Arrangement of Electrons in Atoms Chapter 4 “Energy travels through space as waves, but can be thought of as a stream of particles .“ - Albert Einstein

Development of New Atomic Model Scientist Review Rutherford – Gold Foil Experiment Nucleus – hard, dense, small center of atom containing protons & neutrons Most of the atom is empty space in which the electrons move randomly Did not explain how the electrons filled the empty space around the nucleus. Bohr – Solar System Model Electrons orbit the nucleus maintaining a fixed amount of energy, at constant speeds Energy levels are the circular paths that the electrons travel. Energy levels are arranged like a ladder. Rungs are not evenly spaced…they get closer together the farther from the ground, nucleus. Electrons at higher energy levels have greater energy The amount of energy required by an electron to move from one level to another is a quantum of energy. A quantum leap is the abrupt change in level by the electron. Schrodinger – Quantum Mechanical Model Also called the Electron Cloud Model The electrons travel randomly with no definite path. Electrons are particles that act like waves. The model is based on the mathematical 90% probability of finding an electron within a certain volume of space, called an orbital. Heisenberg Uncertainty Principle – it is impossible to determine both the position and velocity of an electron simultaneously. Development of New Atomic Model

Wave-Particle Duality of Motion Wave Description of Light Visible light is a kind of electromagnetic radiation, which is a form of energy that exhibits wavelike behavior as it travels through space. Together, all forms of electromagnetic radiation form the electromagnetic spectrum. Consists of radiation over a broad range of wavelengths Radio waves Radar waves Microwaves Infrared waves Visible light waves Ultraviolet waves X-rays Gamma rays Wave-Particle Duality of Motion

Wave-Particle Duality of Motion Wave Description of Light Amplitude The height of the wave from origin to crest Wavelength (λ) The distance between waves crests. Units are the same as length, cm. Frequency (v) The number of wave cycles that pass a given point per unit of time. Units of frequency are cycles/second or hertz. Wavelength and frequency are inversely proportional, when one goes up the other goes down. The speed of all forms of electromagnetic radiation in a vacuum is constant at 3.0 x 108 m/s Formula c = λ · v We will do a bunch of calculations for frequency and wavelength at the end of class today!! Wave-Particle Duality of Motion

Wave-Particle Duality of Motion Particle Description of Light When certain frequencies of light strike a metal, electrons are emitted. The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal. Electrons are ejected by metals when light shines on them – light waves act like particles The energy of the light has to be a certain minimum. Light can be absorbed or emitted by matter. Calculators that require light energy as it’s power source. Planck, German Physicist, proposed radiant energy is transferred in units (or quanta) of energy called photons. He wasn’t expected to come up with this – people thought that energy was emitted in waves. Like an hour-glass – small increments Wave-Particle Duality of Motion

Photoelectric Effect absorption spectrum energy p+ no e- ground state e- When a specific or quantized amount of energy is exposed to the atom, the electron jumps from its “ground” or original state to an “excited” state

travels at the speed of light Photoelectric Effect energy photon p+ no e- excited state e- travels at the speed of light (3.00 x 108 m/s) emission spectrum! When the “excited” electron returns to lower energy levels, it releases energy in the form of light

Wave-Particle Duality of Motion Particle Description of Light A quantum of energy is the minimum quantity of energy that can be lost or gained by an atom. Planck proposed the following relationship between quantum energy and the frequency of radiation E = hv E is the energy, in Joules v is the frequency, in s-1 h is the constant, 6.626x10-34 J·s We will do a bunch of calculations at the end of class on this!!! Planck’s constant shows mathematically that the electromagnetic energy is directly proportional to frequency. In 1905, Einstein expanded Planck’s theory by introducing the radical idea that electromagnetic radiation has a dual wave-particle nature. Light exhibits both wavelike properties, but it can also be thought of as a stream of particles. Each particle carries a quantum or energy. These particles, called photons, is a particle of electromagnetic radiation having zero mass and carrying a quantum of energy. Frequency is the energy per photon Intensity is the number of photons Wave-Particle Duality of Motion

Wave-Particle Duality of Motion Electrons exist in very specific energy states. The lowest energy sate of an atom is its ground state. A state in which an atom has a higher potential energy than it has in its ground state is an excited state. There are many possible excited states, each with a unique energy, but only one ground state energy for atoms of a given element. When an excited atom returns to its ground state or lower energy excited state, it gives off the energy it gained in the form of electromagnetic radiation. Every element emits a unique atomic spectrum, the pattern of frequencies obtained by passing energy emitted by atoms of an element in the gas phase through a prism, producing an emission line spectrum. Scientists expected to observe the emission of a continues range of frequencies of electromagnetic radiation, or the continuous spectrum. Attempts to explain emission spectrum led to an entirely new atomic theory called the quantum theory. Wave-Particle Duality of Motion

Bohr Model Niels Bohr e- p+ no Electrons circle around the nucleus on their energy level Energy levels

The Bohr Model of the Atom Electron Orbits, or Energy Levels Electrons can circle the nucleus only in allowed paths or orbits The energy of the electron is greater when it is in orbits farther from the nucleus The atom achieves the ground state when atoms occupy the closest possible positions around the nucleus Electromagnetic radiation is emitted when electrons move closer to the nucleus.

The Bohr Atomic Model

Energy transitions Energies of atoms are fixed and definite quantities Energy transitions occur in jumps of discrete amounts of energy Electrons only lose energy when they move to a lower energy state

Shortcomings of the Bohr Model Doesn't work for atoms larger than hydrogen (more than one electron) Doesn't explain chemical behavior