4.3

Periodic Trends in Chemistry

Overview of Periodic Trends

• Main periodic trends discussed:

  • Effective Nuclear Charge

  • Atomic Size

  • Ionization Energy

  • Electron Affinity

  • Metallic Character

  • Electronegativity

Effective Nuclear Charge (Z_eff)

• Definition: The effective nuclear charge is the net positive charge experienced by valence electrons in an atom. It accounts for the shielding effect caused by inner shell or core electrons.

• Nuclear Charge:

  • Positively charged nucleus due to protons.

  • Symbolized as Z (Z = atomic number = number of protons).

• Valence Electrons:

  • Electrons in the outermost shell.

• Coulombic Attraction:

  • Attraction between positively charged nucleus and negatively charged electrons.

• Core Electrons:

  • Electrons located in inner shells that partially shield the valence electrons from the full effect of the nucleus.

Analogy for Understanding Z_eff

• Nucleus as a clearing in a forest:

  • Trees represent core electrons surrounding a positive clearing (nucleus).

  • Hikers represent valence electrons trying to approach the nucleus but are hindered by trees (core electrons).

  • Distance affects the realization of nuclear charge by valence electrons; the further from the nucleus, the weaker the perceived attraction due to the shielding by core electrons.

Calculation of Effective Nuclear Charge

• Equation: $Z_{eff} = Z - S$

  • Where Z = atomic number (total number of protons)

  • S = number of shielding electrons (core electrons)

Example Calculations

• Oxygen (O):

  • Z = 8 (8 protons)

  • Core electrons (S) = 2

  • Calculation:
    $Z_{eff} = 8 - 2 = 6$

• Magnesium (Mg):

  • Z = 12 (12 protons)

  • Core electrons = 10

  • Calculation:
    $Z_{eff} = 12 - 10 = 2$

• Arsenic (As):

  • Z = 33 (33 protons)

  • Core electrons = 28 (from Argon + filled 3d)

  • Calculation:
    $Z_{eff} = 33 - 28 = 5$

Trends in Z_eff

• Z_eff increases from left to right across a period due to increasing positive charge without an increase in shielding electrons.

• Z_eff remains similar within a group but may show an increase with fewer core electrons between elements at different levels in the periodic table.

• Stronger Z_eff correlates with a greater attraction for electrons, influencing an element's desire to gain or lose electrons to achieve a noble gas configuration.

Atomic Size Trends

• Definition: Atomic size refers to the distance from the nucleus to the outer boundary of the surrounding cloud of electrons.

• Trends:

  • Decreases from left to right:

  • Higher Z_eff pulls electrons closer to the nucleus, thus reducing atomic size.

  • Decreases from bottom to top:

  • As you move up the periodic table, there are fewer electron shells, leading to smaller atomic radii.

Example Comparisons

• Oxygen vs. Magnesium:

  • Magnesium: Further left and lower down on the periodic table.

  • Oxygen: To the right of magnesium.

  • Conclusion: Magnesium is larger than oxygen.

Caddy Corner Relationship (Comparison of Atomic Size)

• Concept: Refers to the tendency to compare elements that are diagonally placed in the periodic table.

• Example: Compare Carbon (higher up) with Phosphorus (lower down, farther left).

  • Phosphorus larger due to being further down (more electron shells).

Cation and Anion Size

• Cations: Positively charged ions that are always smaller than their neutral counterparts due to the loss of an electron leading to increased nuclear attraction on remaining electrons (Example: Mg vs. Mg⁺).

  • Cations result in a decreased electron-electron repulsion after losing electrons.

• Anions: Negatively charged ions that are always larger than their neutral counterparts due to the gain of electrons leading to increased electron-electron repulsion (Example: Br vs. Br^-).

Isoelectronic Series

• Definition: Isoelectronic species are different atoms or ions that have the same number of electrons.

• Characteristics:

  • The element with more protons is smaller due to a stronger positive charge pulling electrons closer (Example: F^- has 9 protons and Ne has 10 protons; Ne will be smaller).

• Example of Isoelectronic Comparisons:

  • Sulfide (S^2-), Chloride (Cl^-), Potassium (K⁺¹), Calcium (Ca⁺²) have 18 electrons each, but different sizes due to differing numbers of protons.

  • Result: Sulfide is largest (16 protons), Calcium is smallest (20 protons).

Final Notes

• Recognizing and applying periodic trends is crucial for predicting chemical behavior, understanding reactivity, and the formation of ions based on effective nuclear charge, atomic size, and charge impacts.