Answer is: particles.
Dalton's atomic theory:
a) All matter consists of indivisible particles called atoms.
b) Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.
c) Atoms cannot be created or destroyed.
d) The ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
e) Atoms of same element can combine in more than one ratio to form two or more compounds.
f) The atom is the smallest unit of matter that can take part in a chemical reaction.