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. 2.0: Prelude to Elements, Atoms, and the Periodic Table The hardest material in the human body is tooth enamel. It has to be hard so that our teeth can serve us for a lifetime of biting and chewing; however, tough as it is, tooth enamel is susceptible to chemical attack. Acids found in some foods or made by bacteria that feed on food residues on our teeth are capable of dissolving enamel. Unprotected by enamel, a tooth will start to decay, thus developing cavities and other dental problems. 2.1: Chemical Elements and Symbols An element is a substance that cannot be broken down into simpler chemical substances. There are about 90 naturally occurring elements known on Earth. Using technology, scientists have been able to create nearly 30 additional elements that do not occur in nature. Today, chemistry recognizes 118 elements—some of which were created an atom at a time. 2.2: 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. 2.3: Early Ideas in Atomic Theory The ancient Greeks proposed that matter consists of extremely small particles called atoms. Dalton postulated that each element has a characteristic type of atom that differs in properties from atoms of all other elements, and that atoms of different elements can combine in fixed, small, whole-number ratios to form compounds. Samples of a particular compound all have the same elemental proportions by mass. 2.4: 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. 2.5: 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. 2.6: Electronic Structure of Atoms Electrons in atoms have quantized energies. The state of electrons in atoms is described by four quantum numbers. Electrons are organized into shells and subshells about the nucleus of an atom. 2.7: Electron Configurations There are a set of general rules that are used to figure out the electron configuration of an atomic species: Aufbau Principle, Hund's Rule and the Pauli-Exclusion Principle. 2.8: Electron Configurations and the Periodic Table The arrangement of electrons in atoms is responsible for the shape of the periodic table. Electron configurations can be predicted by the position of an atom on the periodic table 2.9: Periodic Trends 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.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. 2.S: Elements, Atoms, and the Periodic Table (Summary) To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms and ask yourself how they relate to the topics in the chapter. Template:HideTOC Thumbnail: Ionization energies superimposed on a periodic table. (CC BY-NC-SA; anonymous by request).