Who described atoms that they are small hard particles?

John Dalton.

N.B.: John Dalton [born: September 6, 1776; died: July 27, 1844, at age 67] was an English chemist and physicist who claimed matter was made up of tiny particles and that these particles were identical for any given kind of matter. Moreover, he claimed these particles were indivisible and could neither be created nor destroyed in any chemical process, only be altered in the manner they were grouped.