1.1: The Elements All matter is composed of elements. Chemical elements are represented by a one- or two-letter symbol. 1.2: The Structure of Atoms Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. Protons and neutrons are grouped together in the nucleus of an atom, while electrons orbit about the nucleus. 1.3: Nuclei of Atoms Elements can be identified by their atomic number and mass number. Isotopes are atoms of the same element that have different masses. 1.4: Arrangements of Electrons Electrons are organized into shells and subshells about the nucleus of an atom. 1.5: The Periodic Table The chemical elements are arranged in a chart called the periodic table. Some characteristics of the elements are related to their position on the periodic table. 1.E: Elements, Atoms, and the Periodic Table (Exercises) These are homework exercises to accompany Chapter 2 of the Ball et al. "The Basics of GOB Chemistry" Textmap.