Physics Mod 8

COMPLETED

Models of the atom

Dalton - Billiard Ball
J J Thomson - Plum Pudding
Rutherford - Nuclear model
Bohr - Planetary model
Chadwick - Neutrons in nucleus

cathode rays

beam of electrons

cathode ray experiments

1. gas discharge tubes

2. maltese cross

3. paddle wheel tube

4. curved fluorescent screen

5. addition of electric field between parallel plates

Gold foil experiment (Geiger-Marsden)

Rutherford disproved the "plum pudding" model of the atom

Milikan's oil drop experiment

find charge of e- by measuring stationary oil droplets and equating the magnetic field to the gravitational field to find the charge.

canal ray experiment (Goldstein)

mass of positively charged particles depends upon nature of gas.
q/m ratio of particles depend upon which gas the particles original

alpha particle experiment (Chadwick)

found neutrons in the nucleus

classical physics

physics before the Bohr model (before quantum physics and relativity)

Quantum mechanics

fundamental theory of physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles

Balmer series

A set of spectral lines that appear in the visible light region when a hydrogen atom undergoes a transition from energy levels n>2 to n=2.

Maxwell’s Theory of Electromagnetism

An accelerating charged particle will produce electromagnetic radiation (EM waves)

Limitations of the Rutherford Model of the Atom

• Couldn’t explain spectral lines and why a continous spectra wasn’t produced when excited electrons return to their original energy level

• Can’t explain why accelerating electrons don’t produce EM waves (Maxwell’s Theory of Electromagnetism) → atoms should be unstable based on this model but they are not

Spectral lines

dark or bright lines in a spectrum that correspond to specific wavelengths of light, created when atoms, molecules, or ions absorb or emit photons

obseservable spectra

• continuous spectrum

• emission spectrum 

• absorption spectrum

continuous spectrum

produced: when an incadescent light or sunlight is refracted through a prism

emission spectra

produced: when gas molecules are excited by putting a large DC voltage across them, in a vaccum. Resultant light is refracted through a prism or diffraction grating

result: bright, coloured, distinct lines are produced with a black background

absorption spectra

produced: when white light is passed through a pure gas before passing through diffraction grating or prism

result: spectrum is complete except for the presence of black bands in the same positions (for each gas) as the emission spectrum

Diffraction grating

we did pracs with this, an optical device with a periodic structure that separates light into its constituent wavelengths

Prism

used to refract light, separates into its constituent wavelengths

Bohr’s postulates

Postulate 1: Electrons in an atom exist in stable, circular orbits and these electrons in stable orbits do NOT emit radiation

Postulate 2: Electrons absorb or emit specific quanta of energy when they move from one stable energy level to another. E=hf (for EM waves)

Postulate 3: The electron’s angular momentum is quantised (don’t need to know more than this)

Plank’s quanta

energy of EM waves can be quantised and calculated using E=hf

Rydberg’s mathematical model for hydrogen’s spectral lines

1/𝜆 = R(1/n_f² - 1/n_i²)

Spectral lines → Law of Conservation of Energy

transition of electrons between orbits follows this Law: if an electron drops from a higher energy level to a lower energy level, the energy must be transformed.

Limitations of Bohr’s Model of the Atom

• only predicts hydrogen’s spectrum

• doesn’t explain why the lines in the spectrum vary in intensity/thickness and why some are sharp, dull thin and diffuse

• Can’t explain the ‘zeeman effect’ or the ‘anomalous zeeman effect’

• couldn’t explain why electrons didn’t emit radiation (same as Rutherford) and why they didn’t spiral towards the positive nucleus

The Zeeman Effect

Observed that when the spectrum of a sodium flame burning in a magnetic field was visualised → some lines split into 3 COULDNT BE EXPLAINED BY BOHRS MODEL

The Anomalous Zeeman Effect

Spectral lines could also split into 15 hyperfine lines when observed in a magnetic field. COULDNT BE EXPLAINED BY BOHRS MODEL

Diffraction

the spreading of wavefronts as they pass through a small aperture (small opening) or past an obstacle

Braggs Law

Used for when two reflected x-rays constructively intefere

n𝜆 = 2dsin𝜃

Constructive interference

a phenomenon where two or more waves combine to form a resultant wave with a larger amplitude

