomic just means smaller than an atom. These particles are the proton, which has a positive charge. We talked about what charge is in a previous video. The neutron is neutral. That means it does not have one of these charges. And the electron has a negative electric charge. Electrons and neutrons form the nucleus of the atom, which is in the center. And the electrons move around the outside. But most of the atom is empty space. There are several different models of the atom that we can use to better understand it. The one we use the most often is based on the Bohr model of the atom. And that model looks something like this. Although it's not completely accurate, it's easy to understand, and it's very useful when it comes to understanding how atoms interact. In the Bohr model, the protons and neutrons are in the center, the nucleus of the atom. And the electrons travel in discrete orbits around the nucleus. Each orbit has a different energy level. Electrons in higher energy levels have more energy than electrons closer to the nucleus. The energy levels are like the rungs of a ladder. Electrons are never between these orbits or energy levels. And if they move from one orbit to another, it's more like an instant jump. The more realistic model of the atom is the quantum mechanical model. In this model, electrons travel in clouds of probability rather than in discrete orbits. We can't always know where electrons are at any given moment, but we can predict where they probably are. As in the Bohr model, there are different energy levels, and electrons in higher energy levels have more energy than electrons in lower energy levels. The nucleus of the atom contains the majority of an atom's mass, but the electrons account for the atom's volume, in other words, the total space that the atom takes up. Electrons and protons have electrical charges. The charge on an electron is equal to the charge on a proton, but opposite in directions, and we will be exploring this a lot in the future. Electrons are electrically neutral, which means they don't have a charge. Elements are defined by the number of protons they have. This means that the number of protons determines the identity of the atom. An oxygen atom always has 8 protons. If it doesn't have 8 protons, it's now oxygen. A magnesium atom always has 12 protons, never more, never less. A copper atom always has 29 protons. The periodic table is organized according to atomic number. An atomic number tells us the number of protons in the atom. So you could also say that the periods in the table are arranged by the number of protons, with the lowest number on the left and the highest number on the right. In summary, the parts of the atom are the proton, the neutron, and the electron. The protons and neutrons are in the center of the atom and the electrons surround them. There are several different ways to model the atom, but in the model we use most often, the protons and neutrons are in the center and the electrons orbit them in certain orbits or energy levels.