atomic theory
noun /əˌtɒmɪk ˈθɪəri/
/əˌtɑːmɪk ˈθiːəri/, /əˌtɑːmɪk ˈθɪri/
[uncountable] (chemistry, physics)- the theory that all elements are made up of small particles called atoms, which are made up of a central nucleus surrounded by moving electrons