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Mass of individual atoms Lesson 1 – introduction to project and atomic structure.

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1 Mass of individual atoms Lesson 1 – introduction to project and atomic structure

2 Atomic particleReal mass (g)Relative mass (amu) Proton p x Neutron n x Electron e x

3  The mass of an atom is measured relative to the mass of a specifically chosen atom:  Carbon-12  1 atomic mass unit (amu) = 1/12 th the mass of Carbon-12  1 amu is very close to the mass of one p + or n 0

4 So amu is… the unit of an atom’s mass

5 That’s because…… ……………’s an AVERAGE!



8  (mass x % abundance) + (mass x % abundance) = (35 amu x 0.75)+ (37 amu x 0.25) = amu amu = 35.5 amu Isotopes of Chlorine Mass (amu)Percentage abundance Cl % (75/100 = 0.75) Cl % (25/100 = 0.25)

9 Mg Mass number Atomic number Magnesium-24 written in symbol notation is


11 Nuclear Changes in the atom

12  Chemical reaction: involves electrons, not the nucleus.  Element doesn’t change.  Nuclear reaction: involves the nucleus.  Element changes.

13  Some substances emit particles or rays.  These particles are called radiation.  Radioactivity is the release of these particles

14  Atoms emit radiation when their nucleus is unstable.  Stability is determined by ratio of neutron to protons. Too many or too few neutrons makes an atom unstable.  Spontaneously emitting radiation is radioactive decay

15 Why do radioactive atoms change from one element to another?

16  Alpha (α) or He radiation  the α-particle is the same as the He nucleus  Beta radiation β  β particles are fast moving electrons  Gamma radiation γ  γ rays are high energy radiation  Have no mass or charge  γ rays often emitted during α or β decay


18  Half-life: time taken for ½ the radioactive nuclei to decay into their stable products.  During each half-life, the proportion of parent atoms decreases by ½

19  Measures rate of radioactive decay  Half-life: Time taken for half the radioactive nuclei to decay into their stable products. Mass of Kanorium-136 (g) Time (years)

20  If I have 10g of strontium 90 today,  in 29 years I will have half i.e. 5g  After another 29 years, 2.50 g remains  After another 29 years, 1.25 g remains  After another 29 years, g remains  Decay continues till almost nothing is left  Amount remaining = (initial amount)(1/2) n  n = number of half-lives that have passed.


22  Radioactivity is a powerful tool to measure absolute ages of rocks, past geologic events and  HOW?!?  If something has radioactive material in it. Depending on how much has broken down, we can figure out how old it is.

23  The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable Parents Daughters Half-Life (years) Uranium 238 Lead billion Uranium 234 Lead million Thorium 232 Lead billion Rubidium 87 Strontium billion Potassium 40 Argon billion

24 25 parents 75 daughters Igneous rock Assume: * daughters only produced by decay of parents (no daughters to begin with). * original rock had 100 parents. TODAY 100 parents Orignal Rock 50 parents,50 daughters Rock at some stage 1 half-life 25 parents, 75 daughters Rock Today Another Half-life Rock has experienced decay for two half-lives. How old is that?? If we had been using the Potassium-40 to Argon-40 dating system, the half-life of potassium-40 is 1.3 billion years. In this case, the rock is 2 half-lives x 1.3 b.y./half-life = 2.6 b.y.

25 14 C constantly produced in atmosphere, producing constant 14 C/ 12 C ratio. When organism dies, 14 C/ 12 C begins to decrease due to decay of 14 C. ( 14 C is radioactive, 12 C is stable) Plants and animals incorporate carbon of this constant ratio 14 C has a half life of 5730 years, useful for dating things that are 50,000 years

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