The Lion and the Gazelle

1. The Lion and the Gazelle Every morning in Africa, a gazelle wakes up. It knows it must run faster than the fastest lion or it will be killed. Every morning a lion wakes up. It knows it must outrun the slowest gazelle or it will starve to death. It doesn't matter whether you are a lion or a gazelle. When the sun comes up, you better start running. -African Proverb

2. Section 3-3Counting Atoms Coach Kelsoe Chemistry Pages 77–87

3. Atomic Number • Atoms of different elements have different numbers of protons. • The atomic number of an element is the number of protons in the nucleus of each atom of that element.

4. Atomic Number • Atoms of hydrogen have 1 proton; atoms of helium have 2 protons, and so on. • The atomic number identifies an element. • Since atoms are neutral, an atom will have the same number of protons as electrons.

5. Isotopes • Isotopes are atoms of the same element that have different masses. • Isotopes will have the same number of protons and electrons, but will differ in the number of neutrons. • Tin has the most isotopes (10) • For example, Hydrogen has 3 isotopes: • Protium- one proton, no neutrons • Deuterium- one proton, one neutron • Tritium- one proton, two neutrons.

6. Mass Number • The mass number of an atom is the total number of protons and neutrons in the nucleus of an isotope.

7. Designating Isotopes • Only hydrogen has different names for its isotopes. • There are two ways we designate isotopes: • Hyphen notation: hydrogen-1, uranium-235 • Nuclear symbol: mass # top, atomic # 1H, 235U 1 92 • We find the number of neutrons by subtracting the atomic number from the mass number- Ex: 235U  235-92=143 neutrons 92

8. Sample Problem • How many protons, neutrons, and electrons are found in an atom of Helium-4? • Protons-2, Neutrons-2, Electrons-2 • How many protons, neutrons, and electrons are found in an atom of 120Sn? 50 • Protons-50, Neutrons-70, Electrons-50

9. Nuclides • Nuclide is a general term for any isotope of any element. • Nuclides for hydrogen are protium, deuterium, and tritium.

10. Relative Atomic Masses • Masses of atoms in grams are very small. • When comparing masses of atoms, we use atomic mass units. • An atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.

11. Average Atomic Masses • Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. • To calculate average atomic mass, we multiply the mass of each isotope by the decimal fraction representing its percentage in the mixture.

12. Sample Calculation • If we had 3 red styrofoam balls with a mass of 4.0 grams and 5 black styrofoam balls with a mass of 5.0 grams, what would it’s average mass be? • (3/8)(4.0 g)  (.375)(4) = 1.5 g • (5/8)(5.0 g)  (.625)(5) = 3.125 g • 1.5 g + 3.125 g = 4.625 g average

13. List how many protons, neutrons, and electrons are in each of the following atoms. Nickel-60 Barium-138 Krypton-86 Nitrogen-14 Bellringer • 31P • 201Hg • 42Ca • 184W 15 80 20 74

14. Relating Mass to Numbers of Atoms Mole • There are three important concepts that provide the basis for relating masses in grams to numbers of atoms: • The mole • Avogadro’s number • Molar mass Avogadro's number Molar mass

15. The Mole • The mole is the SI unit for amount of substance. • A mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12. • The mole is a counting unit, like dozens. Not a real mole!

16. Avogadro’s Number • The number of atoms in a mole has been determined to be 6.0221367 x 1023. • Avogadro’s number – 6.022 x 1023 – is the number of atoms in exactly one mole of a pure substance. Avogadro's number: 6.022 x 10 23

17. Molar Mass • The mass of one mole of a pure substance is called the molar mass of that substance. • Molar mass is written in g/mol and is numerically equal to the atomic mass of the element. • Even though the numbers are the same, molar mass and atomic mass aren’t exactly the same. Molar mass is measured in g/mol, atomic mass is measured in amu or g.

18. Gram/Mole Conversions • We can make a conversion factor to convert units from grams to moles and vice versa. For example: 4.00 mol He x 4.00 g He = 16 .0 g He 1 mol He

19. Sample Problems • What is the mass in grams of 2.85 mol of the element potassium, K? 2.85 mol K x 39.10 g K = 111 g K • What is the mass in grams of 5.10 mol of the element oxygen, O? 5.10 mol O x 16.00 g O = 81.6 g O 1 mol K 1 mol O

20. Sample Problems • How many moles of iron, Fe, are in 7.00 g of iron? 7.00 g Fe x 1 mol Fe = 0.125 mol Fe • How many moles of gold, Au, are in 234.0 g of gold? 234.0 g Au x 1 mol Au = 1.188 mol Au 55.85 g Fe 196.97 g Au

21. Conversions with Avogadro’s Number • We can make a conversion factor to convert units from moles to atoms and vice versa. • Here’s a GREAT conversion factor: - 12 National Championships + 5 NCAA probations x smelly cow college =

22. Sample Problems • How many moles of sulfur, S, are in 89700 atoms of sulfur? 89700 atoms x 1 mol S = • How many moles of sodium, Na, are in 6.2 x 104 atoms of sodium? 6.2 x 104 atoms x 1 mol Na = 6.022 x 1023 atoms 6.022 x 1023 atoms

23. Sample Problems • How many atoms of lead, Pb, are in 0.325 mol of lead? 0.325 mol x 6.022 x 1023 atoms = • How many atoms of fluorine, F, are in 0.018 mol of fluorine? 0.018 mol x 6.022 x 1023 atoms = 1 mol 1 mol