1.2: Matter, Radioactive Decay, and Experimentation

• 1.2.1: What is Matter?

  Matter is anything that has mass and volume (takes up space). For most common objects that we deal with every day, it is fairly simple to demonstrate that they have mass and take up space. You might be able to imagine, however, the difficulty for people several hundred years ago to demonstrate that air has mass and volume. Air (and all other gases) are invisible to the eye, have very small masses compared to equal amounts of solids and liquids, and are quite easy to compress (change volume).

• 1.2.2: Classifying Matter According to Its Composition

  One useful way of organizing our understanding of matter is to think of a hierarchy that extends down from the most general and complex, to the simplest and most fundamental. Matter can be classified into two broad categories: pure substances and mixtures. A pure substance is a form of matter that has a consistent composition and properties that are constant throughout the sample. A material composed of two or more substances is a mixture.

• 1.2.3: The Properties 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.

• 1.2.4: Elements- Defined by Their Number of Protons

  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, scientists are always interested in this number, and how this number differs between different elements. 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.

• 1.2.5: Ions - Losing and Gaining Electrons

  Atom may lose valence electrons to obtain a lower shell that contains an octet. Atoms that lose electrons acquire a positive charge as a result. Some atoms have nearly eight electrons in their valence shell, and can gain additional valence electrons until they have an octet. When these atoms gain electrons, they acquire a negative charge.

• 1.2.6: Isotopes - When the Number of Neutrons Varies

  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.

• 1.2.7: The Discovery of Radioactivity

  Henri Becquerel, Marie Curie, and Pierre Curie shared the discovery of radioactivity.

• 1.2.8: Types of Radioactivity- Alpha, Beta, and Gamma Decay

  The major types of radioactivity include alpha particles, beta particles, and gamma rays. Fission is a type of radioactivity in which large nuclei spontaneously break apart into smaller nuclei.

• 1.2.9: Detecting Radioactivity

  Several different devices are used to detect and measure radiation, including Geiger counters, scintillation counters (scintillators), and radiation dosimeters. Probably the best-known radiation instrument, the Geiger counter (also called the Geiger-Müller counter) detects and measures radiation. Radiation causes the ionization of the gas in a Geiger-Müller tube.

• 1.2.10: Natural Radioactivity and Half-Life

  During natural radioactive decay, not all atoms of an element are instantaneously changed to atoms of another element. The decay process takes time and there is value in being able to express the rate at which a process occurs. A useful concept is half-life, which is the time required for half of the starting material to change or decay. Half-lives can be calculated from measurements on the change in mass of a nuclide and the time it takes to occur.