3.4 Atomic Structure Definition Key Points 1. Matter is anything that occupies space and has mass. 2. The smallest particle of matter that can take part in a chemical reaction is called an atom. 3. An atom consists of protons, neutrons and electrons. 4. The protons and neutrons form the nucleus of the atom. 5. Protons and neutrons are collectively known as nucleons. 6. The atomic number of an element indicates the number of protons in the nucleus of its atom. 7. The number of electrons in the outermost orbit is the valence electrons. 8. Electrons carry a negative charge; while protons carry a positive charge. Charged atoms or molecule are called ion...