Atoms are particles with mass. They are the smallest unit of mass that defines the chemical elements. Every solid, liquid, gas, and plasma is made up of atoms. Atoms are very small: the size of atoms is measured in picometers (trillionths of a meter).
John Dalton FRS (6 September 1766 - 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour).
Niels Henrik David Bohr (7 October 1885 - 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter of scientific research.