sentence: Atoms are made of neutrons, protons, and electrons.
sentences_before: A 'quark' is a tiny particle which makes up protons and neutrons.
sentences_after: It was once thought that neutrons, protons and electrons were fundamental particles. Fundamental particles can not be broken up into anything smaller. After the invention of the particle accelerator, it was discovered that electrons are fundamental particles, but neutrons and protons are not. Neutrons and protons are made up of quarks, which are held together by gluons.
concept_name: atom
headings: Quark