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Learner Resource 1: Atomic nomenclature xA z The number x is often called the mass number and gives an indication of the relative mass of the atom. It can also be called the nucleon number because it is equal to the number of neutrons and protons in the nucleus of the atom. The word nucleon covers both neutrons and protons. The number z is the atomic number or proton number and is equal to the number of protons in the nucleus. It is also equal to the number of electrons in the neutral atom. The letter A represents the atomic symbol and this is unique for each element and it is determined by the number of protons in the nucleus (and therefore the number of electrons in the neutral atom). We can find the number of neutrons by subtracting the proton number (number of protons) from the nucleon number (number of protons + neutrons). n=z–x So, 56 26 e.g. Fe This has 26 protons and 56 nucleons (neutrons and protons) so it contains 56 – 26 = 30 neutrons. Fill in the number of nucleons, protons and neutrons in the following atoms. 45 20 Ca nucleons = protons = neutrons = Ti protons = neutrons = protons = neutrons = 47 22 17 8 O nucleons = nucleons = Version 1 Nuclear Particle Physics 1 © OCR 2015 Isotopes Isotopes contain the same number of protons (so they have the same atomic number) but a different number of neutrons (so they have a different mass / nucleon number). e.g. The following are two common isotopes of Chlorine. 35 17 Cl This has 17 protons, 17 electrons and 35-17 = 18 neutrons 37 17 Cl This has 17 protons, 17 electrons and 37-17 = 20 neutrons These two isotopes occur in the ratio of 3 to 1 and so the RMM (relative molecular mass) of Chlorine is 35.5. Questions Work out the number of protons, electrons and neutrons for the following isotopes. 234 92 U 235 92 U 238 92 U Explain which of these isotopes do you think will be the least stable? Version 1 Nuclear Particle Physics 2 © OCR 2015 OCR Resources: the small print OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board, and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. © OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work. OCR acknowledges the use of the following content: Please get in touch if you want to discuss the accessibility of resources we offer to support delivery of our qualifications: [email protected] Version 1 Nuclear Particle Physics 3 © OCR 2015