Atom Condition is a condition on word formation rules proposed in Williams (1981) which says that:
- the attachment of afx to Y can only be restricted by features realized on Y.
English has a class of latinate affixes that can only attach to latinate roots. A clear example is the nominalizing suffix -ion. This suffix shows all kinds of allomorphy, and the choice of the allomorph depends on the choice of the root (deduce:deduc-tion, compose:compos-ition). On the assumption that deduce and compose are formed by prefixing de- to the root duce, and con- to pose, Siegel predicts by her Adjacency Condition that the suffixation rule for -ion will not be sensitive to idiosyncratic features of the embedded roots. But other prefixed forms with the same roots (reduction, production, deposition, proposition) show incontrovertibly that it is the root which determines the allomorphy, even after prefixation. To account for this, Williams assumes that roots such as duce en pose function as the head in prefixed words such as deduce and compose. Given the further assumption that any feature marked on the head of a construction will percolate up to the node that dominates that construction, the prefixed forms deduce and compose will inherit the properties of their heads. By replacing the Adjacency Condition with his Atom Condition, Williams is able to account for the fact the affixation rule which attaches -ion can (indirectly) refer to the root features.
This condition is intended to replace Siegel's (1977) Adjacency Condition, and to explain a class of systematic exceptions to its predecessor.