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Chapter 9

Lecture and Textbook Collaborated Notes - Chapter 9 - CHEM 1A03

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McMaster University
Aadil Merali Juma

Chem 1A03 Chapter 9: The Periodic Table and Some Atomic Properties 9.1 Classifying the Elements: The Periodic Law and the Periodic Table 9.2 Metals and Nonmetals and Their Ions  ns np stable electron configurations o Noble gases o Other elements try to attain this configuration  Groups 1 and 2 – most active metals  Metals tend to lose electrons to attain noble gas electron configurations o Eg/ Group 1 and 2 Metals Alkali metals (Group 1) oxidize more readily than alkaline earth metal (Group 2) Both oxidize easily in water  Groups 16 and 17 – most active non-metals  Non-metals tend to gain electrons to attain noble gas electron configurations o Eg/ Descriptive Halogen Chemistry 1) Ability to oxidize increases up the group YES: Cl2(g)+ 2 I(aq) I2(aq) 2 Cl(aq) NO: Br 2(l)Cl-(aq) no reaction 2) They react with metals to form salts (Eg/ NaCl) 2 Na (s)l 2(g) 2 NaCl (s) 3) Halogen mp/bp increases down group 17A 9.3 The Sizes of Atoms and Ions Atomic Radius  Covalent Radius – one-half the distance between the nuclei of two identical atoms joined by a single covalent bond  Ionic Radius – based on the distance between the nuclei of ions joined by an ionic bond; size must be properly apportioned between the cation and the anion because the ions are not identical in size  Metallic Radius – one-half the distance between the nuclei of two atoms in contact in the crystalline solid metal  Van der Waals Radius – the distance between the centers of neighboring atoms in a solid sample of a noble gas Screening and Penetration  Penetration – a gauge of how close an electron gets to the nucleus o s shell electrons penetrate better than p shell, which penetrate better than d shell electrons  Outer electrons of an atom are screened by the core electrons from feeling the full attractive charge (Z) from the nucleus  Outer electrons feel an effective nuclear charge,effective o Z = Z - S eff o S = # of core electrons  same for all atoms in a given row  In reality, s, p and d electrons are screened to different extents and outer electrons also screen each other o s shell electrons are screened by inner electrons less than p shell electrons, which are screened less than d shell electrons  The Effects of Penetration and Screening o E = kQ r2 E = Electric Field Q = Charge on Nucleus r = Electron Distance  Farther away, electric field   As r  (e- are placed in further shells, n) electric field  and valence e- are not held strongly  As we go  (across) a period, r remains relatively constant but Qnucleus with greater number of protons  Electric field , valence e- are held more strongly Chem 1A03 Atomic Radius 1. Variation of Atomic Radii Within a Group of the Periodic Table o The more electronic shells in an atom, the larger is the atom o Electrons are added to new n shells o Electrons are farther away bueff does not change much down a group, thus valence electrons have weaker attraction to the nucleus o Atomic radius increases from top to bottom through a group of atom  Increases down a group 2. Variation of Atomic Radii Within a Period of the Periodic Table o Z increases, but electrons are added to same n valence shell o These outer electrons feel stronger attraction to nucleus (heffer Z ) o The atomic radius decreases from left to right through a period of elements  Decreases across a period 3. Variation in Atomic Radius Within a Transition Series o Additional electrons go into an inner electron shell, where they participate in shielding outer-shell electrons from the nucleus o At the same time, the number of electrons in the outer shell tends to remain constant  Atomic radii do not change very much Ionic Radius  Cations are smaller than neutral atoms o Same Z but as electrons are removed, there is less repulsion/screening  Anions are larger than neutral atoms o Same Z but as electrons are added, there is an increase in repulsion/screening  Isoelectronic ions/atoms (have equal number of electrons) the more positive the ionic charge, the smaller the ionic radius o Size decreases as Z increases o Eg/ Na and Mg  Na+ is larger, because it’s nuclear charge is smaller (+12 vs. +11) Ionic Radius Chem 1A03  Essential Elements: Tooth Enamel o Principal mineral in tooth enamel is hydroxyapatite, Ca5(Po4)3OH o Crystal Structure o The hydroxide ions reside in channels o Fluoride is added to drinking water  It is a weak base  Same size as –OH  F ions (F-) can substitute into spaced vacated by hydroxide ions (-OH) in tooth enamel o Spaces from plaque cause teeth decay (caused by acids) o F- substitutes so well into spaced vacated by –OH  Charges match  Similar in size -
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