Compound Structure and Properties

Valence electrons are the electrons most directly involved in bonding. A Lewis structure tracks electron pairs; it is not a literal picture of fixed electron positions. Molecular geometry and electron-domain geometry are related but not identical when lone pairs are present.

Count total valence electrons, adjusting for overall charge. Connect atoms, place remaining electrons, and consider multiple bonds when needed. Formal charge is an accounting tool that can help compare plausible structures; it is not the same as a directly measured partial charge. Structures must preserve the total electron count.

Some species require resonance descriptions. Resonance structures are alternative drawings of one delocalized electronic structure, not molecules rapidly switching between ordinary single and double bonds. Equivalent bonds in the actual species can have lengths between typical single and double bonds.

Electron domains repel and tend to arrange to reduce repulsions. A single, double, or triple bond counts as one domain in a basic VSEPR description. Molecular geometry describes the positions of atoms, while electron-domain geometry also includes lone pairs. Two species with the same number of domains can therefore have different molecular shapes.

Lone pairs can compress bond angles relative to idealized values because their electron density occupies space near the central atom. Use the actual electron-domain count and lone-pair arrangement rather than inferring shape from the number of attached atoms alone.

A polar bond has an uneven electron distribution. A molecule’s overall dipole depends on both bond dipoles and geometry. Symmetrically arranged dipoles can cancel; an asymmetric shape can leave a net dipole. The statement “contains polar bonds” is not sufficient to conclude that a molecule is polar.

For example, ideal linear CO₂ has two opposing C–O bond dipoles and no net permanent molecular dipole. Bent H₂O has bond dipoles that do not cancel. Geometry also matters in molecular recognition because a binding site encounters a three-dimensional arrangement of charge and shape.

Network covalent solids have covalent connections across an extended structure and often require substantial energy to disrupt. Molecular solids are held together by intermolecular attractions between discrete molecules, while ionic solids are stabilized by electrostatic interactions among ions. These structural categories help explain melting points and conductivity.

Solid ionic compounds generally do not conduct well because the ions are fixed in place. Melting or dissolving can make ions mobile. A molecular liquid does not become an electrolyte simply because it is liquid; mobile charged particles are needed. Identify the particles that carry charge in each phase.

Count all valence electrons, adding for negative charge and subtracting for positive charge. Choose a reasonable skeleton, connect atoms with single bonds, complete outer-atom octets where appropriate, and place remaining electrons on the central atom. If needed, form multiple bonds while respecting relevant octet exceptions. Recount the electrons at the end.

Formal charge assigns half of each bonding pair to each bonded atom. Compare plausible structures using charge placement and chemical context, but do not confuse formal charge with actual partial charge. Resonance structures differ in electron placement while retaining the same atomic connections. The real molecule is not rapidly switching between drawings; the drawings contribute to a model of delocalized electron density.

A single, double, or triple bond counts as one electron domain in the usual VSEPR model. Lone pairs also count as domains and can alter bond angles. First determine the arrangement of all domains, then describe the positions of atoms to name the molecular shape. For example, four domains can produce tetrahedral, trigonal-pyramidal, or bent molecular shapes depending on lone pairs.

Bond dipoles are vectors. A molecule with polar bonds can be nonpolar when the geometry makes their effects cancel. Conversely, an asymmetric arrangement can give a net dipole. Use the actual three-dimensional geometry rather than a flat Lewis drawing. A claim about polarity should name both the polar bonds and whether their orientations cancel.

Ionic solids consist of extended arrangements of ions. Molecular solids contain discrete molecules attracted to one another. Network covalent solids contain extended covalent bonding, and metals contain mobile electrons in an extended structure. These categories lead to different explanations of melting, conductivity, and mechanical behavior.

An ionic solid can fail to conduct while solid because its ions cannot move freely, yet conduct when molten or dissolved if mobile ions are present. A metal conducts through mobile electrons. Melting a molecular solid generally overcomes intermolecular attractions rather than breaking every covalent bond within each molecule. Select the particles and interactions actually present before explaining a macroscopic property.

