QCEphysics

The atom

describe the ruthford's model of the atom including it's atoms

- Rutherford expected alpha particles to pass right through the (plum pudding) atom, but huge deflections were found. From this, he concluded that:

• The atom is mostly empty space with a tiny, very dense postively charged nucleus surrounded by negatively charged electrons

• Nearly 100% of the atom's mass is contained in the nucleus

• The number of electrons = number of positive charges

• The nucleus is 100 000 times smaller than the atom

- Limitations of Rutherford's model include:

• According to Maxwell's electromagnetic equations, any electron revolving in a circular path should radiate electromagnetic energy. The electron would continuously lose energy and spiral in towards the nucleus, making the atom unstable and implode. The model did not consider this
• The Rutherford model allowed electrons to have any energy, and therefore the light produced from them would be a continuous spectrum (where every wavelength of light is produced). This does not match the emission spectra observed, where only specific wavelengths are seen

describe the bohr model of the atom and how it addresses the limitation of Rutherford's model

- Bohr's postulates include

• Electrons in an atom exist in stationary states

▪ They do not emit energy while orbiting the nucleus

• Transitions between stationary states absorbs or emits electromagnetic radiation (photons)

▪ Energy of the photon is given by Δ E = hf

▪ If the atom absorbs too much energy, the outermost electron will be removed. This is the ionisation energy

• Angular momentum of a stationary electron is quantised

▪ Angular momentum states that for circular motion, the momentum of a particle in which the velocity vector points along the radius of the circular path and is equal to mvr

▪ Electrons must orbit at a fixed radii

explain how the Bohr model of the hydrogen atom integrates light quanta and atomic energy states to explain the specific wavelengths in the hydrogen line spectrum

- Bohn made the following observations

•  The absorption spectrum of hydrogen showed that the hydrogen atom was only capable of absorbing a small number of different frequencies - therefore the absorbed energy was quantised
• Emission spectrum showed that hydrogen atoms emitted quanta of exact energy values
• If the frequency (and therefore energy) of the incident light was below a certain value (ionisation energy) the light would pass through the atom without absorption. Photons of light with energies above the ionisation value for hydrogen are continously absorbed
• Electrons will only be excited if an incoming photon's energy exactly matches the difference in the energy between the two levels

▪ Photons cannot be left with 'leftover energy'

▪ If the atom is bombarded with electrons instead of photons, incoming electrons can give up some of their energy and keep the rest

- Limitations of Bohr's model include that it could not explain:

• The spectra of large atoms
• Relative spectra intensity - why the intensity of spectral lines were not equal, suggesting that some transitions were more favoured than others
• Hyperfine spectral lines
• Zeeman effect - when hydrogen gas was excited in a magnetic field, the emission spectrum showed a splitting of lines (now known due to magnetic field of the electron)
• Stationary states

solve problems involving the line spectra of simple atoms using atomic energystates or atomic energy level diagrams

 Atomic energy level diagram

describe wave-particle duality of light by identifying evidence that supports the wave characteristics of light and evidence that supports the particle characteristics of light.

 - Wave-particle duality states that every particle or quantum entity may be partly described in terms of particles or waves

• In general if light energy is interacting with other forms of light energy, then wave behaviour model is the best explanation
• If light is interacting with matter, particle behaviour model is the best explanation

- de Broglie argued that if light had the properties of a particle but still had a wavelength, particles such as electrons should have wavelength

• Proposed that electron orbitals are circular standing waves
• To avoid destructive interference occurring, the length of the orbital must equal to a whole number of wavelengths. This determines which orbitals are allowed and which are not

- de Broglie's predication was experimentally verified by Davisson and Germer when they showed that electrons scattered by crystals produces a characteristic wave diffraction pattern

Students should be able to recall that waves transfer energy, recall that light
cannot be modelled as a mechanical wave because it can travel through a
vacuum, recall that a wave model of light can explain interference and
define the concept of resonance in a mechanical system (Unit 2
Topic 2: Waves).

- Waves are classified based on how they transfer energy

• Mechanical waves are waves that require an elastic medium for the transfer of energy

▪ Not light as light can travel through a vacuum