Learn how experiments led to the discovery of subatomic particles and how atomic models developed from Dalton and Thomson to Rutherford and Bohr. Understand atomic spectra, energy levels and the limitations of early models.
Atoms are not indivisible. Experiments with discharge tubes showed that atoms contain smaller particles. The three main subatomic particles are electrons, protons and neutrons.
Answer: Cathode rays consist of negatively charged electrons. They are attracted towards a positively charged plate by the electric field.
Answer: Thomson concluded that electrons are common constituents of atoms, because cathode rays showed the same essential properties even when the gas or electrode material was changed.
Answer: Cathode rays are streams of negatively charged electrons. Canal rays are positively charged particles; their properties depend on the gas in the discharge tube.
Answer: The masses of atoms could not be explained by protons and electrons alone. The neutron helped explain the extra nuclear mass and the existence of isotopes.
Answer: It shows that cathode rays carry a negative electric charge.
Answer: Electrons are not unique to one element; they are present in atoms of different elements.
Answer: Neutral rays would not be deflected by an electric field because they have no net electric charge.
Answer: Canal rays contain positively charged ions formed from the gas in the tube. The ions' mass and identity depend on which gas is used.
Answer: A helium nucleus contains two protons but has a mass close to four atomic mass units. Two neutrons account for most of the remaining nuclear mass, so protons and electrons alone cannot explain it.
Answer: The particles had no electric charge. Chadwick identified them as neutrons.
Thomson proposed that an atom was a sphere of positive charge with negatively charged electrons embedded in it. The model explained overall electrical neutrality but did not account for the results of alpha-particle scattering.
Most alpha particles passed through the thin gold foil, showing that most of an atom is empty space. A small number were deflected strongly, indicating a tiny, dense, positively charged nucleus containing most of the atom's mass.
Answer: Most of the atom's volume is empty space, so most alpha particles did not come close enough to the small nucleus to undergo large deflections.
Answer: They passed close to the positively charged nucleus and experienced strong electrostatic repulsion.
Answer: It could not explain (1) why orbiting electrons do not continuously lose energy and fall into the nucleus, and (2) why atoms produce discrete line spectra rather than continuous spectra.
Bohr proposed that electrons occupy only certain allowed energy levels. While an electron remains in an allowed orbit, it does not radiate energy in the Bohr model. Radiation is absorbed or emitted when an electron changes energy levels.
The energy of a photon is related to its frequency by E = hν. When an electron moves to a lower energy level, it emits a photon; when it moves to a higher level, it absorbs energy.
Answer: Electrons can have only certain discrete energy values. Each spectral line corresponds to a photon emitted or absorbed during a transition between two allowed energy levels.
Answer: In classical physics, an orbiting electron accelerating around the nucleus would radiate energy continuously and could have continuously changing energy. That would suggest radiation over a continuous range of frequencies, unlike the observed discrete hydrogen spectrum.
Answer: The evidence suggests the gas is hydrogen, because an element's line spectrum is characteristic of its allowed energy transitions. In practice, measurements would need to rule out mixtures and experimental error.
Answer: A continuous range of energy differences would permit a continuous range of photon frequencies. The observed sharp lines instead support quantised energy levels.
Answer: The statement is incorrect. Bohr's model explained hydrogen's line spectrum and atomic stability better than Rutherford's model, but it could not fully explain multi-electron atoms, fine spectral details, or line splitting in external electric and magnetic fields.
Answer: In Bohr's model, an electron in an allowed stationary orbit does not continuously radiate energy. It therefore does not spiral into the nucleus as predicted by the classical picture.
Answer: Each element has a different nuclear charge and electron arrangement, giving it a distinct set of allowed energy levels. Its possible electron transitions therefore produce a characteristic set of spectral lines.
Answer: Hydrogen has only one electron, so its energy levels can be described relatively simply. In multi-electron atoms, electron-electron repulsion and interactions make the energy structure more complex than Bohr's model can accurately represent.
Answer: It could not explain atomic stability and could not explain the discrete line spectrum of hydrogen.
Answer: It had no mechanism for restricting electrons to discrete energy levels. Bohr's quantised energy levels explained why only specific photon energies—and therefore specific spectral lines—are observed.