Which of the following statements is not true about an atom?

Which of the following postulates of Dalton's atomic theory is challenged by the next generation of scientists?

$(a)$ Calculate the mass of $0.5$ mole of oxygen atoms.
$(b)$ Calculate the number of molecules of glucose present in its $90 \ g$ (molecular mass of glucose is $180 \ u$). (Avogadro constant $= 6.022 \times 10^{23} \ mol^{-1}$)
$(c)$ Calculate the number of moles of water in $2 \ g$ of water.
(Atomic mass of $H = 1 \ u, O = 16 \ u$)

Define formula unit mass of a compound. Calculate the formula unit mass of:
$(a)$ $Na_{2}CO_{3} \cdot 10H_{2}O$
$(b)$ $CuSO_{4}$
(Given atomic mass of $Na = 23 \text{ u}, C = 12 \text{ u}, O = 16 \text{ u}, Cu = 63.5 \text{ u}, S = 32 \text{ u}, H = 1 \text{ u}$)

Give the formulae of the compounds formed from the following sets of elements:
$(a)$ Calcium and fluorine
$(b)$ Hydrogen and sulphur
$(c)$ Nitrogen and hydrogen
$(d)$ Carbon and chlorine
$(e)$ Sodium and oxygen
$(f)$ Carbon and oxygen

Which of the following statements is not true about an atom?