4: Electronic Structure
- Page ID
- 390456
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Atoms act the way that they do because of their structure. We already know that atoms are composed of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, and electrons orbit around the nucleus. But knowing structural details is key to understanding why atoms react the way they do. Virtually everything known about atoms ultimately comes from light. Before the composition of atoms (especially electrons) can be understood, the properties of light need to be understood.
- 4.1: Prelude to Electronic Structure
- A startling conclusion of modern science is that electrons also act as waves. However, the wavelength of electrons is much, much shorter—about 0.5 to 1 nm. This allows electron microscopes to magnify 600–700 times more than light microscopes. This allows us to see even smaller features in a world that is invisible to the naked eye.
- 4.2: Light
- Light acts like a wave, with a frequency and a wavelength. The frequency and wavelength of light are related by the speed of light, a constant. Light acts like a particle of energy, whose value is related to the frequency of light.
- 4.5: Quantum Mechanics
- Quantum mechanics is the study of the motion of objects that are atomic or subatomic in size and thus demonstrate wave-particle duality. In classical mechanics, the size and mass of the objects involved effectively obscures any quantum effects so that such objects appear to gain or lose energies in any amounts. Particles whose motion is described by quantum mechanics gain or lose energy in the small pieces called quanta.
- 4.6: Quantum Mechanical Atomic Model
- The quantum mechanical model of the atom comes from the solution to Schrödinger's equation. Quantization of electron energies is a requirement in order to solve the equation. Solutions to the Schrödinger wave equation, called wave functions, give only the probability of finding an electron at a given point around the nucleus. Electrons do not travel around the nucleus in simple circular orbits.
- 4.8: Orbitals
- We can apply our knowledge of quantum numbers to describe the arrangement of electrons for a given atom. We do this with something called electron configurations. They are effectively a map of the electrons for a given atom. We look at the four quantum numbers for a given electron and then assign that electron to a specific orbital in the next Module.
- 4.9: Aufbau Principle
- to create ground state electron configurations for any element, it is necessary to know the way in which the atomic sublevels are organized in order of increasing energy. The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest. The Aufbau (German: "building up, construction") principle is sometimes referred to as the "building up" principle.
- 4.10: Pauli Exclusion Principle
- The Pauli exclusion principle, which states that no two electrons in an atom can have the same set of four quantum numbers. The energy of the electron is specified by the principal, angular momentum, and magnetic quantum numbers. If those three numbers are identical for two electrons, the spin numbers must be different in order for the two electrons to be differentiated from one another.
- 4.11: Hund's Rule and Orbital Filling Diagrams
- Hund's rule states that orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron and that each of the single electrons must have the same spin. An orbital filling diagram is the more visual way to represent the arrangement of all the electrons in a particular atom. In an orbital filling diagram, the individual orbitals are shown as circles (or squares) and orbitals within a sublevel are drawn next to each other horizontally.
- 4.12: Electron Configurations
- Electron configuration notation eliminates the boxes and arrows of orbital filling diagrams. Each occupied sublevel designation is written followed by a superscript that is the number of electrons in that sublevel.
- 4.13: Valence Electrons
- Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the 1s sublevel are called inner-shell electrons and are not involved directly in the element's reactivity or in the formation of compounds.
- 4.14: Noble Gas Configuration
- A noble gas configuration of an atom consists of the elemental symbol of the last noble gas prior to that atom, followed by the configuration of the remaining electrons.
- 4.17: Electron Shielding
- The concept called "electron shielding" involves the outer electrons are partially shielded from the attractive force of the protons in the nucleus by inner electrons.
- 4.18: Periodic Trends - Electron Affinity
- The energy change that occurs when a neutral atom gains an electron is called its electron affinity. When energy is released in a chemical reaction or process, that energy is expressed as a negative number. The figure below shows electron affinities in kJ/molkJ/mol for the representative elements. Electron affinities are measured on atoms in the gaseous state and are very difficult to measure accurately.
- 4.E: Electronic Structure (Exercises)
- These are exercises and select solutions to accompany Chapter 8 of the "Beginning Chemistry" Textmap formulated around the Ball et al. textbook.