Physics – 4.4 Atomic Structure, 4.4.1 Atoms and Isotopes

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What is the structure of an atom?
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An atom consists of a densely-packed positively charged nucleus, made of protons and neutrons, surrounded by negatively charged electrons at different energy levels/ in different shells.
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What is the relative mass and charge of a proton?
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1, +1
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What is the relative mass and charge of an electron?
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0.0005 / negligible, -1
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What is the relative mass and charge of a neutron?
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1, 0
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What is the atomic number?
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The number of protons in an atom.
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What is the mass number?
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The total number of protons and neutrons in the nucleus.
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What is an isotope?
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The same element with a different mass number. Isotopes of an element have the same number of protons but a different number of neutrons.
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What is the radius of an atom?
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They have a radius of about 1 x 10(-10) metres.
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What size is the radius of a nucleus of an atom compared to the radius of the whole atom?
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About 1/10,000 of the size.
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Where is most of the mass of an atom concentrated?
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In the nucleus.
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Where is the mass and atomic number in relation to an element on the periodic table?
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They are both to the side of it. The mass number is at the top and the atomic number is at the bottom. (The mass number is always bigger).
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What could cause an electron to move energy level/shell?
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Absorption of electromagnetic radiation (more energy = further away from the nucleus) or emission of electromagnetic radiation (less energy = closer to the nucleus).
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What is the number of electrons always equal to in an atom?
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The number of protons in an atom.
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How do atoms turn into ions?
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They gain or lose one or more electrons.
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What were atoms thought to be like before the discovery of the electron?
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Tiny spheres that could not be divided.
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What did the discovery of the electron lead to?
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The plum pudding model.
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What is the plum pudding model?
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A model that suggests the atom is a ball of positive charge with negative electrons embedded in it.
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What is the Rutherford scattering experiment?
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An experiment to test the plum pudding model. A beam of alpha particles was directed at a one-atom-thick sheet of gold inside a vacuum.
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Why was the Rutherford scattering experiment done inside a vacuum?
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Because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.
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Why was gold foil used in the Rutherford scattering experiment?
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It was the only metal that could be rolled out to be so thin.
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What were the expected results of the Rutherford Scattering experiment and why?
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The alpha particles were expected to pass straight through the gold foil. This was because, according to the plum pudding theory, the gold atoms were neutral throughout, and so would neither repel or attract the positively-charged alpha particles.
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What were the results of the Rutherford Scattering experiment and why?
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Whilst most of the alpha particles did pass straight through the gold foil, a small number were deflected by unexpectedly large angles. This provided evidence that atoms have a small,dense, positively-charged nucleus, because: – the charge had to be strong enough to repel the very fast-moving alpha particles, – but small enough the most alpha-particles still passed straight through in the surrounding space.
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What did the results of the Rutherford Scattering experiment lead to?
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The nuclear model.
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What is the nuclear model?
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A model that suggests the atom has a small, dense, positively-charged nucleus around which electrons circulate.
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What happened after the nuclear model was established?
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– Bohr suggested electrons orbit nucleus at specific distances (adapted nuclear model) – Later experiments led to discovery of protons. – James Chadwick proved existence of neutrons in the nucleus.

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