The stuff you scrape off burnt toast is made primarily of atoms of carbon. But what makes up a carbon atom—or any other atom?
The first subatomic particle to be identified was the electron, in 1898. Ten years later, the British physicist Ernest Rutherford discovered that atoms have a very dense nucleus that contains protons. In 1932, another British physicist, James Chadwick, discovered the neutron, another particle located within the nucleus.
With the electron, proton, and neutron, scientists thought they had found the smallest atomic building blocks. This changed in 1963 when American physicist Murray Gell-Mann proposed his quark theory. Gell-Mann believed that each proton and each neutron is made up of three even smaller particles—particles he named quarks.
Physicists have learned a great deal over the past 100 years. For instance, it is now known that in each atom of carbon-12, there is a set number of subatomic particles: six electrons, six protons, and six neutrons. The atom's nucleus and electrons are held together by the electromagnetic force—the positive charges of the protons balance the negative charges of the electrons. Neutrons have no charge.