Chemical Bonds and Formulas

Chemical Bond

• Definition: A chemical bond is the force of attraction between two atoms that overcomes the repulsion of their positively charged nuclei.
• Purpose: Atoms bond to achieve a more stable electron configuration, often resembling a closed shell or noble gas configuration with valence electrons.

Valence Electrons

• Only outer shell electrons are involved in bonding; inner core electrons do not participate due to their already stable configuration.
• The octet rule suggests that atoms strive for eight valence electrons to achieve stability in their outer shells.

Types of Chemical Bonds

• Ionic Bonds

• Involve the transfer of electrons from one atom to another.

• Form between metals and nonmetals or metalloids, creating charged ions.

• Example: Sodium (Na) loses an electron to become Na⁺, and Sulfur (S) gains it to become S²⁻, resulting in the formation of Na₂S through ionic bonding.

• Covalent Bonds

• Involve sharing of electrons between atoms.

• Typically form between nonmetals.

• Example: Two hydrogen atoms (H) share their single electrons to create H₂, having a stable configuration resembling that of helium.

• This sharing creates molecular orbitals, allowing each atom to attain a closed shell.

• Metallic Bonds (not extensively covered)

• Involve a sea of mobile electrons that can move freely from one metal atom to another.

Molecules and Crystalline Structures

• Compounds formed can be discrete molecules (for covalent compounds) or extended arrays (for ionic compounds).
• Example of covalent compound: Dry ice (solid CO₂) made of discrete carbon and oxygen molecules.
• Example of ionic compound: Sodium chloride (NaCl) forms a large crystalline array due to the electrostatic attraction of positive and negative ions.

Representing Chemical Compounds

• Space-Filling Model: Atoms represented as spheres, demonstrating the physical size and position of the atoms (e.g., water molecule H₂O).
• Ball and Stick Model: Atoms represented as spheres connected by sticks to illustrate bonds between them.
• Chemical Symbols and Formulas: Indicate creation of compounds and their proportions using chemical symbols and subscripts for atom counts.

Proportions and Chemical Formulas

• Chemical formulas express composition and proportions of compounds.
• Subscripts reflect the number of atoms (e.g., H₂O indicates 2 hydrogen atoms).
• Parentheses indicate polyatomic groups.

Types of Chemical Formulas

• Molecular Formula: Gives the exact number of atoms in a unit (e.g., C₆H₁₂O₆ for glucose).
• Empirical Formula: Shows the simplest whole number ratio between elements (e.g., for hydrogen peroxide, its empirical formula is HO).
• Structural Formula: Displays how atoms are bonded in a molecule, typically explored in further detail with tools like Lewis structures.

Summary of Fixed Proportions

• Atoms combine in integer ratios to form compounds due to bonding.
• Sodium chloride exists as a formula unit NaCl, indicating a 1:1 ratio of sodium to chlorine atoms in its crystal lattice, despite its larger composition in practice.

Conclusion

• The understanding of chemical bonds is crucial as they dictate the structure and properties of matter, influencing how different substances interact and combine.