Bonding & Structure

• Electrostatic Attraction: All chemical bonds rely on the attraction between positive and negative charges; in ionic bonds, it is between ions; in covalent, between nuclei and shared electrons; in metallic, between ions and delocalized electrons.

• Lattice Energy: The strength of an ionic bond is determined by the charge density of the ions; smaller ions with higher charges (e.g., $Mg^{2+}$ vs $Na^+$) create stronger attractions and higher melting points.

• Bond Polarity: When atoms in a covalent bond have different electronegativities, the electron pair is shared unequally, creating a dipole where one end is partially negative ($\delta-$) and the other partially positive ($\delta+$).

Predicting Bonding Type

• Electronegativity Difference: A large difference (typically $> 1.7$) usually indicates ionic bonding, while a small or zero difference indicates covalent bonding.
• Position in Periodic Table: Metals (left side) reacting with non-metals (right side) generally form ionic compounds, whereas non-metals reacting with each other form covalent compounds.

Determining Structure

• Simple Molecular: Substances consisting of small, discrete molecules held together by weak intermolecular forces (e.g., $H_2O$, $CO_2$).
• Giant Structures: Substances where bonding extends throughout the entire sample in a continuous network (e.g., Diamond, Sodium Chloride, Copper).

• Identify the State: If a substance has a high melting point and conducts electricity as a solid, it is almost certainly a metal.

• Check for 'Molten' Conductivity: If a substance only conducts when liquid or dissolved, it is ionic. If it never conducts, it is likely simple molecular or a giant covalent insulator like diamond.

• Solubility Clues: Simple molecular substances often dissolve in organic solvents (like hexane), while ionic substances are more likely to dissolve in polar solvents (like water).

• Graphite Exception: Always remember that Graphite is a giant covalent structure that does conduct electricity due to delocalized electrons between its layers.

• Breaking Bonds vs. Intermolecular Forces: A common error is stating that covalent bonds break when water boils. In reality, only the weak intermolecular forces (hydrogen bonds) between molecules are overcome; the $O-H$ covalent bonds remain intact.

• Ionic Conductivity: Students often mistakenly say ionic solids conduct electricity because they 'have ions'. Ions are present, but they are locked in a fixed lattice and cannot move to carry a charge until the substance is melted or dissolved.

• Brittleness: Do not confuse malleability with brittleness. Ionic crystals are brittle because shifting the lattice aligns like-charges, causing repulsion and cleavage, whereas metals are malleable because the sea of electrons allows ions to slide past each other without repulsion.