4.1: The Bohr Model - Atoms with Orbits Bohr's model suggests that 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 difference between the two energy levels. The existence of the atomic spectra supports Bohr's model. 4.2: Quantum-Mechanical Orbitals and Electron Configurations We look at the four quantum numbers for a given electron. Electron configuration notation simplifies the indication of where electrons are located in a specific atom. The Aufbau principle gives the order of electron filling in an atom. Hund's rule specifies the order of electron filling within a set of orbitals. Orbital filling diagrams are a way of indicating electron locations in orbitals. 4.3: 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. 4.4: The Explanatory Power of the Quantum-Mechanical Model The chemical properties of elements are determined primarily by the number and distribution of valence electrons. 4.5: Representing Valence Electrons with Dots The Lewis Structure of a molecule shows how the valence electrons are arranged among the atoms of the molecule. Lewis electron dot diagrams use dots to represent valence electrons around an atomic symbol. Lewis electron dot diagrams for ions have less (for cations) or more (for anions) dots than the corresponding atom. From experimentation, chemists have learned that when a stable compound forms, the atoms usually have a noble gas electron configuration—or eight valence electrons. 4.6: Lewis Structures of Ionic Compounds- Electrons Transferred The tendency to form species that have eight electrons in the valence shell is called the octet rule. The attraction of oppositely charged ions caused by electron transfer is called an ionic bond. The strength of ionic bonding depends on the magnitude of the charges and the sizes of the ions. 4.7: Covalent Lewis Structures- Electrons Shared Covalent bonds are formed when atoms share electrons. Lewis electron dot diagrams can be drawn to illustrate covalent bond formation. Double bonds or triple bonds between atoms may be necessary to properly illustrate the bonding in some molecules. 4.8: Writing Lewis Structures for Covalent Compounds Lewis dot symbols provide a simple rationalization of why elements form compounds with the observed stoichiometries. A plot of the overall energy of a covalent bond as a function of internuclear distance is identical to a plot of an ionic pair because both result from attractive and repulsive forces between charged entities. In Lewis electron structures, we encounter bonding pairs, which are shared by two atoms, and lone pairs, which are not shared between atoms. 4.9: Resonance - Equivalent Lewis Structures for the Same Molecule Resonance structures are averages of different Lewis structure possibilities. Bond lengths are intermediate between covalent bonds and covalent double bonds. 4.10: Predicting the Shapes of Molecules The approximate shape of a molecule can be predicted from the number of electron groups and the number of surrounding atoms. 4.11: Electronegativity and Polarity - Why Oil and Water Do not Mix Covalent bonds can be nonpolar or polar, depending on the electronegativities of the atoms involved. Covalent bonds can be broken if energy is added to a molecule. The formation of covalent bonds is accompanied by energy given off. Covalent bond energies can be used to estimate the enthalpy changes of chemical reactions. 4.12: Kinetic Molecular Theory- A Model for Gases The physical behavior of gases is explained by the kinetic theory of gases. An ideal gas adheres exactly to the kinetic theory of gases. 4.13: Pressure - The Result of Constant Molecular Collisions Pressure is a force exerted over an area. Pressure has several common units that can be converted. 4.14: Boyle’s Law - Pressure and Volume Boyle’s Law relates the pressure and volume of a gas at constant temperature and amount. 4.15: Charles’s Law- Volume and Temperature Charles’s Law relates the volume and temperature of a gas at constant pressure and amount. In gas laws, temperatures must always be expressed in kelvins. 4.16: Gay-Lussac's Law- Temperature and Pressure Gay-Lussac's Law states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant. Gay-Lussac's Law is very similar to Charles's Law, with the only difference being the type of container. Whereas the container in a Charles's Law experiment is flexible, it is rigid in a Gay-Lussac's Law experiment. 4.17: The Combined Gas Law- Pressure, Volume, and Temperature There are gas laws that relate any two physical properties of a gas. The Combined Gas Law relates pressure, volume, and temperature of a gas. 4.18: The Ideal Gas Law- Pressure, Volume, Temperature, and Moles The Ideal Gas Law relates the four independent physical properties of a gas at any time. The Ideal Gas Law can be used in stoichiometry problems with chemical reactions involving gases. Standard temperature and pressure (STP) are a useful set of benchmark conditions to compare other properties of gases. At STP, gases have a volume of 22.4 L per mole. The Ideal Gas Law can be used to determine densities of gases. 4.19: Mixtures of Gases - Why Deep-Sea Divers Breathe a Mixture of Helium and Oxygen The pressure of a gas in a gas mixture is termed the partial pressure. Dalton’s Law of Partial Pressures states that the total pressure in a gas mixture is the sum of the individual partial pressures. Collecting gases over water requires that we take the vapor pressure of water into account. Mole fraction is another way to express the amounts of components in a mixture.