Chapter2[1] - 9/1/2011 CHEMISTRY 1307 Chapter II Atoms,...

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9/1/2011 1 1 CHEMISTRY 1307 Chapter II Atoms, Molecules, and Ions 2 Atomic structure Protons ( p + ) positive electrical charge mass = 1.672623 x 10 -24 g mass = 1.007 atomic mass units (u) Electrons ( e ) negative electrical charge relative mass = 0.0005 u Neutrons ( n 0 ) no electrical charge mass = 1.009 u Scientific Notation! (approx = 1 u) (approx = 0 u) (approx = 1 u) 3 Atomic structure The atom is mostly empty space 1) The nucleus contains protons (positively charged) and usually contains neutrons (uncharged). 2) The electrons (negatively charged) are attracted to the nucleus and form a cloud around the nucleus. 3) The nucleus is tiny but contains almost all the mass of an atom. 4) The electron cloud makes up the volume of the atom, but has almost no mass. 5) Extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.
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9/1/2011 2 4 Atomic number and mass number Typical Periodic table entry Each element has a characteristic number of protons in the nucleus. This is the atomic number , Z. The total number of protons and neutrons in the nucleus of an atom is the mass number , A. We use atomic notation to display the number of protons and neutrons in the nucleus of an atom Z A 5 Atomic number and mass number Si The element is silicon (symbol Si). The atomic number is 14: silicon has 14 protons. The mass number is 28: the atom of silicon has 28 protons + neutrons. (A = Z + n) The number of neutrons is: A – Z = n A - Z = 28 – 14 = 14 neutrons. 28 14 6 Atomic mass This tells us the mass of one atom of an element relative to one atom of another element. For example, an O atom is approximately 16 times heavier than an H atom. Define one element as the standard against which all others are measured Standard = carbon The mass of one 12 C atom is 1.99265 10 -23 g. The atomic mass of 12 C atom is defined as exactly 12u. Therefore: 1u = (the mass of one 12 C atom 12) = 1.66054 10 -24 g = 1.66054 10 -27 kg
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9/1/2011 3 7 Isotopes All atoms of the same element have the same number of protons. Most elements occur naturally with varying numbers of neutrons. Atoms of the same element that have a different number of neutrons in the nucleus are called isotopes . Isotopes have the same atomic number but different mass numbers. 8 Hydrogen isotopes 1 1 H 3 1 H 2 1 H 1 proton and 2 neutrons, tritium 1 proton and 0 neutrons, protium 1 proton and 1 neutron, deuterium Norwegian heavy water sabotage http://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage 9 Masses of isotopes Determined with a mass spectrometer (F = m·a)
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9/1/2011 4 10 Isotope abundance The mass spectrometer gives information on the mass and relative abundance of each element s isotopes. Each isotope is represented by a Relative Abundance . 11 Average atomic mass Since not all isotopes of an atom are present in equal proportions, we must use the weighted average.
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This document was uploaded on 10/30/2011 for the course CHEM 1307 at Texas Tech.

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Chapter2[1] - 9/1/2011 CHEMISTRY 1307 Chapter II Atoms,...

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