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0208 Notes - Chemistry Review In the periodic table of...

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2/8/10 Oregon State University PH 202, Lecture 15 1 Chemistry Review : In the periodic table of elements, each element has an atomic number. What does that number mean? 1. It indicates the number of protons in the atom’s nucleus. 2. It indicates the number of neutrons in the atom’s nucleus. 3. It indicates the combined number of protons and neutrons in the atom’s nucleus. 4. Nothing. It was assigned chronologically— listing elements in their order of discovery. 5. None of the above. See the full version of these lecture notes (available online) for a brief review of atomic structure.
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2/8/10 Oregon State University PH 202, Lecture 15 2 Atomic Structure Atoms are the basic building blocks of all chemical substances. Atoms consist of three basic parts: Name Symbol Mass Charge proton p + 1.673 x 10 -27 kg +1.602 x 10 -19 C neutron n 0 1.675 x 10 -27 kg zero electron e - 9.109 x 10 -31 kg -1.602 x 10 -19 C Protons and neutrons bind to each other at the center of an atom, forming the nucleus . The electrons roam around the nucleus. An element is characterized by a distinct con fi guration of these parts. The periodic table not only names each element (and uses a letter symbol for it), it also lists it by atomic number , the number of protons in its nucleus. For example, oxygen (O) is element 8 (i.e. its atomic number is 8), because oxygen atoms have 8 protons. Likewise, iron (Fe) is #26, because iron atoms have 26 protons, etc.
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2/8/10 Oregon State University PH 202, Lecture 15 3 By de fi nition, an atom is electrically neutral (i.e. has a net charge of zero), because it has the same number of protons as electrons (those two particles having equal but opposite charges). Any varia-tion on this, where the overall net charge is not zero (e.g. more electrons than protons, or vice versa)—is called an ion rather than an atom. For example, an atom of chlorine has 17 protons in its nucleus and 17 electrons roaming around that nucleus, forming an electrically neutral body—an atom. But if an 18th electron joins the party (as often occurs), then the whole thing becomes a chlorine ion; it now has a net charge (one more electron than proton): -1.602 x 10-19 C Ions form quite readily and fi gure heavily in chemical actions— interactions between atoms in close proximity due to “sharing” or “borrowing” of electrons. But an ion still has the same nucleus as the corresponding atom.
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2/8/10 Oregon State University PH 202, Lecture 15 4 Isotopes , on the other hand, don’t form so easily. An isotope is an uncommon variant of an atom that differs only in the number of neutrons in its nucleus. This variation doesn’t affect the net charge (as neutrons carry no charge), so it doesn’t make the atom into an
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