Consider the following selected postulates of dalton's atomic theory: (i) each element is composed of extremely small particles called atoms. (ii) atoms are indivisible. (iii) atoms of a given element are identical. (iv) atoms of different elements are different and have different properties. which of the postulates is/are no longer considered valid?

Only postulates II e III are no longer considered valid.

Explanation:

Dalton's atomic theory was the first to tried to explain the composition of the matter. In his theory, Dalton proposed that the elements are composed of small particles, called atoms. Nowadays, the quantum theory (which is the most accurate) confirms this postulate.

The quantum theory and the evidence observed in nature also confirm the postulate IV, each element has its properties (reactivity, boiling point) which are different from the others one.

However, nowadays we know that the atom is divisible in small particles: protons, neutrons and electrons, that's why postulate II is no more valid. We also know the isotopes: atoms from the same element that have different masses: they have different numbers of neutrons, so postulate III is no longer valid.

i do not get it   at all