Download - Atoms
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Atoms
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This is a diagram of a CD. The diameter of its hole is one-eighth of the diameter of the CD.
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Diagrams of atoms look very similar.
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But the nucleus at the centre of an atom is very much smaller.
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The diameter of the nucleus is around 100 000 times less than the diameter of the atom.
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This is equivalent to a CD with a normal-sized hole and an overall diameter of 1.5 km.
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Each atom has a nucleus at its centre, containing sub-atomic particles called protons and neutrons.
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Sub-atomic particles called electrons are arranged around the nucleus.
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What are the properties of these sub-atomic particles?
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Their masses are incredibly small.
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The proton and electron have incredibly small electric charges and the neutron has no charge.
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To make sense of the tiny masses, we use relative masses. The proton’s relative mass is 1.
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The electron is nearly 2000 times lighter than the proton and neutron.
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To make sense of the tiny charges, we use relative charges. The proton’s relative charge is +1.
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Protons and electrons have equal but opposite charges.
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The nuclear symbol for lithium is Li.73
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The bottom number is the atomic number or proton number, Z.
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The top number is the mass number, A.
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These numbers mean that this atom has 3 protons, and ...
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… the number of protons added to the number of neutrons is 7.
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So there are 7 − 3 = 4 neutrons.
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There are an equal number of protons and electrons in an atom, so there are 3 electrons.
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The three protons are found in the nucleus.
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The four neutrons are also found in the nucleus.
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The three electrons are arranged around the nucleus.
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Cl is an isotope of chlorine. It is often called chlorine-35.35 17
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Its mass number is 35.
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Its atomic number is 17.
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So there are 35 − 17 = 18 neutrons.
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Cl is another isotope of chlorine. It is often called chlorine-37.3717
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Its mass number is 37.
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Its atomic number is 17.
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So there are 37 − 17 = 20 neutrons.
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Both isotopes have 17 protons and 17 electrons but different numbers of neutrons.
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Isotopes of an element are chemically identical but have different physical properties.