Why is Dalton's atomic theory not valid?

Because: * Failure to explain the existence of isotopes or isobars- hence atoms of the same element MAY be different. * Failure to explain the affinity of atoms of same or different kind to combine and form a Compound (molecule). read more

John Dalton's atomic theory is indeed valid but just has alot of amendments, that's why it's still taught in school. Ammenents like the fact that he defined atoms as small indivisible particles, where as we all know that, that's how we just our nuclear weapons (by dividing atoms). read more

Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. read more

Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. Dalton based his theory on the law of conservation of mass and the law of constant composition. read more