The present study was conducted for two purposes. The first was to test the relations between the schema of atomism and the concept of density, and to examine the effects of Piagetian stage and M-capacity on these relations. This was accomplished by giving training for the schema of atomism to children at different stages and with varying M- capacities. The second purpose was to test the effects of age (9-10 versus 11-12 years) and IQ (gifted versus nongified children) as variables that would influence the outcome of training. A total of 120 children were tested. The findings were that the effects of the training and the IQ variables were significant while the effect of the age variable was not. When stage and M-capacity were covaried the effects of training and IQ remained significant. Implications for issues of developmental theory and educational practice are discussed.