E.1 Structure of the Atom Flashcards
Rutherford’s Experiment
Alpha particles are fired at a thin gold foil. A screen detected where they went.
Why was gold used in Rutherford’s experiment?
Can be hammered into very thin sheets, large atoms.
Why were alpha particles used in Rutherford’s experiment?
Large enough mass to not be deflected by electrons. Positively charged to investigate charge distribution.
Expected results
If Thomson’s model was correct, alpha particles will pass through undeflected as charge is evenly distributed throughout the atom.
Observation - most of the alpha particles passed through the foil without their path being affected
Most of an atom is empty space
Observation - a significant number of alpha particles had their path deflected
The positive charge of an atom is concentrated at its centre
Observation - some alpha particles bounced straight back from the gold foil
Most of the mass of an atom is in the nucleus
The nucelus forces
Strong nuclear force which acts at a short range between the nucleons. This balances the electrostatic repulsion between protons which acts at a long range.
What happens if there are too many protons in the atom?
Repulsion>strong nuclear force
Strong nuclear force distance
Repulsive below 0.5fm, attractive between 0.5 and 3fm, zero beyond 3 fm.
Why do larger nuclei have more nuetrons?
In order to keep the forces within the nucleus balanced. If not, the nucleus decays to form a more stable smaller nuclei radioactively.
Isotopes
Different nuclei of an element that have a different number of neutrons.
Different forms on a nucleus
Nuclides
Limitations of Rutherford’s model
If electrons were orbiting the nucleus, they would emit electromagnetic radiation and so lose energy and spiral to the nucleus.
J to eV
Divide by 1.6 x 10^-19
eV to J
Multiply by 1.6 x 10^-19
Emission spectrum
When atoms gain energy they emit a range of colours of light. It is best seen when the element is a low pressure gas and gives off thermal or electrical energy.
Absorption spectrum
Light containing all colours is shone through a sample and a range of wavelengths of light will be absorbed by the atoms in the gas.
How do electrons move between energy levels?
Absorbing or emitting photons of light. These contain the precise amount of energy needed to move.
What are the missing lines on an absorption spectrum?
The electrons absorbing photons that contain the specific amount of energy.
Photon
A packet of energy that has proportional energy and frequency
What do emission and absorption spectra allow us to determine?
The chemical composition of substances
How do emission and absorption spectra allow us to chemical composition?
When a substance is heated in its gaseous form, the frequencies of light that it emits depend on the chemical elements that make up the substance.
How do we tell the elements within stars?
We assume that light produced in the centre of a star is a continuous spectrum. As light travels to the surface of the star, different elements absorb different frequencies, leaving behind an absorption spectrum.
