Elements and atomic number | Atoms, isotopes, and ions | High school chemistry | Khan Academy

Everything in our world is made up of atoms. Yep, everything. From the air we breathe, to the water we drink, even the materials inside our cell phones. But what are atoms exactly? What's inside of these atoms? What makes an atom an atom? Atoms are tiny particles that are basic units of matter, like building blocks. But atoms themselves are made up of even smaller subatomic particles called protons, neutrons, and electrons. Let's take a look at this simplified diagram to learn more about these subatomic particles and the structure of an atom. In the center of an atom, we have the nucleus, which is composed of protons and neutrons. Outside the nucleus, we have the electron cloud. This is where electrons are most likely to be found. I've drawn the nucleus much larger than it really is, but an atom's electron cloud can be 100,000 times larger than its nucleus. So the electron cloud actually makes up most of the atom's volume. Protons, neutrons, and electrons differ in terms of their charge and mass. Let's take a look at charge first. Protons have a positive charge of 1+, and electrons have a negative charge of 1-. These charges are equal and opposite, so when protons and electrons are paired in atoms, their charges cancel. Neutrons have no charge, which means they are neutral. What about mass? The unit we use to express the masses of subatomic particles is the Unified Atomic Mass Unit, abbreviated as U. Protons and neutrons have a similar mass, about 1U. In comparison, electrons have a mass of about 0.0005U, which makes them roughly 2,000 times smaller than either a proton or a neutron. This means that nearly all the mass of an atom resides in the nucleus, and not in the electron cloud. Atoms make up everything in our world. Even though the air we breathe, the water we drink, and the cell phones we use all look really different from one another, the atoms that make up these things are all composed of the same subatomic particles, protons, neutrons, and electrons.