Elements are made of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged.
John Dalton's Atomic Theory
What was wrong about his theories?
It was only much later that electrons were discovered, showing that atoms weren't indestructible solid objects, and later still that it was shown that atoms had a specific internal structure, the exact nature of which is still under investigation.
Also Wrong About...
The idea that all atoms of a given element are identical in their physical and chemical properties is not precisely true, as we now know that different isotopes of an element have slightly varying weights.