Physics Subject Test: Atomic Physics 3

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What is an *atom*?
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The smallest unit of an element that displays the properties of that element.
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What is a *subatomic particle*?
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A particle that makes up part of an atom; a smaller part of an atom.
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What is a *proton*?
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A subatomic particle that is positively charged.
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What is an *electron*?
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A subatomic particle that is negatively charged.
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What is a *neutron*?
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A subatomic particle that is neutrally charged.
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What is a *nucleus*?
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The central, dense part of an atom that contains most of the mass.
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Which subatomic particles make up the nucleus?
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Protons and/or neutrons.
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Which subatomic particle determines the *identity* of an element?
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Protons.
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Which subatomic particle determines the *isotope* of an element?
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Neutrons.
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Which subatomic particle determines the *ion* of an element?
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Electrons.
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Who discovered the electron and came up with the “plum pudding” model of the atom?
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Thomson.
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What is the “plum pudding” model of the atom?
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Positive charge is distributed throughout (like pudding) and negative charges are embedded within (like plums). You can also think of this as a spherical chocolate chip cookie: cookie = positive charge, chocolate chips = negative charges (electrons)
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What is an *alpha particle*?
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A helium nucleus, made of 2 protons and 2 neutrons.
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Who came up with the idea of a nucleus and the “planetary” model of the atom?
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Rutherford.
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What did Rutherford do in his famous experiment?
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He shot a alpha particles at thin, gold foil.
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What did Rutherford observe in his experiment?
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That most of the alpha particles passed through the gold foil undeflected, but that a few bounced off.
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What did Rutherford conclude from his observation that *most of the alpha particles passed through the gold foil undeflected*?
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That an atom is mostly made of empty space.
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What did Rutherford conclude from his observation that *a few of the alpha particles bounced off the gold foil*?
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That an atom has a dense, positively charged part. He called this part the nucleus.
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What was the significance of Rutherford’s gold foil (alpha scattering) experiment?
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Led to the idea of the nucleus around which electrons travel.
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What is *planetary model* of the atom?
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Electrons orbit the nucleus.
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What is one major thing that Rutherford’s planetary model didn’t explain?
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The electrons should collapse into the nucleus, but they don’t. The electrons should collapse because opposite charges attract and because the electrons are losing energy as they travel around the nucleus.
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How is Bohr’s model of the atom different from Rutherford’s?
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The orbits in Bohr’s model are quantized – electrons travel in specific orbits that are set distances away from the nucleus. They don’t travel at any random distance.
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What did Bohr observe in his experiments that led to the idea of quantized orbits?
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He saw that hydrogen gas gave off line spectra instead of continuous spectra.
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What did Bohr think the lines in the spectrum he observed represented?
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Each line represented the energy that was given off when electrons changed from a higher energy orbit to a lower energy orbit.
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What is the *ground state*?
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When an electron occupies the lowest energy level (the orbit closest to the nucleus).
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What is the *excited state*?
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When an electron occupies any energy level above the first level (the ground state).
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When an electron jumps from *a higher to a lower* energy level, does it absorb or emit energy?
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Emit.
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When an electron jumps from *a lower to a higher* energy level, does it absorb or emit energy?
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Absorb.
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What happens when an electron absorbs more energy than required to jump to the highest energy level in the atom?
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The electron can escape the atom. Any excess energy is converted to kinetic energy.
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Can an electron jump across more than one energy level at a time?
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Yes.
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What is one major thing that Bohr’s orbital model didn’t explain?
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Electrons in the same energy level aren’t always the same distance away from the nucleus.
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What is the currently accepted model of the atom?
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The quantum mechanical model (electron cloud model).
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How are electron “orbits” pictured in the quantum mechanical model?
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The “orbits” are not well-defined. Instead they’re more like fuzzy clouds.
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What do the varying densities of an electron cloud represent?
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Higher density areas represent higher probabilities of finding an electron at that location.

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