Chemical nucleation involves cluster growth by chemical reactions. In the case where clusters grow via a simple sequence of reversible chemical reactions, a summation expression for the steady-state nucleation rate can be derived. However, in many cases the chemical pathway to cluster growth is more complicated, and requires solving a set of species population balance equations that depend on the specific chemical system. Two examples are considered: soot nucleation in hydrocarbon combustion and nucleation of silicon particles in thermal decomposition of silane. In both cases, chemical kinetic mechanisms have been developed that allow for numerical simulations of particle formation. Soot nucleation is believed to proceed through the formation of polycyclic aromatic hydrocarbons. Models have been developed for the formation of the first aromatic ring and for subsequent growth, either through reaction with small molecules or by coagulation. Silicon nucleation from silane involves a large set of silicon hydride species, which can be grouped into classes according to their structure and reactivity, facilitating estimates of their free energies and reaction rate constants.

Gas-phase nucleation of condensed-phase particles is important in many contexts, including interstellar dust formation, air pollution, global climate change, combustion and fires, semiconductor processing, and synthesis of nanoparticles for practical applications. Nucleation occurs via the growth of atomic or molecular clusters to “critical size” – the size where further growth is irreversible. These critical-size clusters are the nuclei for particle formation, and the growth of clusters to the size of nuclei is the concern of nucleation theory. Various scenarios occur, including single-component homogeneous nucleation from a supersaturated vapor, multicomponent nucleation, ion-induced nucleation, chemical nucleation, and nucleation in plasmas. Classical nucleation theory, which treats small clusters as having the same properties as the bulk condensed phase, is still widely used to estimate nucleation rates for many kinds of systems. However, it is anticipated that atomistic approaches based on computational chemistry will increasingly be used to facilitate more accurate predictions of gas-phase nucleation rates for substances and chemical systems of interest.