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Lecture 2

CHEM 1A03 Lecture 2: Atomic Trends (Chapter 9).docx

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Jeff Landry

Chapter 9: The Periodic Table and Atomic Properties - Goal is to have full shells - As you increase the distance between two charged particles, the force between them decreases - Ns2np6 stable electron configurations - Other elements try to attain these configurations - Considered chemically “inert” until 1962 o Many noble gas compounds known, including eg. KrF , XeF ,2X F ,2XeF3 4XeF ,6XeF ,6 2 XeOF 4 XeO F2 4 - Non –metals: main group (s and p block) non-metals tend to gain electrons – easiest way to gain noble gas configuration, right side of periodic table.. group 15  18 - Descriptive Halogen Chemistry: o 1) Ability to oxidize increase up the group (towards F) eg. Yes: Cl2(g) + 2I-(aq)  I2(aq) + 2Cl-(aq) No: Br2(l) + Cl-(aq)  no rxn o 2) They react with metals to form salts eg. NaCl 2Na(s) + Cl2(g)  2NaCl(s) o 3) Halogen mp/bp increases down group 17A - Metals: main group (s and p block) metals (and H) tend to lose electrons to get to noble gas configurations - Alkali metals (group 1) oxidize more readily than alkaline earth metals (group 2) - Both oxidize easily in water - K in contact with water or air – it burns / flames up - K + H20  KOH + H - General Trends in Physical Properties - Things in centre have the highest melting points ( peaks in the centre) - Metallic properties decrease across a period? Peak in the middle? - Melting point varies with type of bonding - Electrostatic interactions of the nucleus (pos) and electrons (neg) 2 - E = kQ nucleus r o E = electric field, Q = charge on nucleus, r = electron distance nucleus - As you move away from the nucleus, they get easier to remove (inversely proportional) - As r increases, (electrons are placed in further shells, n increases), electric field decreases and valence electrons are not held as strongly - As we go across a period, r ,remains relatively constant, but Qnucleus increases with greater number of protons. Thus, electric field increases and valence electrons are held more strongly - Screening o Outer e- of an atom are screened by the core e- from feeling the full attractive charge (Z) from the nucleus o Outer e- feel an effective nuclear charge, Zeffective o Simply, Zeff = Z-S, where S = #core e- o In reality, s, p, and e- are screened to diff extents; outer e- also screen each other Atomic Radius - radius of atom in gas stage - don`t include noble gases b/c you need a compound and use x ray diffraction.. can`t do it with noble gases - Decreases across a period o Z increases, but e- are added to same n valence shell o These outer e- feel strong attraction to nucleus (higher Zeff) - Increases down a group o E- are added to a new n shells o On average e- are farther away but Zeff does not change much down a group, thurs valence e- have weaker attraction to the nucleus Ionic Radius - Smaller than atomic radius - Left to right – decrease in size - Na+ and Mg2+ are isoelectronic – same # of e- - Cl- has more protons, greater attraction, smaller radius even though it has the same electron # as others - Catio
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