structure of the atom Flashcards
What are cathode rays?
Cathode rays were produced in sealed vacuum cathode ray tubes consisting of two electrodes. The negative electrode is the anode, and the positive electrode is the cathode. When a high voltage is connected between the electrodes, an invisible ray (cathode ray) travels from the cathode to the anode.
What are the charged particle properties of cathode rays?
- Can be deflected by magnetic fields and electric fields
- rays carry energy and momentum
- attracted to positive charges
What are the em wave properties of cathode rays?
- Identical regardless of material used
- emanates from cathode and travels in straight line
- would cast shadow of solid object
- could penetrate thin metal foils
Outline the maltese cross experiment
- A maltese cross was placed in the path of the cathode ray in a cathode ray tube.
- A perfect shadow of the maltese cross was formed on the far end of the tube, indicating that cathode rays travel in straight lines like light.
Outline the electric plates experiment (cathode rays)
With electric plates, the rays always were deflected towards the positive plate, indicating that the cathode ray has a negative charge.
Outline the paddle wheel experiment
When the glass wheel was struck by the cathode ray, the glass wheel slowly rotated, indicating that the cathode rays have momentum and therefore mass. This supported the particle side of the controversy as electromagnetic waves are massless.
Outline JJ Thomson’s charge to mass ratio experiment
He placed electric and magnetic fields at right angles to each other so that their forces would completely negate each other. A fluorescent screen showed where the particle hit. By equating the electric force and the force experienced by the magnetic field, he found v=E/B. Thomson then removed the electric field and applied only the magnetic field, so that the cathode ray would deflect and the radius of the path could be physically determined.
Equating the magnetic force to the centripetal force, mv/r=qB, then q/m=E/rB^2 = 1.76 x 1011 Ckg–1 (same for all materials used)
Outline Milikan’s Oil-drop experiment
- Millikan used an atomiser to spray droplets of oil into a chamber in which the oil droplets eventually would fall into a strong electric field.
- Within this field (between charged metal plates), X-ray was used to ionise gas molecules which produced free electrons that attached themselves to the oil droplets. Thus, the oil droplets acquired negative charge
- The electric field was orientated such that the positive terminal was at the top. Therefore, the electric force that acts on the oil droplets would have an upward direction.
- At the same time, oil droplets would also experience downward gravitational force, upward buoyance force and drag force (direction depends on velocity of oil droplets).
- By manipulating the voltage (strength of the electric field), Millikan was able to determine the electric charge of the electron by equating f=eq and e=v/d—> E=V/d, F=qV/d, mg=qV/d, q=mgd/V
How did Millikan then calculate the mass of the oil droplets?
He measured the terminal velocity of oil droplets when the electric field is switched off, therefore only gravitational and drag force present. Millikan was able to use their velocity to calculate the radius and hence their volume. density =mv
Outline Thomson’s charge to mass ratio conclusion
Thomson determined the cathode ray (electron)’s charge to mass ratio which proved that it was a particle rather than a wave.
Outline Milikan’s oil-drop conclusion
Millikan determined the charge of the electron with very high accuracy and showed that charge is quantised.
Outline Rutherford’s proposed model of the atom (nuclear model)
- He proposed that an atom consists of a positive centre (nucleus) and is surrounded by smaller and negatively charged electrons.
- He suggested that an atom consists of mostly empty space.
Outline Rutherford’s gold foil experiment
Rutherford, Geiger and Marsden conducted a series of experiments in which positively charged alpha particles (helium nuclei) were fired at a thin sheet of gold foil which was surrounded by a ring-shaped fluorescent screen for detecting the alpha particles.
What were the results of the gold foil experiment?
Most of the projected alpha particles passed through the gold foil (indicating the largely empty atom) but there were a few which were deflected or reflected back (indicative of the small positive nucleus).
What was the significance of gold foil experiment?
Thomson proposed that an atom only consists of a sea of electrons scattered within a positive ‘cloud’. Had his model been correct, all the alpha particles should have passed through the gold foil with minimal scattering.