Destructive interference

a phenomenon where two waves combine to cancel each other out, resulting in a wave with a reduced or zero amplitude

De Broglie Hypothesis/The Theory of electron waves

“all moving matter exhibits wave-like properties and that the wavelength of this matter wave is inversely proportional to its momentum according to the equation: 𝜆 = h/p = h/mv

includes evidence from wave-particle duality theory of matter, Plank’s work and Einstein’s work on light (mod 7)  → as quanta were now accepted and the dual theory of light

standing waves

a wave pattern that oscillates in place without moving through space, appearing to "stand still” → De Broglie waves were this type

nodes

the points of the wave that do not vibrate

antinodes

the points that vibrate between maximum and minimum positions

de Broglian wavelengths

standing waves that allowed electron quantum states (shells) circumferences would always contain a multiple of these wavelengths (integer)

-→ used incrememnts to explain why certain energy levels were stable, ie. the ones that has an integer multiple of de Broglian wavelengths

equation:  C = n𝜆

— The stable orbits of the hydrogen atom are those where the circumference  is exactly equal to a whole number of electron wavelengths

Electron spin

Pauli Exclusion principle

• any atomic orbital (or other quantum state) can contain at most two electrons, and they must have opposite spin directions

• This means that no two electrons can have all four quantum numbers the same

No two electrons in a single atom can have the same set of four quantum numbers (applies to electrons, and all fermions)

Quantum numbers

Proposed in order to distinguish electrons in an atom from one another by considering 4 quantum numbers:

1. The energy level

2. Shape of the orbital

3. Orientation of the sub-orbital

4. Spin of the electron

NO TWO ELECTRONS COULD BE IDENTICAL

Heisenberg’s uncertainty principle

The more exactly we know a particle’s position, the less certain we become of its velocity (and momentum) and vice versa

“electron cloud” model of the atom (The Quantum mechanical model of the atom)

Schrodinger’s model of the atom → currently accepted model of atom

nucleon

the collective name for the two main subatomic particles in the nucleus; protons and neutrons

atomic number (Z)

number of protons in an atom

standard atomic weight

average weight of an element (considers that many elements have isotopes

mass number (A)

number of protons and neutrons

charge on an atom

results from a loss or gain of electrons

Strong nuclear force

the force that holds the nucleus together. it has properties of:

• acts only over small distances

• force is between nucleons (p→p, p→n etc.)

• at very close distances, nucleons repel each other (not a linear relation)

• at around 3×10^-15 m the strong nuclear force drops to zero

radioactive

way to describe a nuclide that emits some kind of radiation (if it is unstable)

radioactive decay

an unstable nuclide emits radioactive particles in process to become stable

nuclide

a specific type of atom defined by the exact number of protons and neutrons in its nucleus, and its energy state

3 ideas to predict nuclear stability

1. Neutron to proton ratio

2. The band of stability

3. Magic numbers

Transmutation

the change of one chemical element into another by nuclear decay or radioactive bombardment

spontaneous radioactivity

decay/emission of radioactive particles which is NOT caused by human intervention or accelerators

ionisation

removal of a bound electron from an atom to produce a free electron and a positive ion - THIS IS NOT RADIOACTIVE DECAY

radioisotopes

unstable atoms that emit particles that undergo nuclear reactions (decay) to become more stable

• daughter products have a greater binding energy per nucleon than the original parent nuclide

Decay series

It is called a ______ _____ when one radioactive isotope decays into another, and the daughter is also unstable and further decays occur until a stable nucleus is created

half life

the time it takes for half of a sample of a radioactive substance to undergo radioactive decay or the time required for the number of unstable atomic nuclei in a sample to decrease by half

hadrons

heavy, composite (made up of multiple quarks) that are affected by the strong nuclear force

• e.g. protons

leptons

fundamental particles (not made up of other particles) which are NOT affected by the strong nuclear force

