Unformatted text preview: 10/2/11 Atomic Diversity: The Elements Each element has dis�nct physical and chemical proper�es caused by diﬀerences among atoms. All atoms are composed of electrons, protons and neutrons so elements diﬀer from one another because their atoms contain diﬀerent numbers of these subatomic par�cles. Mass number A Protons and neutrons in nucleus Elemental symbol X Atomic number Z Protons in nucleus Isotopes Isotopes The iden�ty of an atom is determined by its nuclear charge but its mass is the sum of the contribu�ons from all of its atomic building blocks. Thus, the mass of an atom depends on the number of protons and neutrons in its nucleus. Two atoms with the same number of protons but diﬀerent numbers of neutrons are called isotopes. Uranium-‐238 or 238U Atomic Mass The Mole An isotope is usually speciﬁed by giving its mass number. Some elements (F and P) occur naturally with just one isotope. It is diﬃcult to count atoms and molecules but we can weigh substances rou�nely. From the mass of a sample we can calculate the number of molecules it contains if we know the mass of an individual molecule. As chemists we can do this using modern instrumenta�on. Unfortunately, there is a big problem with this in that there is an enormous diﬀerence between the mass of one molecule and the masses that we measure in the laboratory. Mass #
5.2 1 10/2/11 The Mole The Mole To avoid having to work with HUGE numbers, we use a unit called the mole (mol) which is the number of atoms in a reference sample having a convenient mass. One mole is the number of atoms in exactly 12 g of the pure isotope carbon-‐12. Given the mass of one 12C is 1.992648 X 10-‐23 g/atom, then 12 g 12C/mol / 1.992648 X 10-‐23 12C/atom = 6.022 X 10-‐23 atoms / mol The Mole Mass-‐Mole-‐Atom Conversions Elemental molar mass = (frac�onal abundance) (isotopic molar mass) Isotope
58Fe Isotopic Molar Mass (g/mol)
57.933 The mass of 1 mol of any naturally occurring element is the sum of the contribu�ons from each of its isotopes. Abundance%
0.33 Elemental molar masses can be thought of as conversion factors between masses in grams and # of moles. To determine the amount of an elemental sample, we can meausre its mass and convert to moles by dividing by the molar mass of that element. Similarly, if we wish to know the mass of a par�cular # of moles of an element, we can mul�ply that # of moles by the elemental molar mass. Problem Mt. St. Helens erupted in 1980. Gas samples from the plume were collected and analyzed for toxic heavy metals. To collect mercury (Hg) from the plume, unﬁltered gas samples were passed over a piece of gold metal, which bind Hg atoms �ghtly. The mass of the metal increased as it absorbed from the plume. From a plume-‐gas sample containing 200 g of ash, 3.60 µg of Hg was deposited on the Au. How many moles of Hg were present in the gas sample? How many atoms is this? 2 ...
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