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Transcript
13.2 THE NUCLEUS 1 - ATOMIC NUMBER, ATOMIC MASS AND ISOTOPES Atomic number. The atomic number (Z) of an element is the number of protons in the nucleus of one atom of that element. This is also the number of electrons orbiting the nucleus. Every element has its own atomic number. Hydrogen is the lightest element and has an atomic number of one. Hydrogen atoms have one proton in the nucleus and one electron in orbit. The second lightest element is the inert gas helium which has an atomic number of two (two protons and two electrons). For the element carbon, which is the basis of all living things, Z = 6 (six protons and six electrons) and for the heaviest naturally occurring element uranium, Z = 92. neutron proton electron Hydrogen (Z=1, A=1) Helium (Z=2, A=4) Lithium (Z=3, A=7) Carbon (Z=6, A=12) Simple models of four atoms (with their atomic numbers) Atomic mass. The atomic mass (A) of an element used to be defined as the mass of one atom of that element compared to the mass of one atom of hydrogen. On that standard, the atomic mass of hydrogen was exactly one, and the atoms of all other elements were compared to that. Now the standard has been adjusted slightly so that an atom of carbon Other terms and standards. Atomic mass, relative with 6 protons, 6 neutrons and 6 atomic mass and atomic weight are alternative terms that electrons, has an atomic mass of have been used in various circumstances and times. On exactly 12. For most purposes this the hydrogen standard, the atomic mass of hydrogen was makes very little difference (see the exactly 1. On the present carbon standard, the atomic text box on the right) so we will use mass of hydrogen is 1.008 so the difference is very small. the old standard in this book. Look at the simple models of the atoms shown above. Remember that protons and neutrons are equally heavy, and that the mass of the electrons is so tiny that we can ignore it. You can see that hydrogen has an atomic mass (A) of 1 (the nucleus contains one proton only), helium has an atomic mass of 4 (2 protons and 2 neutrons), lithium has an atomic mass of 7 (3 protons and 4 neutrons), and carbon has an atomic mass of 12 (6 protons and 6 neutrons). Now study Appendix B and check out the atomic numbers and atomic masses of your favourite elements. Isotopes. Isotopes are atoms of the same element that have different atomic masses; they have the same number of protons, but different numbers of neutrons. Look at the element chlorine in Appendix B. Its atomic number Z is 17 so all atoms of chlorine have 17 protons. About 75% of chlorine atoms have 18 neutrons (so the atomic mass A of these atoms is 35), while the rest have 20 neutrons (so A = 37). We say that chlorine has two isotopes, one with an atomic mass of 35, and the other with an atomic mass of 37. The atomic mass given in Appendix B is 35.5 which is the average mass of chlorine atoms as compared to a standard hydrogen atom. 99.99% 0.01% a trace Z=1, A=1 Z=1, A=2 Z=1, A=3 The three isotopes of hydrogen 1. Define (i) atomic number, (ii) atomic mass, (iii) isotopes. 2. What element has fewest neutrons in its atoms? 3. What elements have Z numbers: 1, 6, 8, 14, 79? Many elements have two or more isotopes, but one is usually much more common than the rest. The diagram on the left shows the three isotopes of hydrogen. 99.99% of all hydrogen atoms are the first isotope with one proton only in the nucleus (A = 1); 0.01% are the second isotope which has a neutron as well as a proton (A = 2); only a very few are the third isotope with two neutrons (A = 3). 13 - 2 4. What do you get if you subtract Z from A? 5. The element zinc has an atomic number of 30 and an atomic mass of 65.4. Explain all that you can work out about zinc atoms from this information.