John Dalton

He studied the history of the atom and unsderstood the Greek theory of the atom. He even made his own theory that atoms can't be divided into smaller pieces

Dimitri Mendeleev

He was a Russia scientist and the first person to organize the elements into a Periodic table. In 1872 he organized the elements by mass, but not all elements were known so he left spaces for elements the were expected to be discovered.

Henry Moseley

He was a British scientist and he took Mendeleev's work of the Periodic table and improved it. In 1913 he organized the elements by atomic number and this was better because isotopes were later found. His is the Periodic table that we use today.

Niehls Bohr

HE is a Danish scientist that discovered energy levels of electrons in the electron cloud. The Bohr diagram is named after him.