Count total valence electrons, adjusting for overall charge. Establish atom connectivity, place bonds, complete appropriate outer shells, and then check the central atom and total electron count. Hydrogen has a duet, not an octet. A plausible drawing must account for every electron and the stated net charge. Formal charge is a bookkeeping model that helps compare contributors; it is not generally identical to a measured localized ionic charge.

Resonance contributors preserve atom positions and differ in electron placement. The molecule does not switch back and forth between isolated drawings. Equivalent contributors can predict equivalent bond lengths intermediate between simple single- and double-bond expectations. Do not move an atom or change total charge while claiming merely to draw another resonance contributor.

Before applying a trend, identify the species. A molecular ion, a neutral molecule, a metal and an ionic lattice are different structural situations. In an ionic solid, a formula such as NaCl describes a ratio throughout an extended structure, not a collection of isolated diatomic molecules.

VSEPR uses regions of electron density around a central atom. A single, double or triple bond each counts as one domain for the basic geometry. Lone pairs count in the electron-domain arrangement but are omitted when naming the arrangement of atoms. Thus an atom with three bonds and one lone pair has four domains, a tetrahedral domain arrangement and a trigonal-pyramidal molecular geometry.

Bond polarity follows unequal electron sharing, but molecular polarity is a vector sum. Symmetrically arranged equal bond dipoles can cancel. CO₂ has polar bonds yet no permanent molecular dipole in its linear structure; a bent arrangement can prevent cancellation. A formula alone may not reveal the geometry, so establish the Lewis model first.

Use the complete argument: electronegativity difference gives bond dipoles; geometry determines their directions; their vector sum determines a net dipole. Merely stating “it has a lone pair” or “it contains oxygen” is not a generally sufficient polarity explanation. Consider whether the actual arrangement has symmetry and whether the surrounding atoms are equivalent.

Conductivity requires mobile charged particles. Ions in an ordinary ionic crystal are constrained, while melting or dissolving can allow them to move. A metal conducts through mobile electrons in its bonding model. A molecular substance can dissolve without producing appreciable ions, so solubility and electrical conductivity are not synonyms.

Melting a molecular solid usually overcomes attractions between molecules; breaking a covalent network disrupts bonding throughout an extended structure. This distinction explains why a small molecular substance and a network solid made from similar elements can have very different properties. Avoid calling all attractions “bonds” without specifying which particles are interacting.

A potential-energy curve has a minimum at a preferred separation because attractive and repulsive contributions balance there. At much shorter distances, repulsion becomes large. The depth of the energy well relates to separation energy, whereas the position of the minimum relates to bond length. Neither axis alone establishes every macroscopic property of a substance.

Count total valence electrons, adjusting for ionic charge. Choose a plausible skeleton, connect atoms, complete appropriate outer shells, and place remaining electrons. If needed, use multiple bonds to satisfy electron requirements while evaluating formal charges. A structure must conserve the total electron count.

Formal charge is an accounting tool, not the same as measured partial charge. Resonance structures share atom positions and differ in electron placement; the actual molecule is not rapidly switching between separate drawings. Equivalent resonance can make bonds intermediate between simple single and double descriptions.

Electron-domain geometry counts bonding regions and lone pairs around a center, while molecular geometry describes atom positions. A double bond counts as one domain for this purpose. Lone pairs affect shape and bond angles, and molecular polarity depends on vector cancellation of bond dipoles in the actual geometry.

An ionic solid’s rigid lattice prevents ions from moving freely, while molten or dissolved ions can carry current if mobile charged species are present. A metal conducts through mobile electrons. A molecular solid can contain polar molecules yet be a poor conductor because polarity alone does not supply freely mobile charge carriers.

Network covalent solids contain extended bonded structures rather than separate small molecules held together only by intermolecular forces. Their properties depend on the network and bonding; avoid assigning all solids to the same melting or conductivity rule.

For a bond-energy curve, the minimum indicates a stable separation in the model. Shorter distances produce strong repulsion; separating the pair from the minimum requires energy. The depth of the potential well and the position of its minimum represent different properties, so a shorter bond is not simply the same statement as a deeper well.