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# 9: Electrons in Atoms and the Periodic Table

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Atoms act the way 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 we need to know the structural details to understand why atoms react the way they do. Virtually everything we know about atoms ultimately comes from light. Before we can understand the composition of atoms (especially electrons), we need to understand the properties of 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.
• 9.2: 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.
• 9.3: The Bohr Model: Atoms with Orbits
Electrons in atoms have quantized energies. The state of electrons in atoms is described by four quantum numbers.
• 9.4: 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.
• 9.5: 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.
• 9.6: 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.
• 9.7: The Explanatory Power of the Quantum-Mechanical Model
The chemical properties of elements is determined primarily by the number and distribution of valence electrons.
• 9.8: Periodic Trends: Atomic Size, Ionization Energy, and Metallic Character
Certain properties—notably atomic radius, ionization energy, electron affinity and metallic character—can be qualitatively understood by the positions of the elements on the periodic table.