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Transcript
5/6 Oct 2015 Agenda Objectives: Atomic number Mass number Isotopes - Radioisotopes Half-life Slip Quiz Return Isotopes and Av. Atomic Mass POGILS Analysis Activity - need own paper and a ruler Radioactivity and Nuclear Chemistry introduction Homework Slip Quiz 1.How many neutrons in an atom of neon-21 (21Ne). State how you know. 2. How many electrons in a neon-21 atom? How do you know? Slip Quiz 1.How many neutrons in an atom of neon-21 (21Ne). State how you know. I looked up neon’s atomic number on periodic table, neon has atomic number 10. Therefore a neon-21 atom has 10 protons in its nucleus and the remainder of the particles in the nucleus are neutrons. Mass number - atomic number = number of neutrons 21 - 10 = 11, 11 neutrons Slip Quiz 2. How many electrons in a neon-21 atom? How do you know? I know that for an electrically neutral atom, the number of protons in the nucleus is equal to the number of electrons in the electron cloud around the nucleus. For question 1 I found the atomic number of neon-21 is 10, hence the atoms have 10 protons and must have 10 electrons. Turn in Av. Atomic Mass Pogil Per 3 Return Isotopes Pogil Per 2 and 3 Return Av. Atomic Mass Pogil Per2 Share Qu. 17. Boron has two naturally occurring isotopes: boron-10 and boron-11. Which isotope is more abundant on Earth? The expected mass of boron-10 would be close to 10amu, while the expected mass of boron-11 would be close to 11amu. On the periodic table we see that the average atomic mass of boron is 10.811 amu, which is numerically closer to 11 than it is to 10. Qu. 17. Boron has two naturally occurring isotopes: boron-10 and boron-11. Which isotope is more abundant on Earth? (Explain). The average atomic mass value for boron reported on the periodic table is a weighted average of all the naturally occurring isotopes of boron and so it will be closest to the mass of the most abundant isotope. I can conclude that boron-11 is the more abundant isotope on Earth. Notes Atomic Mass Units (amu) The masses of protons, neutrons and electrons in g are very small and difficult to work with. Chemists developed a method of measuring the mass of an atom relative to the mass of a specifically chosen atomic standard. Atomic Mass Units (amu) …atomic standard Carbon-12 atom One carbon-12 atom was assigned a mass of exactly 12 atomic mass units (amu). One atomic mass unit (amu) is defined as th 1/12 (one twelfth) of the mass of a carbon-12 atom. 1 amu is close to the mass of a single proton or single neutron. See p 102 Check you have on your “Periodic Table of The Elements” Add to your “Periodic Table of The Elements” • Atomic number is the number of protons in the nucleus of each atom of the element • unique name for element All atoms are neutral have no net charge Number of protons (+) = number of electrons (-) np + (ne-) = 0 Atomic Number, Mass Number and relationships to number of protons, neutrons and electrons On a new piece of paper that can be turned in when you have completed the exercise MLA heading Title Centered on top line Spacious table, lines drawn using straight edge Isotopes and Atomic Structure Isotope Atomic Mass Number number Hydrogen-1 1 1 fluorine-19 19 9 Number protons Number of electrons number of neutrons neon-20 10 10 sodium-23 11 12 aluminum27 13 cobalt-___ 27 krypton-__ 36 uranium-__ 92 32 48 146 Neutron/ proton ratio Section 4.4 Unstable Nuclei and Radioactive Decay Main Idea Radioactivity Use with pages 105-106 Nuclei is plural of nucleus Details • In chemical reactions, atoms may be rearranged, but their identities do not change. The rearrangement affects only the electrons of the atoms, not the nuclei. Nuclear reactions are different. In nuclear reactions, unstable nuclei gain stability by emitting radiation. Section 4.4 Unstable Nuclei and Radioactive Decay (continued) Main Idea Radioactivity Use with pages 105-106 Alchemists dream come true – changing atoms into other atoms! Details • . As a result of changes in the nuclei, the atom’s identities change. Section 4.4 Unstable Nuclei and Radioactive Decay (continued) Main Idea Radioactivity Use with pages 105-106 Alchemists dream come true – changing atoms into other atoms! Unfortunately usually lead not gold! Details • As a result of changes in the nuclei, the atom’s identities change. • Unstable nuclei will continue emitting radiation in a process called radioactive decay, until stable nuclei, often of a different element, are formed. Section 4.4 Unstable Nuclei and Radioactive Decay (continued) Main Idea Radioactivity Use with pages 105-106 Details The steps of a nuclear reaction: 1. An atom has an unstable nucleus. 