Honors Chemistry
Chapter 8: Periodicity
8.1 The Periodic Table
• John Newlands (1864)
• Arrange elements by mass
• Properties repeat every
8th element
• Law of Octaves
• Ridiculed for musical
reference
8.1 The Periodic Table
• Dmitri Mendeleev (1869)
• Periodic Law
• Grouped elements by
repeating properties
• Predicted properties of
undiscovered elements
• Problem – switched pairs of elements
• Moseley – count by atomic #, not by mass
8.2 Periodic Classification
• Old vs. new numbering system
• Representative Elements (main group)
• Groups IA – VIIA, Unfilled s or p sublevels
• Noble Gases
• Group VIIIA, Filled p sublevel
• Transition Metals
• “B” Groups, Unfilled d sublevel
• Lanthanides and Actinides
• Unfilled f sublevel
8.2 Periodic Classification
• Valence Electrons
• Electrons in outermost shell
• Highest n quantum number reached
• Involved in chemical bonding
• Representing Free Elements
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Metals – always use single atom (Na, Fe, Cu)
Most nonmetals – also monatomic (C, As, Kr)
Sulfur – actually S8, but we usually use just S
Phosphorous – exists as P4
Diatomics – H2, N2, O2, F2, Cl2, Br2, I2
8.2 Electron Configurations of Ions
• Representative Elements
• Try to get ns2 np6 (isoelectronic with noble gas)
• Metals lose electrons (cations)
• Nonmetals gain electrons (anions)
• Find electron configurations of...
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K+
Al3+
S2C4Sn2+
8.2 Electron Configurations of Ions
• Transition Metals
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End with s2 dn
Remove the higher-level s electrons first = 2+
Remove d’s one at a time until d5 or d0
Often not isoelectronic with noble gas
• Predict oxidation states of...
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Fe
V
Cd
Ag
8.3 Effective Nuclear Charge
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Z = # of protons = nuclear charge
Decreases with distance from nucleus
Decreased by shielding effect
Zeff = Z – s
s = shielding constant
s increases with intervening shells
8.3 Atomic Radius
• “Size of an atom”
• Half distance between
adjacent nuclei
• Radius increases down a group
• Caused by increasing number of shells
• Radius decreases across a period
• Caused by increased Z on the same shells
• Which is bigger: Ca or Sr? Ca or Zn?
8.3 Atomic Radius
8.3 Ionic Radius
• Radius of a cation or anion
• Anion (negative charge) – size increases
• More electron repulsion
• Cation (positive charge) – size decreases
• Less electron repulsion
• Isoelectronic ions
• Size decreases with increasing positive charge
• Size increases with increasing negative charge
• Compare sizes of O2-, F-, Ne, Na+, Mg2+
8.4 Ionization Energy
• Minimum energy needed to remove an
electron from an atom in its ground state
• Measured in kJ/mol
• Measures how tightly electrons are held
• Multiple ionization energies
• increases across a period
• Increased nuclear force
• decreases down a group
• Increased dist. from nucleus
8.4 Ionization Energy
8.5 Electron Affinity
• Measure of the energy change when an
atom gains an electron
• Measure of attraction for electrons
• Also measured in kJ/mol
• More difficult to measure than Eion
• Same trends as Eion
• increases across period
• decreases down group
8.6 Periodicity of Chemical
Properties
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Eion = atom’s attraction for its own eEA = atom’s attraction for other eMetals – low attraction – form cations
Nonmetals – high attraction – form anions
Diagonal relationships
• Similarities between pairs in different groups
• Likely caused by small size at the top of the group
• Similar charge densities in cations
8.6 Properties Across a Period
• Need to compare similar compounds
• Use oxides for comparison
• Metal oxides
• Ionic, High melting/boiling point
• Basic
• Nonmetal oxides
• Molecular, Low melting/boiling
• Acidic
• Amphoteric
• both acidic and basic prop.