The document defines several key terms related to atoms and their structure. It explains that atoms are the fundamental building blocks of matter, consisting of protons, neutrons, and electrons. The nucleus of an atom contains protons and neutrons, while electrons orbit the nucleus in shells or orbitals. Valence electrons are located in the outermost orbital. The periodic table arranges the elements and groups them according to their chemical properties and number of valence electrons. Ions are atoms that have gained or lost electrons, giving them a positive or negative charge. Electronegativity refers to an atom's ability to attract electrons towards itself.