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Studying Atoms 1899-Modern Atomic Theory Book pages 104-105 & 113-116 ISN pg 43 Rutherford’s Atomic Theory • 1899, Rutherford discovered that uranium emits fastmoving particles that have a positive charge called alpha particles Rutherford’s Atomic Theory • Hypothesis: (asked student- Marsden, to find out what happens to alpha particles when they pass through a thin sheet of gold) Predicted that most particles would travel in a straight path from their source to a screen that lit up when struck. Rutherford’s Atomic Theory • The Gold Foil Experiment – Aimed narrow beam of alpha particles at gold, the material around the gold produced a flash of light when struck by fast-moving alpha particles – Could figure out path of alpha particles by observing the flash after they had passed through the gold – Did NOT support Rutherford’s prediction Rutherford’s Atomic Theory • Gold Foil Experiment continued… – More particles were deflected than he expected – About 1 out of every 20,000 was deflected by more than 90 degrees – Some alpha particles behaved as though they had struck an object and bounced straight back Gold Foil Experiment Rutherford’s Atomic Theory • Discovery of the Nucleus – Alpha particles whose paths were deflected must have come close to another charged object – The closer they came, the greater the deflection – Concluded: the positive charge of an atom is not evenly spread throughout the atom, it is concentrated in a very small central area- nucleus – Nucleus: a dense, positively charged mass located in the center of the atom Rutherford’s Atomic Theory • According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus Properties of Subatomic Particles • Neutrons – 1932, English physicist J. Chadwick designed an experiment to show that neutrons exist; concluded particles were neutral because a charged object did not deflect their path – A neutral subatomic particle that is found in the nucleus of an atom – Mass is almost exactly equal to that of a proton Bohr’s Model of the Atom • Bohr was a Danish physicist who worked with Rutherford • Bohr agreed with Rutherford’s model, but Bohr’s model focused on the electrons • Energy Levels – In Bohr’s model, electrons move with constant speed in fixed orbits around the nucleus (like planets around a sun) Bohr’s Model of the Atom • Energy Levels – Each electron in an atom has a specific amount of energy – If an atom gains or loses energy, the energy of an electron can change – The possible energies that electrons in an atom can have are called energy levels – An electron cannot exist between energy levels Bohr’s Model of the Atom • Energy Levels – An electron in an atom can move from one energy level to another when the atom gains or loses energy – The size of the jump between energy levels determines the amount of energy gained or lost • Evidence for Energy Levels – Scientists can measure the energy gained or released when an electron absorbs or releases energy and moves to higher or lower energy levels Bohr’s Model of the Atom Electron Cloud Model • An electron cloud is a visual model of the most likely locations for electrons in an atom • The cloud is denser at those locations where the probability of finding an electron is high • Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus