Everything around you is made of atoms: the air, this screen, your hands. Atoms are far too small to see, but chemists have worked out what is inside them.
An atom has a tiny nucleus in the middle, made of protons (which carry a positive charge) and neutrons (which carry no charge). Around the nucleus are electrons, which carry a negative charge.
What makes an atom carbon is its number of protons. Every carbon atom has exactly 6 protons. That number is carbon's atomic number. An atom with 7 protons is nitrogen, and one with 8 is oxygen.
Most carbon atoms also have 6 neutrons (chemists call these carbon-12). About 1 in 100 has 7, and a very rare few have 8. A carbon atom has 6 electrons too: 2 close to the nucleus and 4 in its outer shell. Remember that 4. It is the most important number in this whole course.