Periodic Trends Trends in Properties on the Periodic Table

The Plan Essential Question: What causes the trends of the properties in the periodic table? Discuss Some Properties of Atoms Discuss Some Properties of Atoms Explore the Trends Explore the Trends Try to Explain the Trends Try to Explain the Trends

Effective Nuclear Charge Effective Nuclear Charge - (Z eff ) – the charge an electron “feels” from the nucleus Effective Nuclear Charge - (Z eff ) – the charge an electron “feels” from the nucleus Z eff = Z – S, where Z is the atomic number and S is the shielding experienced by the outer electrons Z eff = Z – S, where Z is the atomic number and S is the shielding experienced by the outer electrons The electrons in the outer energy levels don’t feel the full charge of the nucleus because the core electrons help shield them from the nucleus. The electrons in the outer energy levels don’t feel the full charge of the nucleus because the core electrons help shield them from the nucleus.

Atomic Radius Atomic radius – the radius of an atom Atomic radius – the radius of an atom Several methods of determining the radius - usually measure the distance between two atoms Several methods of determining the radius - usually measure the distance between two atoms Not all values are completely consistent with each other. Not all values are completely consistent with each other.

Ionization Energy Ionization Energy – the energy required to remove one electron from an atom Ionization Energy – the energy required to remove one electron from an atom Essentially this is the process of making a positive ion. Essentially this is the process of making a positive ion. X  X + + e -

Electronegativity Electronegativity – the ability of an atom in a compound to attract electrons to itself Electronegativity – 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. Electrons in compounds spend the most time around atoms with high electronegativities. Elements with high electronegativities have partial negative charges in compounds. Elements with high electronegativities have partial negative charges in compounds.

Periodic Trends Lab In 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 charge Atomic radius Ionization energy Electronegativity Record on the index card for discussion later

Effective Nuclear Charge As you travel left to right across a period: As you travel left to right across a period: Atomic number increases Atomic number increases Shielding stays roughly the same Shielding stays roughly the same Therefore Z eff increases significantly across a period Therefore Z eff increases significantly across a period As you travel down a family As you travel down a family Atomic number increases sharply Atomic number increases sharply Shielding increases sharply Shielding increases sharply Z eff does increase down a family but not as much as you might expect. Z eff does increase down a family but not as much as you might expect.

Atomic Radius As you travel left to right across a period: As you travel left to right across a period: Effective nuclear charge increases Effective nuclear charge increases Electrons are being added to the same n shell Electrons are being added to the same n shell n values have somewhat to do with distance from the nucleus. n values have somewhat to do with distance from the nucleus. Size of atoms shrink as you move to the right. Size of atoms shrink as you move to the right. As you travel down a family As you travel down a family Effective nuclear charge increases more slowly Effective nuclear charge increases more slowly Electrons are added to n shells farther away Electrons are added to n shells farther away Size of atoms increase down the family Size of atoms increase down the family

Ionization Energy As you travel left to right across a period As you travel left to right across a period Effective nuclear charge increases Effective nuclear charge increases Harder to remove an electron therefore… Harder to remove an electron therefore… Ionization Energy increases Ionization Energy increases As you travel down a family As you travel down a family Effective nuclear charge increases slowly Effective nuclear charge increases slowly Outer electrons are farther away from the nucleus Outer electrons are farther away from the nucleus Ionization Energy decreases Ionization Energy decreases

Ionization Energy

Electronegativity As you travel left to right across a period: As you travel left to right across a period: Effective nuclear charge increases strongly Effective nuclear charge increases strongly Electronegativity increases Electronegativity increases As you travel down a family As you travel down a family Effective nuclear charge increases slowly Effective nuclear charge increases slowly Outer electrons are being added in shells farther and farther away Outer electrons are being added in shells farther and farther away Electronegativity decreases Electronegativity decreases

Electronegativity

Ionic Radius Ionic Radius – size of an ion Ionic Radius – size of an ion Consider two different relationships Consider two different relationships Relationship between atom and ion Relationship between atom and ion Relationship between ions and atoms with the same number of electrons Relationship between ions and atoms with the same number of electrons “isoelectronic series” “isoelectronic series”

Ionic Radius

Ionic vs. Atomic Number of protons always stays the same Number of protons always stays the same For negative ions (anions) For negative ions (anions) Electrons are added to the same outer shell Electrons are added to the same outer shell Electron-electron repulsions increase Electron-electron repulsions increase Anions are larger than their atoms Anions are larger than their atoms For positive ions (cations) For positive ions (cations) Electrons are removed to leave one less shell Electrons are removed to leave one less shell n shells describe distance from nucleus n shells describe distance from nucleus Cations are smaller than their atoms Cations are smaller than their atoms

Ionic Radius

Isoelectronic series All have the same number of electrons All have the same number of electrons Number of protons changes Number of protons changes Therefore effective nuclear charge increases Therefore effective nuclear charge increases Electrons are pulled in tighter Electrons are pulled in tighter The negative ions are larger than the positive ions with the same number of electrons. The negative ions are larger than the positive ions with the same number of electrons.

Practice Problems Arrange the following elements in order of decreasing atomic radius: Arrange the following elements in order of decreasing atomic radius: As, O, Sn, Ge, Ne, Ba, He As, O, Sn, Ge, Ne, Ba, He Arrange the following elements in order of increasing electronegativity Arrange the following elements in order of increasing electronegativity Al, Mg, P, Sr, O, F, Rb Al, Mg, P, Sr, O, F, Rb

Ticket Out the Door Arrange the following elements in order of increasing ionization energy: Arrange the following elements in order of increasing ionization energy: Cs, Ba, Y, In, Ga, Si, P, F Cs, Ba, Y, In, Ga, Si, P, F

Test Review Problem Challenge Problem Challenge Problem Place the following in order of increasing size: Ne, Cl -, Ar, Na + Place the following in order of increasing size: Ne, Cl -, Ar, Na +