2.1: What is Radiation What type of radiation do you experience in your daily life? This section will provide information on the the types of radiation that are relevant in understanding the foundation of chemistry and the atom. 2.2: The Discovery of Radioactivity Henri Becquerel, Marie Curie, and Pierre Curie shared the discovery of radioactivity. 2.3: Nuclear Equations Nuclei can undergo reactions that change their number of protons, number of neutrons, or energy state. Many different particles can be involved in nuclear reactions. The most common are protons, neutrons, positrons (which are positively charged electrons), alpha (α) particles (which are high-energy helium nuclei), beta (β) particles (which are high-energy electrons), and gamma (γ) rays (which compose high-energy electromagnetic radiation). 2.4: Biological Effects of Radiation We are constantly exposed to radiation from naturally occurring and human-produced sources. This radiation can affect living organisms. Ionizing radiation is the most harmful because it can ionize molecules or break chemical bonds, which damages the molecule and causes malfunctions in cell processes. Types of radiation differ in their ability to penetrate material and damage tissue, with alpha particles the least penetrating but potentially most damaging and gamma rays are most penetrating. 2.5: 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. 2.6: Fission and Fusion Nuclei that are larger than iron-56 may undergo nuclear reactions in which they break up into two or more smaller nuclei. These reactions are called fission reactions. When a neutron strikes a UU -235 nucleus and the nucleus captures a neutron, it undergoes fission producing two lighter nuclei and three free neutrons. The production of the free neutrons makes it possible to have a self-sustaining fission process - a nuclear chain reaction. 2.7: Radioactivity in Medicine and Other Applications Compounds known as radioactive tracers can be used to follow reactions, track the distribution of a substance, diagnose and treat medical conditions, and much more. Other radioactive substances are helpful for controlling pests, visualizing structures, providing fire warnings, and for many other applications. Hundreds of millions of nuclear medicine tests and procedures, using a wide variety of radioisotopes with relatively short half-lives, are performed every year in the US. 2.8: The Electromagnetic Spectrum Electromagnetic waves have an extremely wide range of wavelengths, frequencies, and energies. The highest energy form of electromagnetic waves are gamma (γ) rays and the lowest energy form are radio waves. 2.9: The Bohr Model - Atoms with Orbits Bohr's model suggests each atom has a set of unchangeable energy levels and electrons in the electron cloud of that atom must be in one of those energy levels. Bohr's model suggests that the atomic spectra of atoms is produced by electrons gaining energy from some source, jumping up to a higher energy level, then immediately dropping back to a lower energy level and emitting the energy different between the two energy levels. The existence of the atomic spectra is support for Bohr's model, 2.10: The Quantum-Mechanical Model- Atoms with Orbitals Quantum mechanics involves the study of material at the atomic level. This field deals with probabilities since we cannot definitely locate a particle. Orbitals are mathematically derived regions of space with different probabilities of having an electron. 2.11: 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. 2.12: Arrangements of Electrons Electrons are organized into shells and subshells about the nucleus of an atom. 2.13: Periodic Trends Certain properties-notably atomic radius, IE, and EA-can be qualitatively understood by the positions of the elements on the periodic table.