An aggregate of a small number of atoms, molecules or ions. An embryo of such a size that the Gibbs energy at constant pressure and temperature is a maximum is called a critical embryo. An embryo larger than a critical embryo is called a homogeneous nucleus. @[email protected] may be formed by condensation methods. When a condensation method is applied, molecules (or ions) are deposited on nuclei which may be of the same chemical species as the @[email protected] (@[email protected]) or different (@[email protected]).