Chemistry of Groups on the Periodic Table

Alkali Metals (Group 1A or Group 1)

• Possess classic metal properties.
• Lower densities than other metals.
• One loosely bound electron in the outermost shell.
• Low $Z_{eff}$ values.
• Largest atomic radii in their periods.
• Low ionization energy.
• Low electron affinities.
• Low electronegativities.
• Easily lose one electron to form univalent cations.
• React readily with nonmetals, especially halogens (e.g., NaCl).

Alkaline Earth Metals (Group 2A or Group 2)

• Possess metal characteristics.
• Share characteristics with alkali metals.
• Slightly higher effective nuclear charges.
• Slightly smaller atomic radii.
• Two electrons in the valence shell.
• Easily lose two electrons to form divalent cations.
• Alkali and alkaline earth metals are active metals due to high reactivity; not found naturally in elemental neutral state.

Chalcogens (Group 16 or Group 4A)

• Eclectic group of nonmetals and metalloids.
• Crucial for biological functions, though less reactive than halogens.
• Six electrons in the valence electron shell.
• Small atomic radii and large ionic radii (due to proximity to metalloids).
• Oxygen:
  • Most important element in the group.
  • Constituent of water, carbohydrates, and other biological molecules.
• Sulfur:
  • Important component of certain amino acids and vitamins.
• Selenium:
  • Important nutrient for microorganisms.
  • Role in protection from oxidative stress.
• Remainder of the group:
  • Primarily metallic.
  • Generally toxic to living organisms.
  • Note: High concentrations of these elements can be toxic, regardless of biological usefulness.

Halogens (Group 17 or Group 4A)

• Highly reactive nonmetals.
• Seven valence electrons.
• Desperate to complete octets by gaining one electron.
• Variable physical properties:
  • Gaseous: $F<em>2$ and $Cl</em>2$
  • Liquid: $Br_2$
  • Solid: $I_2$
• Uniform chemical reactivity.
• High electronegativities and electron affinities.
• Especially reactive toward alkali and alkaline earth metals.
• Fluorine (F) has the highest electronegativity of all elements.
• Not naturally found in elemental state; exist as ions (halides) or diatomic molecules.

Noble Gases (Group 18 or Group 8A)

• Inert gases with minimal chemical reactivity due to filled valence shells.
• High ionization energies.
• Little or no tendency to gain or lose electrons.
• No measurable electronegativities (for He, Ne, and Ar).
• Extremely low boiling points.
• Exist as gases at room temperature.
• Commercial niche as lighting sources due to lack of reactivity.

Transition Metals (Groups 3-12 or Group 1B to 8B)

• Metals with low electron affinities, ionization energies, and electronegativities.
• Hard, with high melting and boiling points.
• Malleable and good conductors.
• Loosely held electrons fill d orbitals in valence shells.
• Unique property: Can have different charge forms or oxidation states.
• Capable of losing different numbers of electrons from s and d orbitals.
  • Example: Copper (Cu) can exist as $Cu^{+1}$ or $Cu^{+2}$.
  • Example: Manganese (Mn) can exist as $Mn^{+2}$, $Mn^{+3}$, $Mn^{+4}$, $Mn^{+6}$, or $Mn^{+7}$.
• Form many different ionic compounds.
• Different oxidation states often correspond to different colors; solutions with transition metal complexes are often vibrant.
• Complex ions associate in solution with water molecules (hydration complexes) or nonmetals.
  • Example: $CuSO<em>4 "." 5H</em>2O$
  • Example: $[Co(NH<em>3)</em>6]Cl_3$
• Ability to form complexes contributes to variable solubility.
  • Example: $AgCl$ is insoluble in water but soluble in aqueous ammonia due to the formation of the complex ion $[Ag(NH<em>3)</em>2]^+$.
• Formation of complexes causes d orbitals to split into two energy sublevels.
• Complexes absorb certain frequencies of light, raising electrons from lower to higher energy d orbitals.
• Frequencies not absorbed (subtraction frequencies) give the complexes their characteristic colors.
• Perception of color:
  • Color is not absorbed, but reflected by the object.
  • Brain mixes subtraction frequencies and perceives the complementary color of the frequency that was absorbed.
  • Example: Carotene absorbs blue light and reflects other colors; brain interprets color as white light minus blue light, which is yellow light.
• Complementary colors: Important relationship.
• Additive and subtractive color mixing differences are outside the scope of the MCAT.

Conclusion

• Understand, don't just memorize, the trends of physical and chemical properties of elements.
• A foundational understanding of the elements will help in understanding general and particular behaviors.
• Understanding the properties of the elements will help make topics in general chemistry understandable.
• Elements from the groups are critical or detrimental to biological function.
• The human body utilizes certain elements for specific purposes, taking advantage of the discussed periodic table information.