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GOSSMONT CHEM 141 - Atoms, Elements, and the Periodic Table

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Atoms, Elements, and the Periodic Table (Chapter 2)Atomic Structure1. Historical View - Dalton's Atomic TheoryBased on empirical observations, formulated as Laws of:Conservation of MassDefinite ProportionsMultiple ProportionsSummary of Dalton's theory:- matter is composed of small particles called atoms- in chemical reactions atoms are indestructible- atoms of different elements have different masses- atoms of the same and/or different elements combine to form newsubstances (i.e., compounds)- in a given compound, the constituent atoms are always combined inthe same fixed numerical ratio2. "Modern" Viewatom consists of: central nucleus containing protons (+) and neutronswith outer shells of electrons (-)most of an atom's mass is in the nucleus -- relative masses:neutron - proton - 1 amuelectron - 10-4 amu (amu = atomic mass unit = 1.66 x 10-24 g)Page 13. Atomic Masses and IsotopesXAZYZ = atomic number of an element = # protons in nucleuse.g., carbon is atomic number 6 -- all carbon atoms have 6 protonsA = mass number (often omitted) = # protons + # neutronsY = charge (on an ion) = # protons - # electronsisotopes - atoms of the same element with different mass numberse.g., natural carbon has 2 isotopes (2 types of C atoms):Isotope symbolrelativeabundance protons neutronsmassnumbercarbon-1212C 99% 6 6 12carbon-1313C 1% 6 7 13atomic mass (atomic weight) of an element is based on the 12C scale:1 atom of 12C is defined as exactly 12 amusince most elements are actually mixtures of isotopes,the atomic mass (or weight) is really an "average" atomic masse.g., atomic mass of chlorine, Cl, is 35.4527 amu35Cl 75.77% 34.9689 amu37Cl 24.23% 36.9659 amu"weighted average"(0.7577 x 34.9689) + (0.2423 x 36.9659) = 35.4527 (- 35.45)Normally, chemists are concerned with such "average" atomic massesPage 2The Mole Concept and Molar Mass1. Avogadro's Number -- The Chemist's "Dozen"No= number of atoms in exactly 12 grams of carbon-12= 6.022 x 1023 "things" {a very large number ! }this is a conversion factor, just like 12 things per dozen, e.g.:mass of one atom of carbon-12 = (12 g) / (6.022 x 1023 atoms)= 1.99 x 10-23 g/atom2. The Mole -- How Chemists "Count"One Mole of a substance contains an Avogadro's Number of formula units1 mole = 6.022 x 1023 formula unitse.g., 1 mole of iron contains 6.022 x 1023 Fe atoms3. Molar MassMolar Mass = the mass (in grams) of one mole of a substanceSince 12 grams of 12C is defined as 1 mole of 12C and the atomic masses ofother elements are defined relative to that, then…..the molar mass of an element = its atomic mass in grams/mole e.g., from the periodic table, the "atomic mass" of Al is 26.98What does this mean?- mass of one Al atom = 26.98 amu- molar mass of Al = 26.98 g/mole (mass of one mole of Al)Page 3The Periodic Table1. General arrangement - increasing atomic number within:groups: vertical columns (also called families)periods: horizontal rows2. Terminology - parts of the periodic tablerepresentative elements..........the longer columns (the "A" groups)transition elements..................shorter, central columns ("B" groups)alkali metals..............................group IA (1): Li, Na, K, ...alkaline earth metals................group IIA (2): Be, Mg, Ca, ...halogens...................................group VIIA (17): F, Cl, Br, ...noble gases..............................group 0 (18): He, Ne, Ar, ...lanthanide elements................# 58 - 71 (1st row at bottom)actinide elements.....................# 90 - 103 (2nd row at bottom)3. Types of elements - by physical propertiesmetals: shinny, malleable, ductile solids with high mp and bpgood electrical conductorsnonmetals: gases, liquids, or low-melting solidsnon-conductors of electricitydiatomic elements: H2, O2, N2, F2, Cl2, Br2, I2metalloids: intermediate properties, often semiconductors (e.g., Si)Page 4Ions and the Periodic Table1. Elements combine to form compounds -- two general typesMolecular Compounds -- more later in Chapter 3!atoms linked together by "chemical bonds" in discrete electricallyneutral particles called moleculese.g., H2O CO2 PCl3 C12H22O11Ionic Compoundsresult from transfer of one or more electrons from one atom to anotherto yield oppositely-charged particles called ionscation = positive ion anion = negative ionthere are not discrete molecules -- the ions are held together byelectrostatic forces in a regular, 3-dimensional pattern called acrystalline latticee.g., LiF lithium fluorideLi + FLi+ + F-LiFe-MgCl2magnesium chlorideMgClCle-e-Mg2+ + 2 Cl-MgCl2Page 52. Relationship to Periodic TableGeneral trends (Figure 2.14)Ionic compounds usually involve metals and nonmetalsgroup IA (1) +1 cations Li+, Na+, K+, .....group IIA (2) +2 cations Be2+, Mg2+, Ca2+, .....other metals may form more than one cation, e.g.:Fe2+ and Fe3+Sn2+ and Sn4+group VIA (16) -2 anions O2-, S2-, Se2-, .....group VIIA (17) -1 anions F-, Cl-, Br-, .....Molecular compounds usually result from the combination of twononmetals and/or metalloidse.g., PH3 AsF5 HBrsome nonmetallic elements actually exist as molecular compoundse.g., the diatomics (H2, O2, N2, etc. as listed before)also: P4, As4, S8, Se8Page


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