From the universe to the atoms

The Big Bang Theory

The universe has a hot and dense beginning and has been expanding ever since.

Began 13-8 billion years ago.

7 stages of Big Bang

1. The big bang event and inflation - singularity

2. Hot soup of electrons, photons, and subatomic particles

3. Nucleons from (protons & neutrons)

4. Nuclei form (but no atom yet), and universe is too hot for light to shine without scattering.

5. Atoms finally form (mostly hydrogen and helium) and light can shine without scattering

6. Gravity pulls the atom into stars and galaxies

7. Present day

Hubble’s law

Hubble’s constant (H) = gradient of the line

• inverse of gradient = value of time (estimation of age of universe)

v= H_od

V = recession velocity

Lifecycle of star

Matter and Anti-matter

• particles are always created in pairs

• For every matter there is an antimatter particle

• At the instant they form and because they are so close tgt, they immediately annihilate each other and convert to pure energy. During this rapid expansion and cooling of the universe slightly more matter was created than antimatter. This is why most of the universe that we detect and observe is now matter.

Fundamental forces in BBT

• gravitational

• electromagnetic

• strong and weak nuclear forces

The particles which meditate these forces are known as field particles

Helium and hydrogen ratio - BBT evidence

In BBT the ratio of hydrogen to helium is 3:1. Matches the composition of the universe: 75% hydrogen and 25% helium

Explains the abundance of hydrogen and helium - can be only formed by nuclear fusion rxn in the centre of the stars.

Evidence for expansion of universe

Red shift of light from galaxies.

Galaxies from all direction from our galaxies are moving away from us and those which are further away are travelling at greater speeds.

cosmic microwave background radiation (cmbr) /afterglow

• After the big bang, the universe began to cool, after 300000 years the universe was sufficiently cool enough for radiation to able to travel through space for the first time.

• The heat radiated from this point in time as gamma rays but since the universe has expanded the radiation is now detected as microwave rays.

• Explains not only that all masses began as dense form and been expanded infinitely since then, but it can determine the time of exploasion.

Nucleosynthesis/fusion for each star

stars produce energy via nuclear fusion.

Process in which atomic nuclei physically combine to form one or more diff atomic nucleus and additional sub-atomic particles such as neutron.

Obey energy-mass conversation but requires the application of energy-mass equivalence.

proton-proton chain (fusing H to HE)

Stars with the same or smaller mass than our sun will produce energy by fusing protons to obtain Helium nuclei in the chain reaction

In the process, energy is released. The product therefore has less mass than the sum of the reactants, according to E = mc2.

• most common in main sequence

carbon-nitrogen oxygen cycle (CNO)

Carbon acts as a catalyst in the reaction as it transforms into nitrogen and then oxygen before returning back to carbon to start the cycle again.

• More common in supergiants

Main sequence star

• very greatly in luminosity, mass, size, and surface temperature.

• TOP LEFT = heavier, larger, more luminous and higher surface temp

  • turn to super giants - consume their fuel very quickly.

• BOTTOM RIGHT = light, smaller, less luminous, lower surface temp

  • Turn to red giants - consume fuel very slowly and star as main star for a very long time

Red giant

• evolve from smaller main-sequence stars when they deplete their hydrogen source for nuclear fusions.

• Outer layers of gases expand due to higher temp

• Larger surface area compared to preceding main-sequence

Super giant

• Evolve from larger main-sequence stars when their hydrogen source are depleted.

• Greater surface and area

• However, due to their much larger surface area, the intensity of radiation is relatively low at the surfaces of super giants.

• Relatively low surface temp despite their great luminosity

White dwarfs

• the final stage of evolution following a red giant

• there are no more nuclei remaining for nuclear fusion

• when nuclear fusion cannot longer occur, the star contracts under the influence of gravitational force.

• They shed their outer layer exposing their core which increases temp

H-R diagram

Graphs the luminosity of stars against their surface temp. Matching luminosity and surface temp helps to determine the size/mass of a star.

A star in a particular region of a H-R diagram depends on its evolutionary stage. These stages are controlled by the nucleosynthesis rxn occuring in the star’s core.

Charged particle properties of Cathodes

• deflected by electric and mag field

• ray carries energy and momentum

  • attracted to positive charge

Electromagnetic wave properties of cathode ray

• identical, regardless of material used

• It emanates from the cathode and travel in a straight line

• would cast a shadow of solid object

• could penetrate thin metal foils

Maltese Cross experiment - cathode ray

What was done: An anode in the shape of a maltese cross was placed in the path of cathode ray.

Observation: A shadow of maltese cross was formed directly behind anode.

Conclusion: Cathode rays travel in a straight line and can cast a shadow.

Cathode ray containing electric and magnetic field experiment

What was done: A metal plate coated with fluorescent material was used to visualise the trajectory of a cathode ray. The cathode ray was passed through a uniform electric and mag field.

Observation: In the presence of an electric field, the path of the cathode ray was deflected in a direction that was consisted with a negatively charged mass.

Conclusion: Cathode rays are stream of negatively charged magnetic particles.