• e.g. the electron, the muon

fundamental particles

particles which are not made up of other particles

elementary particles

fundamental particle that is not made up of any other particles

• e.g. quarks, electrons

composite particles

a particle that is composed of two or more elementary particles

quarks

an elementary particle that are the building blocks of hadrons and have a fractional charge , interact with the strong force and obey the Pauli Exclusion Principle

magnetic moment

the magnetic strength and orientation of a magnet -→ mangetic moment is parallel to the spin axis

positron

antimatter to the electron

cloud chamber

a simple device that contains air or gas supersaturated with water or alcohol vapour that can be used to detect charged particles by the condensation trails which they produce

composition of a proton

two up quarks and one down quark = +1 charge

(2/3 + 2/3 - 1/3 = +1)

composition of a neutron

one up quark and two down quarks = 0 (neutral) charge

(2/3 - 1/3 -1/3 = 0)

bosons

fundamental force carriers which have an integer spin

• e.g. photons, gluons

baryons

type of hadron that is specifically made up of 3 quarks

• e.g. protons and neutrons

fermions

subatomic particles with a half-integer spin

Four fundemental forces of interaction

(listed in order of magnitude; strong → weaker)

1. Strong nuclear force

2. Electromagnetic

3. Weak nuclear force

4. Gravitational

Nuclear fission

A heavy nucleus splits to form to or more lighter nuclei, each of which is more stable than the original nucleus (release of 1-3 neutrons)

critical mass

smallest amount of fissible material needed for a sustained nuclear chain reaction

Fuel (nuclear reactor)

enriched uranium fuel (5-20% U-235)

Moderator (nuclear reactor)

slows down neutrons

• e.g. water, heavy water (deuterium oxide), graphite rods (more economical option)

Control Rods (nuclear reactor)

used to absorb neutrons, controls/adjusts the rate of the nuclear reaction

• made of steel with boron or cadmium

Coolant (nuclear reactor)

extracts the heat energy from generator system

• e.g. water, heavy, air, helium or liquid sodium

Protection (nuclear reactor)

sheilding from gamma radiation and keeps neutrons inside

• graphite and lead reflect neutrons back into the core

• thick wall of concrete to absorb the gamma radiation

Mass-Energy Equivalence

E=mc²

relativistic mass

mass of object + kinetic energy

invariant mass

rest mass

Special Theory of Relativity

certain types of matter may be created or destroyed (due to mass-energy equivalence). Total mass and energy associated with such matter remains unchanged in quantity.

Binding energy

the energy required to separate an atomic nucleus completely into its constituent protons and neutrons

Fusion

nuclei with small mass numbers need to combine to become heavier, more stable nuclei

mass defect

the difference between the actual mass of a nucleus and the total combined mass of its individual constituent nucleons (protons and neutrons). 

Linear accelerators

electric field created between cylindrical electrodes (force applied to charged particle)

Cyclotrons

D-shaped hollow metal accelerator

• perpendicular magnetic field

• Uses F_C = mv² / r (centripital motion)

• Faster than linear accelerators

Synchrotrons

contains a linear accelerator and then a ‘booster ring’ of electromangets and high voltage and accelerating cavities

• Faster than cyclotrons and linear accelerators

Large Hadron Collider (LHC) → special synchrotron

most powerful particle accelerator

• two high energy particle beams travel close to the speed of light before they are made to collide

Hadrons

any particle made of quarks

consists of subcategories: Baryons and Mesons

Baryons

made up of three quarks (e.g. protons and neutrons)

Mesons

are made up of one quark and one antiquark (with zero or 1 spin)

Leptons

• can exist on their own

• are fundamental particles

• have half-integer spins (follow Pauli’s Exlusion Principle)

theory

a set of concepts, claims and/or laws that can be used to explain and predict a wide range of related observed phenomena. ___ are typically founded on clearly identified assumptions, are testable, produce reproducible results and have explanatory power

Big Bang Theory

proposes that the Universe began with an ‘event’ that produced an enormous amount of energy in a single position (singularity) and that all matter has condensed from this energy (E=mc²) as it expanded outward and cooled

Processes leading to transformation of radiation into matter

1. Inflation

2. Pair Production

3. Annihilation

4. Big Bang Nucleosynthesis

5. Recombination

6. Decoupling

Inflation

caused the vast expansion of space filled with energy (just after Big Bang)

Pair Production

occurred when extremely high energy photons would collide, producing a range of particles and their antiparticles (E=mc²)

Annihilation

all mass is converted back into energy as the particles and antiparticles collide, producing gamma rays

Big Bang Nucleosynthesis

first atomic nuclei started forming (after the Big Bang)