Structure: Classification of Matter

A homologous series is a family of compounds sharing the same general formula and functional group, with each member differing by a $-{\text{CH}}_2-$ unit. Physical properties like boiling point change gradually along the series, but chemical behaviour stays essentially constant — because reactivity is controlled by the functional group, not the hydrocarbon backbone. Recognising the functional group instantly tells you which reactions and tests apply.

Find the longest continuous chain that contains the principal characteristic group, number it to give that group the lowest possible locant, then cite substituents as prefixes in alphabetical order. When multiple functional groups are present, only the highest-priority one gets the suffix: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine, with alkenes, alkynes, and halogens always cited as prefixes only.

Structural isomers share a molecular formula but differ in atom connectivity, split into three types: chain isomers (different branching), position isomers (functional group at a different locant), and functional group isomers (an entirely different group — ${\text{C}}_3{\text{H}}_6\text{O}$ can be propanal or propanone). The index of hydrogen deficiency (IHD) tells you how many rings or π bonds a formula must contain, which narrows down possible structures fast.

A primary (1°) carbon is attached to only one other carbon, secondary (2°) to two, and tertiary (3°) to three — for alcohols, a tertiary carbon has no H left to remove, which is exactly why tertiary alcohols resist oxidation completely. Primary alcohols oxidise stepwise: distilled off as it forms stops the reaction at the aldehyde, while reflux with excess oxidant pushes all the way to the carboxylic acid. Diagnostic tests confirm structure: Na metal effervescence means O–H present, Tollens'/Fehling's positive means an aldehyde, and a positive iodoform test means a ${\text{CH}}_3\text{CO-}$ or ${\text{CH}}_3\text{CH(OH)-}$ group.

Elements are arranged by increasing atomic number into periods (rows = number of occupied electron shells) and groups (columns = number of valence electrons), so elements in the same group share predictable chemistry. Four blocks are named after the sub-shell being filled last: s-block (groups 1–2, plus He), p-block (groups 13–18 except He), d-block (groups 3–12), and f-block (lanthanides/actinides). Metals sit to the left and centre, non-metals to the upper right, separated by the staircase line (B, Si, Ge, As, Sb, Te, At), with metalloids straddling that boundary.