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Average Atomic Mass. GPS Standards: SC3. Students will use the modern atomic theory to explain the characteristics of atoms. d. Explain the relationship of isotopes to the relative abundance of atoms of a particular element.
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Average Atomic Mass GPS Standards: SC3. Students will use the modern atomic theory to explain the characteristics of atoms. d. Explain the relationship of isotopes to the relative abundance of atoms of a particular element. SC2. Students will relate how the Law of Conservation of Matter is used to determine chemical composition in compounds and chemical reactions. c. Apply concepts of the mole and Avogadro’s number to conceptualize and calculate mass, moles and molecules relationships. 8/12/10
Part I: Average Atomic Mass • atomic mass (or weight) = weighted average of all the mass numbers of all the isotopes of an element. Not written on symbol, but is found on PT. • atomic mass is calculated by multiplying the mass number of each isotope by its abundance in the environment (a percentage). • Ex: hydrogen has three isotopes… Carbon 6 C 12.01 NOTE: numbers given as percentages must be convertedbefore they can be entered into the calculator. Move the decimal 2 places to the left, then enter the number.
Ex: hydrogen has three isotopes… 1. protium (1 p+, 0 n0) @ 99.985% 1.0073 amu x 0.99985 = 1.007148905 amu 2. deuterium (1 p+, 1 n0) @ 0.0149% 2.0160 amu x 0.000149 = 0.000300384 amu 3. tritium (1 p+, 2 n0) @ 0.0001% 3.0247 amu x 0.000001 = 0.0000030247 amu 1.007452314 amu ≈ 1.01 amu • let’s try another one….. Hydrogen 1 H 1.01
39.5524614 • 15.64193593 • 0.688520967 • 2.223227621 • 0.581744509 • 58.69 Nickel 28 Ni 58.69 • atomic mass tells you two very important things about an element:
atomic mass tells you two very important things about an element: 1. the mass of one atom of an element, measured in amu. • hydrogen = 1.01 amu per atom • carbon = 12.01 amu per atom • gold = 196.97 amu per atom
2. the mass of one mole of an element, measured in grams. • hydrogen = 1.01 grams per mole • carbon = 12.01 grams per mole • gold = 196.97 grams per mole • So what is a mole? Remember that molecules and atoms are extremely small units of matter. A small drop of water contains approx. 1.7 x 1022 molecules of H2O!
Part II: The Mole Concept • So what is a mole? Remember that molecules and atoms are extremely small units of matter. A small drop of water contains approx. 1.7 x 1022 molecules of H2O! • since extremely large numbers of molecules are a bit cumbersome to work with (meaning calculate with), scientists simplify this process by using a counting unit called a mole. • counting units are often used in everyday life. Examples: 1 dozen = 12, a pair = 2, a gross = 144, 1 ream = 500. • 1 mole = 6.022 x 1023or … 602,200,000,000,000,000,000,000 This number is known as Avogadro’s number.
1 mole = 6.022 x 1023or … 602,200,000,000,000,000,000,000 This number is known as Avogadro’s number. • Avogadro was a scientist who figured out a way to tie the weight of individual atoms to the weight of their compounds in grams. • mole = a counting unit representing the number of particles (6.022 x 1023) it takes for a substance’s atomic weight to equal it’s weight in grams. • why tie these two quantities together? Grams (a measure of weight) are way easier to work with! Weighing out a few grams of a compound is much easier to do than counting out individual atoms or molecules! • just to drive home the point:
just to drive home the point: • 1 mole of dollar bills, stacked, would reach 3,800,000,000,000,000 miles (3.8 x 1016) into outer space! This distance is like going to the sun and back 160,000,000 times! • 1 mole of regular M&Ms would cover the 48 contiguous United States to a depth of 17,000 feet (3.2 miles) thick! • 1 mole of sand grains would be more than all the grains of sand found on Miami Beach (which is several miles long and quite wide as well)! • 1 mole of blood cells is more than all the blood cells found in all the humans alive on the earth (over 6 billion people)!
1 mole of blood cells is more than all the blood cells found in all the humans alive on the earth (over 6 billion people)! • 1 mole of any object, counted at 1 object per second, would take 1 x 1019 years to count out! That’s older than the universe itself, which is approx. 15 billion (1.5 x 109) years old!
