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# Chapter 3.3 : Counting Atoms

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### Chapter 3.3 : Counting Atoms

1. 1. Counting Atoms<br />Chapter 3.3<br />
2. 2. Objectives<br />Explain what isotopes are<br />Define atomic number and mass number, and describe how they apply to isotopes.<br />Given the identity of a nuclide, determine its number of protons, neutrons, and electrons.<br />Define mole in terms of Avogadro’s number, and define molar mass.<br />Solve problems involving mass in grams, amount in moles, and number of atoms of an element<br />
3. 3. Atomic Number<br />Symbol – Z<br />Number of protons in nucleus of atom<br />Determines the identity of the atom!!!!!<br />C - 6<br />He - ___<br />F - ___<br />Pb - ___<br />Use periodic table to find these!<br />Since atoms are neutral atomic number is also number of electrons<br />So C also has 6 electrons<br />He - ____ electrons<br />
4. 4. Isotopes<br />All Hydrogen atoms contain 1 proton<br />Many naturally occurring elements can contain different numbers of neutrons.<br />Isotope – at0ms of the same element that have different masses.<br />n<br />n<br />n<br />Protium<br />99.985 %<br />Deuterium<br /> 0.015 % <br />Tritium<br />Trace<br />Three isotopes of Hydrogen<br />
5. 5. Mass Number<br />Mass number – total number of protons and neutrons in the nucleus of an isotope<br />Symbol – A<br />Protium has 1 proton and 0 neutrons <br />Mass number = #p + #n<br /> = 1 + 0<br /> = 1<br /><ul><li>What is the mass number deuterium and tritium?
6. 6. Find on periodic table</li></li></ul><li>Designating Isotopes<br />Hyphen Notation <br />Name of atom – mass number<br />Uranium - 235<br /><ul><li>Nuclear symbol
7. 7. A X A : mass number, Z : atomic number, X : symbol of element
8. 8. 235U</li></ul>Z<br />92<br /><ul><li>Number of protons Neutrons Electrons</li></ul> 92 143 92<br /><ul><li>Nuclide – general term for any isotope of any element</li></li></ul><li>Relative Atomic Mass<br />Oxygen – 16 , mass of 2.657 x 10-23 g<br />Easier to use relative scale<br />Need arbitrarily chosen standard<br />All others are compared to carbon - 12<br /><ul><li>One atomic mass unit (1 amu) – exactly 1/12 the mass of a carbon – 12 atom.
9. 9. All others are compared to carbon -12
10. 10. Hydrogen – 1 , about 1/12 the mass of carbon -12
11. 11. Precise value is 1.007825 amu
12. 12. Magnesium – 24 , Slightly less than twice that of carbon -12
13. 13. Precise value is 23.985042 amu</li></li></ul><li>Average Atomic Mass of Elements<br />Most elements occur naturally as mixtures of isotopes<br />Average atomic mass : weighted average of the atomic masses of the naturally occurring isotopes of an element<br />Ex. of weighted average<br />Box contains two types of marbles, 25% have mass of 2.00g, and 75% have a mass of 3.00 g.<br />25 marbles x 2.00 g = 50 g<br /> 75 marbles x 3.oo g = 225 g<br />so 50 g + 225 g = 275 g (TOTAL MASS)<br /> 275 g ÷ 100 = 2.75 g average marble mass <br />
14. 14. Calculating average atomic mass<br />Or<br />(2.00 g x 0.25) + (3.00 g x .75) = 2.75 g<br /><ul><li>Calculate the average atomic mass of copper
15. 15. Use table 3-4 pg. 80 in text
16. 16. Copper - 63 69.17% with mass of 62.929599 amu</li></ul> Copper – 65 30.83% with mass of 64.927793 amu<br />(0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu)<br />=63.55<br />Match with atomic mass on periodic table <br />Round to two decimal places in calculations<br />
17. 17. Relating Mass to Numbers of Atoms<br />The MOLE<br />SI base unit for amount of a substance<br />Abbreviated as mol<br />Defined as : amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon – 12<br />Counting unit just like a dozen<br />We don’t order 12 or 24 eggs, we order 1 or 2 dozen<br />In same way, a chemist may want 1 mol of carbon or 2 mol of iron.<br />63.55 g of copper or 1 mol of copper<br />
18. 18. Avogadro’s Number<br />The number of particles in a mole – AVOGADRO’S NUMBER<br />6.022 x 1023particles in exactly one mole of a pure substance<br /><ul><li>How big is this number?
19. 19. If every person on earth (5 billion people) counted 1 atom per second, it would take 4 million years to count all the atoms.</li></li></ul><li>Molar Mass<br />Molar mass – mass of one mole of a pure substance<br />Units : g/mol<br />Numerically equal to atomic mass of element<br />Mass of 6.02 x 1023 atoms of element<br />One mole He 6.02 x 1023atoms 4.oo g<br />One mole Li 6.02 x 1023atoms6.94 g<br />One mole Cu 6.02 x 1023atoms63.5o g<br />One mole Fe 6.02 x 1023 atoms 55.78 g<br />
20. 20. Gram/Mole Conversions<br />Mass <br />In grams<br />Moles <br />In mol<br />Atoms <br />In atoms<br />Avogadro’s Number<br />Molar mass<br />What is the mass in grams of 3.50 mol of the element copper, Cu?<br />A chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced? <br />How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver?<br />What is the mass in grams of 1.20 x 1023 atoms of copper, Cu?<br />
21. 21. The Ten Commolements<br />(found at http://members.tripod.com/~smalls42/commolements.html)<br />Thou shalt not use 6.02 x 1023 in vain. <br />Thou shalt not use the term mole if thou has no true knowledge of the term mole. <br />Thou shalt not kill a mole. <br />Thou shalt not covet your neighbor's mole. <br />Thou shall always remember to celebrate Mole Day. <br />Thou shalt not disparage Mole Day. <br />Thou shalt not use a mole out of season. <br />Thou shalt always honor the one who introduced thou to Mole Day. <br />You shalt always keep sacred 10/23. <br />Thou shalt always remember these commolments or thou will never properly celebrate Mole Day. <br />