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

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

Atomic particleReal mass (g)Relative mass (amu) Proton p + 1.673 x 10 -24 1 Neutron n 0 1.675 x 10 -24 1 Electron e - 9.109 x 10 -28 0

 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

So amu is… the unit of an atom’s mass

That’s because…… ……………..it’s an AVERAGE!

 (mass x % abundance) + (mass x % abundance) = (35 amu x 0.75)+ (37 amu x 0.25) = 26.25 amu + 9.25 amu = 35.5 amu Isotopes of Chlorine Mass (amu)Percentage abundance Cl-353575% (75/100 = 0.75) Cl-373725% (25/100 = 0.25)

Mg 12 24 Mass number Atomic number Magnesium-24 written in symbol notation is

Nuclear Changes in the atom

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

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

 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

Why do radioactive atoms change from one element to another?

 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 4242 0 0

 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

 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)

 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, 0.625 g remains  Decay continues till almost nothing is left  Amount remaining = (initial amount)(1/2) n  n = number of half-lives that have passed.

 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.

 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 206 4.5 billion Uranium 234 Lead 207704 million Thorium 232 Lead 20814 billion Rubidium 87 Strontium 8748.8 billion Potassium 40 Argon 401.3 billion

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.

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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