Transcript File
Isotopes & Atomic Mass Noadswood Science, 2012 Wednesday, April 6, 2016 Isotopes & Atomic Mass To understand isotopes and atomic mass 1 proton; 0 neutrons; 1 electron 1 proton; 1 neutron; 1 electron 1 proton; 2neutrons; 1 electron The Periodic Table Periodic table shows all the known elements (with spaces left for undiscovered ones) It has vertical groups (I – VIII) with each group containing elements which have similar properties It has horizontal periods with each new period representing another full shell of electrons Metals are to the left of the step, with non-metals to the right of it It also shows some key information about an element’s atomic mass and atomic number… Atomic Mass & Number Atomic mass Number of protons + neutrons Atomic number Number of protons (therefore number of electrons too in a neutral atom) 6 protons 6 neutrons 6 electrons Atomic Mass & Number How many protons, neutrons and electrons are there in the following… 1 1 H 1 protons 0 neutrons 1 electrons 23 11 Na 11 protons 12 neutrons 11 electrons 11 5 B 5 protons 6 neutrons 5 electrons 35 17 Cl 17 protons 18 neutrons 17 electrons 16 8 O 8 protons 8 neutrons 8 electrons 238 92 U 92 protons 146 neutrons 92 electrons Nucleus Isotopes An atom is made from a nucleus surrounded by electrons – the nucleus contains protons and neutrons Isotopes are atoms that have the same number of protons, but different numbers of neutrons – the nuclei of some isotopes are unstable, emitting radiation and breaking down to form smaller nuclei… Isotopes Isotopes are the atoms of an element with different numbers of neutrons – they have the same proton number, but different mass numbers… Look at the isotopes of hydrogen: - 1 proton; 0 neutrons; 1 electron 1 proton; 1 neutron; 1 electron 1 proton; 2neutrons; 1 electron Radioactive The nuclei of some isotopes are unstable – they can split up or ‘decay’ and release radiation Such isotopes are called radioactive isotopes or radioisotopes When a radioactive isotope decays, it forms a different atom with a different number of protons Carbon Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14 Carbon Dating What is carbon dating, and how does it work? Carbon Dating Carbon-14 is created at a constant rate in the upper atmosphere by cosmic rays acting on nitrogen The carbon-14 which is formed is radioactive and decays producing nitrogen again – there is therefore a fixed amount of carbon-14 in the environment which is a balance between the rate at which it is formed and the rate at which it decays All living things take carbon into themselves: plants take in carbon during photosynthesis; and animals take in carbon when they eat their food All living things therefore have carbon-14 in them at the same amount which is present in the environment – this amount is small (1 in 850 billion are carbon-14, the rest are mainly carbon-12 which are not radioactive) Carbon Dating When a living thing dies, it stops taking in carbon from its environment., so he amount of carbon-14 in it will start to decrease as the carbon-14 slowly decays The further back in time that something died, the less carbon-14 it will have – measuring the amount of carbon-14 can tell you how long ago the thing died and therefore the "age" of the sample Carbon Dating – Limitations Carbon dating can be used on anything which used to be alive, e.g. animal substances (remains including skin, fur and bone); and plant substances (remains including wood, seeds, pollen, cloth, rope etc…) Some fossils can be dated this way if they still contain some of the original carbon of the plant or animal Carbon dating cannot be used on things which have never lived or for substances older than around 50’000 years (as the carbon-14 sample is very small after 9/10 half-lives) The method of carbon dating uses an assumption that the amount of carbon-14 present in the past is the same as that present in the environment today