Question bank — Dalton's atomic theory — postulates and limitations

False ::: Dalton assumed the atom was indivisible, but atoms are built from electrons, protons and neutrons (see Discovery of the Electron (Thomson)). The atom is only the smallest unit that keeps an element's chemical identity.

False ::: Conservation of mass follows from postulate 3 (atoms are neither created nor destroyed) plus postulate 2 (each atom's mass is fixed). Whole-number ratios (postulate 4) instead explain definite and multiple proportions.

False ::: Only the clause "atoms of an element are identical in mass" fails. The core claim — matter is atomic and combines in fixed ratios — remains the foundation of chemistry. A theory can be refined without being discarded.

False ::: That is exactly what isotopes violate: same proton count (same element) but different neutron count, hence different mass — e.g. ${}^1\text{H},{}^2\text{H},{}^3\text{H}$. See Isotopes and Isobars.

${}^{40}\text{Ar}$ and ${}^{40}\text{Ca}$ have the same mass number, so Dalton would call them the same element. (T/F) True (and that is the flaw) ::: They are isobars — same mass number $40$, but different elements (18 vs 20 protons). Dalton's postulate 2 linked "same mass" to "same element", which isobars break.

False ::: Non-stoichiometric solids like wüstite ${\text{Fe}}_{0.95}\text{O}$ have fractional formulae because of lattice vacancies. Dalton's postulate 4 holds for most compounds but not these.

False ::: In nuclear reactions a tiny mass converts to energy via $E=m{c}^2$, so rest-mass is not conserved. This breaks postulate 3. See Mass–Energy Equivalence (E=mc²). (In ordinary chemical reactions the change is far too small to measure, so Dalton looks correct there.)

False ::: Definite proportions concerns one compound (water is always $1:8$ H:O). Multiple proportions compares two or more compounds of the same elements (CO vs CO${}_2$ give $1:2$).

False ::: The ratio is a fingerprint of the compound, independent of sample size — it stays $1:8$. Only the absolute masses double.

Wrong link ::: Conservation of mass comes from postulate 3 (atoms not created/destroyed). Postulate 4 explains the ratio laws (definite & multiple proportions), not conservation.

Error ::: Multiple proportions needs two compounds. You compare oxygen masses per fixed carbon: CO gives 16 g, CO${}_2$ gives 32 g, ratio $16:32=1:2$. A single compound cannot show the law.

Error ::: Chemistry is governed by electrons (proton number), which isotopes share. Different neutron count barely affects chemistry — that is why Dalton's identical-atom assumption worked so well in practice.

Error ::: Chemical processes cannot split an atom, but nuclear processes (fission, radioactivity) can. Dalton's "indivisible" claim is a chemical truth, not a universal one.

Error ::: He said atoms of the same element are identical, and atoms of different elements differ. That distinction is the whole point of postulate 2.

${\text{Fe}}_{0.95}\text{O}$ proves atoms can be broken into fractions." Error ::: The atoms are still whole; the formula is fractional because some iron sites in the crystal lattice are empty (vacancies). It breaks the whole-number-ratio postulate, not the indivisibility of atoms.

require that matter be made of discrete atoms? Because you can only combine countable, indivisible units in integer amounts ::: You cannot use "half a lump" of an element, so the observed small-integer ratios only make sense if matter comes in discrete pieces.

$m_B$ cancel out when deriving multiple proportions? Because both compounds are built from the same kind of B atom ::: In $\frac{x{m}_B}{y{m}_B}$ the identical $m_B$ divides out, leaving the pure count ratio $x/y$ — the whole-number signature the law describes.

quantitative for the first time? It turned vague "mixing" into countable atoms with fixed masses ::: Once matter is fixed-mass countable units, mass measurements translate directly into atom counts and ratios — the seed of the mole concept.

$E=m{c}^2$ exists? The mass converted in chemical bonds is immeasurably tiny ::: Chemical bond energies are far too small to produce a detectable mass change, so for lab chemistry Dalton's postulate 3 is effectively exact.

Because its core mechanism can survive while specific clauses get refined ::: Dalton's clauses about identical mass and indivisibility failed, but the atomic, fixed-ratio core underlies all of modern chemistry (see Modern Atomic Theory).

atomic mass a whole number? Usually not ::: The measured atomic mass is a weighted average over isotopes (e.g. chlorine $\approx 35.5$), so it is generally non-integer even though each individual atom's mass number is integer.

No — the system was not closed ::: Mass is conserved for the whole system; the gas carried mass away. Dalton's law needs a sealed system to be verified.

mixture like salt water? No ::: Definite proportions applies to compounds, which have fixed composition. Mixtures can have any ratio, so they are outside the law's scope.

$2.5:1$ — does multiple proportions fail? No — rescale it ::: $2.5:1=5:2$, still small whole numbers. Always reduce to lowest integer terms before judging.

no reaction occurs? Mass is trivially unchanged ::: With no rearrangement, every $N_i$ and $m_i$ stays put, so ${\sum }_i{N}_i{m}_i$ is identical before and after — the law holds in the degenerate "nothing happens" case too.