General Chemistry: Molecules and Compounds

Chapter 3: Molecules and Compounds

Fundamental Building Blocks of Chemistry

• Atoms: The fundamental and indivisible unit of matter, consisting of a nucleus (protons and neutrons) surrounded by electrons. Each element is defined by the number of protons in its atoms.

• Molecules: Formed when two or more atoms chemically bond together, acting as a discrete entity. These bonds can be either covalent (sharing electrons) or ionic (transferring electrons, leading to electrostatic attraction).

• Chemistry: The scientific study of matter's composition, structure, properties, and the changes it undergoes during chemical reactions, along with the energy changes that accompany these processes.

Classification of Pure Substances

Pure substances possess a uniform and definite composition and are categorized into elements and compounds based on their atomic and molecular structures.

• Elements

  • Atomic Elements: These elements exist in nature as individual, unbonded atoms as their most stable and fundamental units. They do not typically form molecules with themselves under normal conditions.

    • Examples: Many metals like $Na$ (Sodium), $K$ (Potassium), $Mg$ (Magnesium), and noble gases such as $Ne$ (Neon), $Ar$ (Argon), which are characterized by their stable electron configurations.

  • Molecular Elements: These are elements that do not exist as single atoms in their natural state but rather as molecules, which are composed of two or more atoms of the same element chemically bonded together. This bonding typically occurs to achieve a more stable electron configuration.

    • Diatomic Elements: There are seven elements that naturally exist as molecules consisting of two atoms of the same element covalently bonded together to form a diatomic molecule. They are often remembered as "BrINClHOF".

      • $H_2$ (Hydrogen): A colorless, odorless, highly flammable gaseous substance.

      • $N_2$ (Nitrogen): The most abundant gas in Earth's atmosphere, diatomic nitrogen is very stable due to a triple bond.

      • $O_2$ (Oxygen): Essential for respiration, forming a double bond.

      • $F_2$ (Fluorine): A highly reactive pale yellow gas, the most electronegative element.

      • $Cl_2$ (Chlorine): A greenish-yellow gas, toxic and used as a disinfectant.

      • $Br_2$ (Bromine): A red-brown liquid at room temperature, one of the few elements that are liquid at standard conditions.

      • $I_2$ (Iodine): A lustrous dark-purple solid that readily sublimes to a purple gas.

    • Polyatomic Elements: Some elements form molecules with more than two atoms.

      • Examples: P4$(Phosphorus) typically exists as a tetrahedral molecule in its white allotrope, while$S8$(Sulfur) commonly forms a crown-shaped eight-atom ring molecule.</p></li></ul></li></ul></li></ul></li><li><p><strong>Compounds</strong></p><ul><li><p><strong>Molecular Compounds</strong>: These compounds are formed when two or more different nonmetal atoms share electrons through covalent bonds, creating distinct, discrete molecules. They typically have lower melting and boiling points compared to ionic compounds and are generally poor conductors of electricity.</p><ul><li><p>Example:$H2O$(Water), where two hydrogen atoms share electrons with one oxygen atom;$CO2$(Carbon Dioxide), a linear molecule with double bonds;$CH_4$(Methane), a tetrahedral molecule.</p></li></ul></li><li><p><strong>Ionic Compounds</strong>: These compounds are formed between a metal and a nonmetal (or polyatomic ions) through the complete transfer of electrons from the metal to the nonmetal. This transfer generates positively charged ions (cations) and negatively charged ions (anions) which are then held together by strong electrostatic forces in a repeating, extended crystalline lattice structure, rather than forming discrete molecules. They possess high melting points, are typically solid at room temperature, and can conduct electricity when molten or dissolved in water.</p><ul><li><p>Example:$NaCl$(Sodium Chloride, common table salt), where a sodium atom transfers one electron to a chlorine atom, forming$Na^+$and$Cl^-$ions that arrange into a crystal lattice. Other examples include$MgO$(Magnesium Oxide) and$K2SO4$$ (Potassium Sulfate).