# 2: Atomic Structure

Just as a language has an alphabet from which words are built, chemistry has an alphabet from which matter is described. However, the chemical alphabet is larger than the one we use for spelling. You may have already figured out that the chemical alphabet consists of the chemical elements. Their role is central to chemistry, for they combine to form the millions and millions of known compounds.

• 2.1: The Elements
All matter is composed of elements. Chemical elements are represented by a one- or two-letter symbol.
• 2.2: Atomic Theory
Atoms are the ultimate building blocks of all matter. The modern atomic theory establishes the concepts of atoms and how they compose matter.
• 2.3: 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.
• 2.4: 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.
• 2.5: Arrangement of Electron (Shell Model)
An electron shell is the outside part of an atom around the atomic nucleus. It is a group of atomic orbitals with the same value of the principal quantum number $$n$$. Electron shells have one or more electron subshells, or sublevels. The name for electron shells comes from the Bohr model, in which groups of electrons were believed to go around the nucleus at certain distances, so that their orbits formed "shells".
• 2.6: 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.
• 2.7: Chapter Summary
These are homework exercises to accompany Chapter 2 of the Ball et al. "The Basics of GOB Chemistry" Textmap.