Paddle wheel

What was done: A glass paddle wheel that could move and rotate freely was placed in the path of cathode ray.

Observation: When the glass paddle wheel was struck by a cathode ray, it rotated and moved towards the cathode ray.

Conclusion: Cathode rays have momentum and kinetic energy - they have mass and are particles in nature.

Thomson’s charge-to-mass experiment

A pair of charged metal plates was used to create a uniform electric field. When a cathode ray travels through this field, it was deflected towards a positive field.

Next Thomson applied a uniform magnetic field using current-carrying coils. The direction o f this magnetic field was perpendicular to the electric field and positioned such that it would cause the cathode ray to deflect downwards.

The mathematical calculation for Thomson’s charge-to-mass experiment

Thomson’s interpretation of his charge-to-mass experiment

Proved cathode rays have mass and a negative charge, identifying them as electrons. their constant charge-to-mass ratio across all materials proved they are fundamental to all matter. Their ratio being 1800x greater than hydrogen ions implied electrons are 1800x lighter.

Thomson’s ‘plum pudding’ model

Negatively charged electrons are dispersed in a positive mass due to electrostatic repulsion. The electrons are held together in the atom due to their attraction to the positive mass.

Limitations: Lack of explanation for the positive mass.

Milikan’s Oil experiment

Millikan sprayed tiny oil droplets into a chamber between two charged plates. As the droplets passed through the spray nozzle, they gained charge from friction. When a voltage was applied across the plates, the electric force on the charged droplets could balance their weight, causing some droplets to remain suspended in mid-air.

Charge is quantised - milikan’s oil experiment

• Discovered that some oil drops required a higher voltage to be applied in order for them to be suspended between the plates.

• This was due to the oil drop picking up extra charge as it came out of the spray nozzle.

• Concluded that when he calculate the charge he always got a multiple of 1.6×10-19C

Milikan Oil Experiment - Mathematical calculation

Geiger-Marsden Experiment

Fired alpha particles at a thin gold foil. The gold foil was surrounded by a screen that would cause scintillations when alpha particles hit it.

Geiger-Marsden Experiment Observations

• Most alpha particles went through undeflected as most scintillations were observed directly behind the gold foil. Supported Rutherford’s postulate that an atom is mostly empty space.

• Few alpha particles were deflected and some were reflected back. Supported the presence of a region in the atom of highly positive charge (nucleus). In an atom of gold, the charge and mass of the nucleus are substantially greater than that of an alpha particle. As a result, when an alpha particle collided with the nucleus it was reflected.

Why can’t JJ Thomson explain: Geiger-Marsden Experiment

its dispersed positive charge could not explain the significant deflections of alpha particles observed in the gold-foil experiment

Rutherford’s model of an atom

• nucleus = a highly concentrated positively charged region in the centre of the atom

• Most of the atom is empty space

• Electrons orbit with the nucleus

Rutherford’s proton discovery

Fired alpha particles at a sample of nitrogen gas which resulted in a transmutation rxn producing protons.

Conducted similar experiment to Thomson's to determine the value of the charge to mass ratio of a proton. Showed that a proton is positively charged and much heavier than an electron.

Limitation of Rutherford’s model

•He could not explain what exactly was inside the nucleus. The number of protons were less than the atomic mass. This led to the speculation that there is something else inside the atom that scientists weren’t aware of.

•Did not know how the electrons were arranged around the nucleus.

•Could not explain how electrons did not collapse into positive charges.

•Electrons are moving= moving objects have centripetal acceleration and ccelerating charges produce EMR (energy), which should lead to loss of energy.

•Therefore, every atom should be unstable and short lived. But it isn’t!

Chadwick’s discovery of neutron

Alpha particles were fired at berylium and unknown particles were observed coming out of the berylium.

Conclusions of chadwick’s discovery

• same mass as protons

• Neutrally charged

Limitation

He had the same problem as Rutherford when it came to electrons.

• did not know how electrons were arranged around the nucleus

• Could not explain how electrons did not collapse into positive charge.

Chadwick’s model diagram

Bohr’s postulate

1.An electron moves in circular orbit around the nucleus. (Electrostatic attraction provides centripetal force.)

2.Electrons can only exist in particular energy levels and orbits. When an electron is in a stable orbit it will not emit or absorb energy.

3.When an electron moves into a different orbit energy will be absorbed or emitted . The energy absorbed or emitted will relate to the energy difference between the shells according to: E = hf

4.The angular momentum of electrons is quantized.

Problems with Bohr’s mdoel

1. it was a mixture of classical and quantum physics.

  2. it could not explain the Zeeman effect (when an external magnetic field interacts with the electron’s energy state.)

  3. it could not explain the variable intensity/brightness of the spectral lines. (the probabilities need quantum modelling)

  4. it could not account for the hyperfine spectral lines (the magnetic field of the nucleus interacts with the electron’s magnetic properties).

  5. it only worked for hydrogen and not larger atoms.

The Balmer series + formula

The hydrogen spectrum consists of a series which correspond to specific colours and wvaelengths. These are described as discrete, single line spectra

Electrons relationship to energy