Transcript Ch4_S3 - ib
4.3 Modern Atomic Theory Bohr’s Model of the Atom • Nucleus surrounded by large volume of space –focused on electrons –constant speed –fixed orbits • Each electron has a specific amount of energy 4.3 Modern Atomic Theory Bohr’s Model of the Atom Energy Levels • Possible energies of electrons • An electron cannot exist between energy levels. • Size of jump between energy levels determines amount of energy gained or lost. • No two elements have the same set of energy levels. 4.3 Modern Atomic Theory Bohr’s Model of the Atom An electron can move from one energy level to another when the atom gains or loses energy. Electron Electrons gain or lose energy when they move between fixed energy levels Nucleus Bohr Model 4.3 Modern Atomic Theory Bohr’s Model of the Atom Analogy: staircase • The landing at the bottom is the lowest level. • Each step up represents a higher energy level. • The step height represents an energy difference between levels. • You can only move in whole numbers of stairs. 4.3 Modern Atomic Theory Bohr’s Model of the Atom Evidence for Energy Levels • Scientists can measure the energy absorbed or released • Light: form of energy that can be observed. • Example- Fireworks: • Heat Some electrons move to higher energy levels • Electrons move back to lower energy levels and release energy • visible light • Different colors: no elements have the same energy levels 4.3 Modern Atomic Theory Electron Cloud Model • Electrons do not move like planets • Probability used to establish estimated electron location: - Model - Most likely locations for electrons The electron cloud is a visual model of the probable locations of electrons in an atom. The probability of finding an electron is higher in the denser regions of the cloud. The nucleus contains protons and neutrons Electron Cloud Model 4.3 Modern Atomic Theory Electron Cloud Model Analogy: airplane propellers • Propeller at rest: you can see the location of the blades. • Propeller is moving: you see only a blur (“electron cloud”) 4.3 Modern Atomic Theory Electron Cloud Model Analogy: your path through the day • Imagine a map of your school. Mark your exact location with a dot once every 10 minutes over one week. • The dots on your map are a model of your “orbital.” • They describe your most likely locations. 4.3 Modern Atomic Theory Electron Cloud Model • Lowest energy level: has only one orbital • Higher energy levels: have more than one orbital 4.3 Modern Atomic Theory Electron Configurations • Electron configuration: arrangement of electrons • Ground state: all electrons have lowest possible energies • Excited state: electrons move to an orbital with higher energy 4.3 Modern Atomic Theory Electron Configurations Example: A lithium atom has three electrons. • Ground state: two electrons are in the orbital of the first energy level, third electron is in an orbital of the second energy level. • Absorbs enough energy one of its electrons can move to an orbital with a higher energy: excited state • less stable than the ground state. • Eventually, the electron that was promoted to a higher energy level loses energy, and the atom returns to the ground state. 4.3 Modern Atomic Theory Electron Configurations • Analogy: Gymnast • The ground state of a person is on the floor. • A gymnast on a balance beam is like an atom in an excited state • When she dismounts, the gymnast will return to a lower, more stable energy level. 4.3 Modern Atomic Theory 1. What did Bohr focus his model on? 2. What do we use to determine the likely locations of electrons in the electron cloud model? 3. If all electrons are in the lowest possible energy levels, what state is the atom in? 4. If electrons move from the excited state to the ground state, what type of energy might we observe being released? 5. Why are there different colors seen when different elements move from excited to ground states?