4: Atoms and Elements
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- 4.1: Cutting Aluminum Until You Get Atoms
- Take some aluminum foil. Cut it in half. Now you have two smaller pieces of aluminum foil. Cut one of the pieces in half again. Cut one of those smaller pieces in half again. Continue cutting, making smaller and smaller pieces of aluminum foil.
- 4.2: Early Atomic Theory
- You learned earlier how all matter in the universe is made out of tiny building blocks called atoms. All modern scientists accept the concept of the atom, but when the concept of the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.
- 4.3: Discovery of the Nucleus
- While Dalton's Atomic Theory held up well, later scientists demonstrated that his theory was not the entire story. J. J. Thomson suggested that the small, negatively charged particles making up the cathode ray were actually pieces of atoms now known as electrons. Ernest Rutherford developed the nuclear model of the atom based on the scattering of alpha particles shot at a sheet of gold foil.
- 4.4: Protons, Neutrons, and Electrons
- Atoms are composed of protons, neutrons, and electrons. Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of −1, which is equal but opposite to the charge of a proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges "cancel out", making atoms electrically neutral.
- 4.5: Chemical Symbols and the Atomic Number
- Scientists distinguish between different elements by counting the number of protons in the nucleus. Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, the number of protons in an atom is called its atomic number (Z). This number is very important because it is unique for atoms of a given element. Each element is also identified by a unique name and a one- or two-letter symbol based on that name.
- 4.6: The Periodic Table
- The periodic table provides a way to organize elements based on their atomic number and chemical properties. Elements can then be classified based on where they are located on the table. Certain elemental properties also become apparent in a survey of the periodic table as a whole. Thus, every element can also be classified as either a metal, a nonmetal, or a metalloid (or semi metal).
- 4.7: Isotopes and Mass Numbers
- All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally.
- 4.8: Atomic Mass
- In chemistry we very rarely deal with only one isotope of an element. We use a mixture of the isotopes of an element in chemical reactions and other aspects of chemistry, because all of the isotopes of an element react in the same manner. That means that we rarely need to worry about the mass of a specific isotope, but instead we need to know the average mass of the atoms of an element.
- 4.9: Ion Formation
- Metal atoms may lose valence electrons to form a more stable arrangement of electrons. Atoms that lose electrons acquire a positive charge as a result because they are left with fewer negatively charged electrons to balance the positive charges of the protons in the nucleus. These positively charged ions are called cations. In contrast, nonmetal atoms tend to gain electrons and thus become negatively charged. Negatively charged ions are called anions.
- 4.10: For Future Use
- 4.E: Exercises
Thumbnail Chapter 4: Periodic Table (2012rc via Wikimedia Commons).