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CHEMISTRY, M1 EQ-Bank 6

Describe the process by which emission line spectra are formed.   (4 marks)

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  • Electrons in atoms exist in discrete energy levels.
  • When an atom absorbs energy (e.g. from heat or electricity), an electron is excited to a higher energy level.
  • The excited electron is unstable and will eventually fall back to a lower energy level.
  • As it does so, it releases energy in the form of a photon of light.
  • The energy of the photon corresponds to the difference between the two energy levels, so only specific wavelengths of light are emitted.
  • Passing this light through a spectroscope produces a series of discrete coloured lines known as the emission spectrum.
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  • Electrons in atoms exist in discrete energy levels.
  • When an atom absorbs energy (e.g. from heat or electricity), an electron is excited to a higher energy level.
  • The excited electron is unstable and will eventually fall back to a lower energy level.
  • As it does so, it releases energy in the form of a photon of light.
  • The energy of the photon corresponds to the difference between the two energy levels, so only specific wavelengths of light are emitted.
  • Passing this light through a spectroscope produces a series of discrete coloured lines known as the emission spectrum.

CHEMISTRY, M1 EQ-Bank 2 MC

Which scientist developed the quantum mechanical model of the atom, describing electrons as existing in orbitals `(s, p, d, f)` defined by regions of space with the highest probability of finding an electron?

  1. Niels Bohr
  2. J. J. Thomson
  3. Erwin Schrödinger
  4. John Dalton
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\(C\)

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  • Schrödinger used wave mechanics to describe electrons as occupying orbitals `(s, p, d, f)`, regions of space with the highest probability of finding an electron rather than fixed paths.

\(\Rightarrow C\)

CHEMISTRY, M1 2010 HSC 35a

Identify the element in period 3 of the periodic table that has the highest electronegativity and justify your choice.   (3 marks)

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  • Most electronegative element in period 3 is chlorine \(\ce{(Cl)}\).
  • The electronegativity is a measure of an atom’s ability to attract electrons to itself.
  • Electronegativities increase across periods in the periodic table from left to right or as the number of valence electrons increases.
  • Fluorine is the most electronegative element with all other electronegativities relative to this.
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  • Most electronegative element in period 3 is chlorine \(\ce{(Cl)}\).
  • The electronegativity is a measure of an atom’s ability to attract electrons to itself.
  • Electronegativities increase across periods in the periodic table from left to right or as the number of valence electrons increases.
  • Fluorine is the most electronegative element with all other electronegativities relative to this.

CHEMISTRY, M1 2012 HSC 37e

Evaluate the contribution of the Bohr model to the development of our understanding of the structure of the atom.   (7 marks)

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  • The Bohr model of the atom was developed to use the quantisation of energy to explain the emission and absorption spectrum of hydrogen.
  • It proposed that the single electron of the atom was confined to defined orbits around the nucleus, somewhat analogous to a planet’s orbit around its star.
  • For the electron to change to a different orbit, energy had to be absorbed (to go to a higher energy orbit) or emitted (to go to a lower energy orbit).
  • The energy absorbed or emitted is well-defined, giving rise to sharp absorption or emission lines. No other changes in energy are allowed.
  • The model was an important step towards understanding and accepting the quantum view of the atom and introduced the idea of energy levels (shells) to describe the electronic configuration of atoms.
  • The Bohr model could not be applied to atoms other than hydrogen and did not provide any explanation for the quantisation.
  • These restrictions and limitations of the model were recognised by Bohr and the model was not meant to be used beyond the hydrogen atom, but the attractiveness of the simplicity of the model has ensured continued use and propagates the incorrect concept of electrons as particles orbiting a nucleus.
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Evaluation Statement

  • The Bohr model was partially effective in advancing atomic understanding.
  • This evaluation is based on: explaining hydrogen spectra, introducing quantum concepts and applicability to other atoms.

Explaining Hydrogen Spectra

  • Bohr’s model successfully addressed hydrogen’s emission and absorption lines.
  • Evidence supporting this includes: correctly predicted wavelengths of spectral lines using quantised energy levels.
  • The model clearly fulfilled its primary purpose by mathematically linking electron transitions to observed spectra.
  • This strongly meets the criterion as it solved a major scientific puzzle of the time.

Applicability Beyond Hydrogen

  • The model fails to achieve accuracy for multi-electron atoms.
  • Evidence indicates that predictions become increasingly inaccurate with more electrons.
  • Bohr himself recognised these limitations, restricting the model to hydrogen.
  • This criterion is insufficiently met, revealing fundamental flaws in the model’s underlying assumptions.

Introduction of Quantum Concepts

  • The model effectively introduced revolutionary ideas like energy levels (electron shells) and quantisation.
  • These concepts became foundational for quantum mechanics development.
  • While the orbital concept was incorrect, it adequately fulfilled the need for a transitional model.
  • This satisfactorily meets the criterion of advancing theoretical understanding.

Final Evaluation

  • Weighing these factors shows the Bohr model was highly effective as a stepping stone but limited as a complete theory.
  • The strengths outweigh the weaknesses because it successfully bridged classical and quantum physics.
  • Although inadequate for complex atoms, it proves essential for teaching atomic concepts.
  • The overall evaluation demonstrates its crucial but transitional role in atomic theory development.