2: Atomic Structure

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2.1: Modern Atomic Theory and the Laws That Led to It
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.
2.2 The Structure of the Atom
Chemists consider atoms to be made of three sub-atomic particles: protons, neutrons, and electrons. The protons and neutrons make up most of the mass of an atoms, and are located in the small, dense nucleus in the center of an atom. Electrons do have mass, but are about 2000 times lighter than protons or neutrons. The electrons are found outside the nucleus, and the volume of space in which they are likely found determines the volume of an atom. Note that the electrons do not move in orbits!
2.3 Molar Mass - Counting Atoms by Weighing Them
The chemical changes we observe always involve discrete numbers of atoms that rearrange themselves into new configurations. These numbers are far too large in magnitude for us to count , but they are still numbers, and we need to have a way to deal with them. We also need a bridge between these numbers, which we are unable to measure directly, and the masses of substances, which we do measure and observe. The mole concept provides this bridge, and is key to all of quantitative chemistry.
2.4 Nuclear Reactions
Nuclei can undergo reactions that change their number of protons, number of neutrons, or energy state. Many different particles can be involved and the most common are protons, neutrons, positrons, alpha (α) particles, beta (β) particles (high-energy electrons), and gamma (γ) rays (which compose high-energy electromagnetic radiation). As with chemical reactions, nuclear reactions are always balanced. When a nuclear reaction occurs, the total mass (number) and the total charge remain unchanged.
2.5 The Belt of Stability - Predicting the Type of Radioactivity
Many elements have at least one isotope whose atomic nucleus is stable indefinitely, but all elements have isotopes that are unstable and decay, at measurable rates by emitting radiation. Some elements have no stable isotopes and eventually decay to other elements. In contrast to the chemical reactions that were the main focus of earlier chapters and are due to changes in the arrangements of the valence electrons of atoms, the nuclear decay results in changes within atomic nuclei.
2.6 Half-lives and the Rate of Radioactive Decay
Unstable nuclei decay in a very specific way, following what is called a first-order process. In a first-order decay process, the rate of the decay depends directly on the number of radioactive nuclei. Because of this direct concentration dependence, the decay process can be described by its half life, the time it takes for half of the radioactive nuclei to decay. Half-lives range from less than 0.000001 seconds to longer than 1,000,000 years.
2.7 Mass Defect - The Source of Nuclear Energy
The energy released from nuclear reactions comes directly from the conversion of matter into energy. That is, the mass of the products is smaller than the mass of the reactants in both fusion reactions and fission reactions. The difference in mass, the mass defect, multiplied by the speed of light squared, is the energy released by the process.
2.8 Nuclear Energy - Fission and Fusion
Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger binding energies per nucleon. Sometimes neutrons are also produced. This decomposition is called fission, the breaking of a large nucleus into smaller pieces. The breaking is rather random with the formation of a large number of different products. Fission usually does not occur naturally, but is induced by bombardment with neutrons.
2.9 The Nature of Light
Understanding the electronic structure of atoms requires an understanding of the properties of waves and electromagnetic radiation. A basic knowledge of the electronic structure of atoms requires an understanding of the properties of waves and electromagnetic radiation. A wave is a periodic oscillation by which energy is transmitted through space. All waves are periodic, repeating regularly in both space and time. Waves are characterized by several interrelated properties.
2.10 Quantum Mechanics and The Atom
There is a relationship between the motions of electrons in atoms and molecules and their energies that is described by quantum mechanics. Because of wave–particle duality, scientists must deal with the probability of an electron being at a particular point in space. To do so required the development of quantum mechanics, which uses wavefunctions to describe the mathematical relationship between the motion of electrons in atoms and molecules and their energies.
2.11 Trends of the Periodic Table
The arrangement of electrons in the atoms of the various elements allow us to understand many periodic trends. The size of an atom (atomic radius), the energy needed to remove one electron from an atom (ionization energy), the energy change associated with adding one electron to an atom (electron affinity), the attraction that an atom has for the electrons that it is sharing with another atom (electronegativity), and the number of reactive electrons (valence electrons) are trends we will study.
2.12 Example Problems