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Counting Atoms

Chapter 3.3. Counting Atoms. Objectives. Explain what isotopes are Define atomic number and mass number, and describe how they apply to isotopes. Given the identity of a nuclide, determine its number of protons, neutrons, and electrons.

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Counting Atoms

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  1. Chapter 3.3 Counting Atoms

  2. Objectives • Explain what isotopes are • Define atomic number and mass number, and describe how they apply to isotopes. • Given the identity of a nuclide, determine its number of protons, neutrons, and electrons. • Define mole in terms of Avogadro’s number, and define molar mass. • Solve problems involving mass in grams, amount in moles, and number of atoms of an element

  3. Atomic Number • Symbol – Z • Number of protons in nucleus of atom • Determines the identity of the atom!!!!! • C - 6 • He - ___ • F - ___ • Pb - ___ • Use periodic table to find these! • Since atoms are neutral atomic number is also number of electrons • So C also has 6 electrons • He - ____ electrons 2 9 82 2

  4. Isotopes • All Hydrogen atoms contain 1 proton • Many naturally occurring elements can contain different numbers of neutrons. • Isotope – at0ms of the same element that have different masses. n n n Protium 99.985 % Deuterium 0.015 % Tritium Trace Three isotopes of Hydrogen

  5. Mass Number • Mass number – total number of protons and neutrons in the nucleus of an isotope • Symbol – A • Protium has 1 proton and 0 neutrons • Mass number = #p + #n • = 1 + 0 • = 1 • What is the mass number deuterium and tritium? • Find on periodic table

  6. Designating Isotopes • Hyphen Notation • Name of atom – mass number • Uranium - 235 • Nuclear symbol • A X A : mass number, Z : atomic number, X : symbol of element • 235U Z 92 • Number of protons Neutrons Electrons 92 143 92 • Nuclide – general term for any isotope of any element

  7. Sample Problem A • How many protons, electrons, and neutrons are there in an atom of chlorine-37? • atomic number = number of protons = number of electrons • mass number = number of neutrons + number of protons • # protons • # electrons • #neutrons Atomic number from periodic table 17 17 # protons = # electrons 20 Mass # = # protons + # electrons

  8. Practice Pg. 80 • How many protons, electrons, and neutrons make up an atom of bromine-80? • Write the nuclear symbol for carbon-13. • Write the hyphen notation for the isotope with 15 electrons, and 15 neutrons. Answer : 35 protons, 35 electrons, 45 neutrons Answer : 13 C 6 Answer : phosphorus – 30

  9. Relative Atomic Mass • Oxygen – 16 , mass of 2.657 x 10-23 g • Easier to use relative scale • Need arbitrarily chosen standard • All others are compared to carbon - 12 • One atomic mass unit (1 amu) – exactly 1/12 the mass of a carbon – 12 atom. • All others are compared to carbon -12 • Hydrogen – 1 , about 1/12 the mass of carbon -12 • Precise value is 1.007825 amu • Magnesium – 24 , Slightly less than twice that of carbon -12 • Precise value is 23.985042 amu

  10. Average Atomic Mass of Elements • Most elements occur naturally as mixtures of isotopes • Average atomic mass : weighted average of the atomic masses of the naturally occurring isotopes of an element • Ex. of weighted average • Box contains two types of marbles, 25% have mass of 2.00g, and 75% have a mass of 3.00 g. • 25 marbles x 2.00 g = 50 g 75 marbles x 3.oo g = 225 g so 50 g + 225 g = 275 g (TOTAL MASS) 275 g ÷ 100 = 2.75 g average marble mass

  11. Calculating average atomic mass • Calculate the average atomic mass of copper • Use table 3-4 pg. 80 in text • Copper - 63 69.17% with mass of 62.929599 amu • Copper – 65 30.83% with mass of 64.927793 amu • (0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu) • =63.55 • Match with atomic mass on periodic table • Or • (2.00 g x 0.25) + (3.00 g x .75) = 2.75 g Round to two decimal places in calculations Visual Concept – Click Here

  12. Relating Mass to Numbers of Atoms • The MOLE • SI base unit for amount of a substance • Abbreviated as mol • Defined as : amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon – 12 • Counting unit just like a dozen • We don’t order 12 or 24 eggs, we order 1 or 2 dozen • In same way, a chemist may want 1 mol of carbon or 2 mol of iron. 63.55 g of copper or 1 mol of copper Visual Concept – Click Here

  13. Avogadro’s Number • The number of particles in a mole – AVOGADRO’S NUMBER • 6.022 x 1023particles in exactly one mole of a pure substance • How big is this number? • If every person on earth (5 billion people) counted 1 atom per second, it would take 4 million years to count all the atoms. Visual Concept – Click Here

  14. Molar Mass • Molar mass – mass of one mole of a pure substance • Units : g/mol • Numerically equal to atomic mass of element • Mass of 6.02 x 1023 atoms of element • One mole He 6.02 x 1023 atoms 4.oo g • One mole Li 6.02 x 1023 atoms 6.94 g • One mole Cu 6.02 x 1023 atoms 63.5o g • One mole Fe 6.02 x 1023 atoms 55.78 g Visual Concept – Click Here

  15. Gram/Mole Conversions Mass In grams Moles In mol Atoms In atoms Avogadro’s Number Molar mass Sample Problem B What is the mass in grams of 3.50 mol of the element copper, Cu? 63.55 g Cu x 3.30 mol Cu 222 g Cu = 1 mol Cu Step 1: Step 2: Step 3: Step 4: Use molar mass from periodic table Write the given value Cancel out units Calculate Always round to 2 places after the decimal

  16. Sample Problem C A chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced? 1 mol Al 11.9 g Al 0.441 mol Al x = 26.98 g Al Sample Problem D How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver? 1 mol Ag = x 3.01 x 1023 atoms Ag 0.500 mol Ag 6.02 x 1023 atoms Ag Avogadro’s number

  17. Sample Problem E What is the mass in grams of 1.20 x 108atoms of copper, Cu? 63.55 g Cu 1 mol Cu x x 1.20 x 108atoms Cu 1 mol Cu 6.02 x 1023 atoms Cu 1.27 x 10-14 g Cu = Avogadro’s # ****Requires 2 conversions Molar mass

  18. The Ten Commolements (found at http://members.tripod.com/~smalls42/commolements.html) • Thou shalt not use 6.02 x 1023 in vain. • Thou shalt not use the term mole if thou has no true knowledge of the term mole. • Thou shalt not kill a mole. • Thou shalt not covet your neighbor's mole. • Thou shall always remember to celebrate Mole Day. • Thou shalt not disparage Mole Day. • Thou shalt not use a mole out of season. • Thou shalt always honor the one who introduced thou to Mole Day. • You shalt always keep sacred 10/23. • Thou shalt always remember these commolments or thou will never properly celebrate Mole Day.

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