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Chapter 5 Electrons in Atoms Chemistry Section 5.1 Light and Quantized Energy • At this point in history, we are in the early 1900’s. Electrons were the 1st subatomic particle to be discovered. • Chemists have a Rutherford Model of the atom. There is a small, dense, positively charged center of the atom called a nucleus. Electrons move around outside the nucleus. The Atom and Unanswered Questions • In Rutherford's model, the atom’s mass is concentrated in the nucleus and electrons move around it. • The model doesn’t explain how the electrons were arranged around the nucleus. • The model doesn’t explain why negatively charged electrons aren’t pulled into the positively charged nucleus. • In the early 1900s, scientists observed certain elements emitted visible light when heated in a flame. • Analysis of the emitted light revealed that an element’s chemical behavior is related to the arrangement of the electrons in its atoms. The Wave Nature of Light • Visible light is a type of electromagnetic radiation, a form of energy that exhibits wave-like behavior as it travels through space. • All waves can be described by several characteristics. • The wavelength (λ) is the shortest distance between equivalent points on a wave. • The frequency (ν) is the number of waves that pass a given point per second. • The amplitude is the wave’s height from the origin to a crest. What relationship do you see between λ, v, and c? • The speed of light (3.00 108 m/s) is the product of it’s wavelength and frequency. c = λν What relationship do you see between λ and ν? • Sunlight, or visible light, contains a continuous range of wavelengths and frequencies. • A prism separates sunlight into a continuous spectrum of colors – pg. 138. • The separation of white light by a prism • The electromagnetic spectrum includes all forms of electromagnetic radiation – pg. 139. The Electromagnetic Spectrum Visible Light … Note the trends: Blue light has shorter λ, higher v, and more energy. Red light has longer λ, lower v, and less energy. Brain Pop - The EM Spectrum • The wave model of light cannot explain all of light’s characteristics. • An example is the photoelectric effect , when electrons are emitted from a metal’s surface when light of a certain frequency shines on it (how solar calculators work). • Photoelectric Effect Simulation • Albert Einstein proposed in 1905 that light has a dual nature. • Einstein suggested a beam of light has wavelike and particlelike properties. • A photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy. Ephoton = h Ephoton represents energy, h is Planck's constant (6.626 x 10-34 J-s), & represents frequency. Atomic Emission Spectra • Light in a neon sign is produced when electricity is passed through a tube filled with neon gas and excites the neon atoms. • The excited atoms emit light to release energy. Emission Spectrum for Hydrogen • The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by the atoms of the element. • Each element’s atomic emission spectrum is unique – they have their own fingerprints! Absorption and Emission Spectra's Section 5.2 Quantum Theory and the Atom Bohr's Model of the Atom… • Bohr suggested that an electron moves around the nucleus only in certain allowed circular orbits Planetary Atomic Model. • The lowest allowable energy state of an atom is called its ground state. • When an atom gains energy, it is in an excited state. Planetary Atomic Model Bohr Model • Bohr assigned a quantum number for each principal energy level. • The highest quantum number can be found by the period the element is in on the Periodic Table. • He tried to predict the spectral lines for elements following Hydrogen, but was not successful. • The behavior of electrons is still not fully understood, but it is known they do not move around the nucleus in circular orbits. Big Discoveries… • Louis de Broglie hypothesized that particles, including electrons, could also have wavelike behaviors. • Werner Heisenberg showed it is impossible to take any measurement of an object without disturbing it. • The Heisenberg uncertainty principle states that it is fundamentally impossible to know precisely both the velocity and position of a particle at the same time. • The only quantity that can be known is the probability for an electron to occupy a certain region around the nucleus. Our Current Atomic Theory… • Erwin Schrödinger treated electrons as waves in a model called the quantum mechanical model of the atom. Most people call this the Electron Cloud Model. • This model applied to all elements!!! • The Electron Cloud is a 3-D arrangement of electrons around the nucleus. • Within the cloud, electrons are arranged by energy level, sublevel, and orbital shape. • When combined, a spherical shape is the result. • There is an attraction between the positively charged nucleus and the negatively charged electrons in the electron cloud. Orbital Shapes… Section 5.3 Electron Configuration Valence Electrons • Valence electrons are defined as electrons in the atom’s outermost orbitals—those associated with the atom’s highest principal energy level. • An element’s valence electrons determine the chemical properties of the element. • The number of valence electrons can be found using its group number on the periodic table. • Electron-dot structure consists of the element’s symbol representing the nucleus and inner electrons, surrounded by dots representing the element’s valence electrons.