2. Radiation is emitted. 3. Process continues. 4. A stable non-radioactive atom is formed (usually a new element). 55 Section 4.4 Unstable Nuclei and Radioactive Decay (continued) Main Idea Radioactivity Use with pages 105-106 Neutron: proton ratio, close to 1 is associated with stable nuclei for light elements, close to 1.5 for heavier elements Details Discuss why some elements are radioactive while most elements are not. Atoms are unstable when there are too many or too few neutrons in their nuclei. They undergo radioactive decay until the nucleus becomes stable. Most atoms have already decayed into stable atoms. Section 25.1 Nuclear Radiation Types of Radiation 3 Common types of radiation 4 4α alpha He helium nucleus 2 Mass no Charge 2 0 -1 β beta γ gamma 0 0 0 e -1 particle (ray) Section 25.2 Radioactive Decay Nuclear Stability The strong nuclear force acts on protons and neutrons that are very close together in a nucleus to overcome electrostatic repulsion between the protons. Section 25.2 Radioactive Decay Nuclear Stability Neutron-to-proton (n/p) ratio and nuclear stability. To some degree, the stability of a nucleus can be correlated with its neutron-to-proton ratio. For light atoms 1/1 ratio, for heavier atoms closer ot 1.5/1 Section 25.2 Radioactive Decay Nuclear Stability As atomic number increases, more neutrons are needed to produce the strong nuclear force to balance the electrostatic repulsion forces between the protons. Plotting the number of neutrons versus the number of protons for all stable nuclei illustrates the band of stability. The Band of Stability Ratio gradually increases to maximum of 1.5: 1 for largest atoms For atoms with low atomic number (<20) 1:1 n/p ratio The Discovery of Radioactivity • • • • • 1895 Roentgen 1895 Becquerel 1898 The Curies (Marie and Pierre) 1903 The Curies and Becquerel 1911 Marie Curie Today we call isotopes that are radioactive, radioisotopes. Types of Radiation – Penetrating Power Gamma rays are very dangerous because they penetrate living tissue Skin can stop alpha radiation Beta radiation – usually only penetrates 1-2cm beneath the skin Alpha, beta and gamma are “ionizing radiation” – they have enough energy to break bonds in molecules which ionizes them, which makes them unstable, and very reactive inside the organism Find out more at Ionizing vs. nonionizing radiation at the EPA website. Properties of Alpha, Beta and Gamma Radiation Radiation type Charge Mass Alpha (α ) 2+ 6.64 x 10-24kg Beta (β) 1- 9.11 x 10-28kg Gamma (γ) 0 0 (photons) Relative penetrating power Blocked by paper (least penetrating) Blocked by thin metal sheets (foil) Not completely blocked by lead or concrete (most penetrating) Section 25.3 Transmutation Vocabulary Half-life the time required for one half of a radio-isotope’s nuclei to decay into its products Radiochemical dating – process of determining the age of an object by measuring the amount of certain radioisotopes remaining in the object Section 25.3 Transmutation Half-life demonstration (model) 10 pennies – heads up Represent 10g of a radioisotope – all radioactive (unstable nuclei) Section 25.3 Transmutation Half-life demonstration Put all pennies in cup Shake carefully and tip pennies out onto the table-top. Section 25.3 Transmutation Half-life demonstration Record the number of heads left Put the tails in a separate pile, these represent nuclei that have decayed and are now stable Heads are still radioactive Section 25.3 Transmutation Half-life demonstration Carry on until no radioactive (heads up) Trial Number of pennies pennies remaining Number remaining 0 Compile class results 1 2 3 4 5 6 7 8 9 10 10