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?

The postulates ii) and iii) are no longer considered valid.

Explanation:

Nowadays, the postulate ii) Atoms are indivisible is no longer valid, because it is known now that atoms are formed by subatomic particles (protons, neutrons and electrons).

The postulate iii) Atoms of a given element are identical is currently no valid due to the fact that there are isotopes of certain elements (atoms of the same chemical element, with the same number of protons, but different number of neutrons). For example, hydrogen (with 1 proton alone, without neutrons) has two isotopes: deuterium (with 1 proton and 1 neutron) and tritium (with 1 proton and 2 neutrons).

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.