# 1.1: Atoms and the Periodic Table

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This chapter will describe some of the fundamental chemical principles related to the composition of matter, including those central to the concept of molecular identity.

• 1.1.0: Evolution of Atomic Theory
Although no one has actually seen the inside of an atom, experiments have demonstrated much about atomic structure. Thomson’s cathode ray tube showed that atoms contain small, negatively charged particles called electrons. Millikan discovered that there is a fundamental electric charge—the charge of an electron. Rutherford’s gold foil experiment showed that atoms have a small, dense, positively charged nucleus; the positively charged particles within the nucleus are called protons.
• 1.1.1: Atomic Structure and Symbolism
An atom consists of a small, positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons; its diameter is about 100,000 times smaller than that of the atom. The mass of one atom is usually expressed in atomic mass units (amu), which is referred to as the atomic mass. An amu is defined as exactly $$1/12$$ of the mass of a carbon-12 atom and is equal to 1.6605 $$\times$$ 10−24 g.
• 1.1.2: The Periodic Table
The discovery of the periodic recurrence of similar properties among the elements led to the formulation of the periodic table, in which the elements are arranged in order of increasing atomic number in rows known as periods and columns known as groups. Elements in the same group of the periodic table have similar chemical properties. Elements can be classified as metals, metalloids, and nonmetals, or as a main-group elements, transition metals, and inner transition metals.
• 1.1.3: Memorizing Element Names and Symbols
You are responsible for identifying these elements by name or symbol for CHE 101 at Oregon Tech.
• 1.1.4: Development of Quantum Theory
Macroscopic objects act as particles. Microscopic objects (such as electrons) have properties of both a particle and a wave. but their exact trajectories cannot be determined. The quantum mechanical model of atoms describes the 3D  position of the electron in a probabilistic manner according to a mathematical function called a wavefunction, often denoted as ψ. The squared magnitude of the wavefunction describes the distribution of the probability of finding the electron in a particular region in
• 1.1.5: Electron Configurations
Thumbnail: Spinning Buckminsterfullerene ($$\ce{C60}$$). (CC BY-SA 3.0; unported; Sponk).