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Periodic TrendsTrends in Properties on the Periodic Table
The PlanEssential Question: What causes the trends of the properties in the periodic table?
Discuss Some Properties of Atoms
Explore the Trends
Try to Explain the Trends
Effective Nuclear ChargeEffective Nuclear Charge - (Zeff) the charge an electron feels from the nucleusZeff = Z S, where Z is the atomic number and S is the shielding experienced by the outer electronsThe electrons in the outer energy levels dont feel the full charge of the nucleus because the core electrons help shield them from the nucleus.
Atomic RadiusAtomic radius the radius of an atomSeveral methods of determining the radius - usually measure the distance between two atomsNot all values are completely consistent with each other.
Ionization EnergyIonization Energy the energy required to remove one electron from an atom
Essentially this is the process of making a positive ion.X X+ + e-
ElectronegativityElectronegativity the ability of an atom in a compound to attract electrons to itself
Electrons in compounds spend the most time around atoms with high electronegativities.
Elements with high electronegativities have partial negative charges in compounds.
Periodic Trends LabIn your lab groups, predict what trends you think might occur down a family and across a period for each of the properties we discussed:Effective nuclear chargeAtomic radiusIonization energyElectronegativityRecord on the index card for discussion later
Effective Nuclear ChargeAs you travel left to right across a period:Atomic number increasesShielding stays roughly the sameTherefore Zeff increases significantly across a periodAs you travel down a familyAtomic number increases sharplyShielding increases sharplyZeff does increase down a family but not as much as you might expect.
Atomic RadiusAs you travel left to right across a period:Effective nuclear charge increasesElectrons are being added to the same n shelln values have somewhat to do with distance from the nucleus.Size of atoms shrink as you move to the right.As you travel down a familyEffective nuclear charge increases more slowlyElectrons are added to n shells farther awaySize of atoms increase down the family
Ionization EnergyAs you travel left to right across a periodEffective nuclear charge increasesHarder to remove an electron thereforeIonization Energy increasesAs you travel down a familyEffective nuclear charge increases slowlyOuter electrons are farther away from the nucleusIonization Energy decreases
ElectronegativityAs you travel left to right across a period:Effective nuclear charge increases stronglyElectronegativity increasesAs you travel down a familyEffective nuclear charge increases slowlyOuter electrons are being added in shells farther and farther awayElectronegativity decreases
Ionic RadiusIonic Radius size of an ion
Consider two different relationshipsRelationship between atom and ionRelationship between ions and atoms with the same number of electronsisoelectronic series
Ionic vs. AtomicNumber of protons always stays the sameFor negative ions (anions)Electrons are added to the same outer shellElectron-electron repulsions increaseAnions are larger than their atomsFor positive ions (cations)Electrons are removed to leave one less shelln shells describe distance from nucleusCations are smaller than their atoms
Isoelectronic seriesAll have the same number of electronsNumber of protons changesTherefore effective nuclear charge increasesElectrons are pulled in tighterThe negative ions are larger than the positive ions with the same number of electrons.
Practice ProblemsArrange the following elements in order of decreasing atomic radius:As, O, Sn, Ge, Ne, Ba, He
Arrange the following elements in order of increasing electronegativityAl, Mg, P, Sr, O, F, Rb
Ticket Out the DoorArrange the following elements in order of increasing ionization energy:Cs, Ba, Y, In, Ga, Si, P, F
Test Review ProblemChallenge ProblemPlace the following in order of increasing size: Ne, Cl-, Ar, Na+