Modern & Atomic Physics

An atom is the smallest particle of an element that takes part in a chemical reaction, and it is built from three fundamental sub-atomic particles arranged around a tiny central nucleus.

• The proton carries a positive charge and sits inside the nucleus.
• The neutron carries no charge (it is neutral) and also sits inside the nucleus.
• Protons and neutrons together are called nucleons because both are found in the nucleus.
• The nucleus is positively charged (due to protons) and contains almost the entire mass of the atom.
• An atom as a whole is electrically neutral, as the number of protons equals the number of electrons.

• The electron was discovered by J. J. Thomson in 1897 through his cathode-ray (discharge tube) experiments.
• The proton was discovered through Goldstein's anode-ray (canal-ray) experiment, and was named and established by Rutherford.
• The neutron was discovered by James Chadwick in 1932, the last of the three to be found because it has no charge.
• The charge on an electron = −1.6 × 10⁻¹⁹ coulomb, measured by Millikan's oil-drop experiment.
• The proton has the same magnitude of charge as the electron, but positive.

Over time scientists proposed several pictures of how the atom is arranged inside, each model improving on the one before it.

• The atom is a sphere of positive charge with electrons embedded in it, like seeds in a watermelon or plums in a pudding.
• The positive and negative charges are equal, so the atom is neutral.
• It could not explain how the charge was actually distributed inside the atom.

• Given by Ernest Rutherford from his famous alpha-particle scattering (gold-foil) experiment.
• Most of the atom is empty space, since most alpha particles passed straight through the foil.
• The whole positive charge and almost the entire mass are concentrated in a tiny central nucleus.
• Electrons revolve around the nucleus like planets revolve around the Sun.
• Drawback: it could not explain the stability of the atom — a revolving electron should lose energy and spiral into the nucleus.

• Given by Niels Bohr in 1913, improving on Rutherford's model.
• Electrons revolve only in certain fixed orbits (energy levels / shells) called stationary orbits, without radiating energy.
• Each orbit has a fixed (quantised) energy, and these shells are named K, L, M, N….
• An electron emits or absorbs energy only when it jumps from one orbit to another — absorbing energy to jump up, releasing energy (as light) to fall down.
• This model successfully explained the stability of the atom and the line spectrum of hydrogen.

Two simple numbers identify any atom — one counts its protons, the other counts its protons and neutrons together.

• Mass number A = number of protons + number of neutrons (i.e. total nucleons).
• A = Z + N, where N = number of neutrons.
• Therefore number of neutrons N = A − Z.
• The atomic number Z decides the identity of an element — every element has its own unique atomic number.
• An atom is written as ᴬ𝒁X (e.g. carbon = ¹²₆C, meaning Z = 6, A = 12, so N = 6).

By comparing the proton count and neutron count of different atoms, we classify them into three important families.

• Isobars = same mass number A, different atomic number Z — different elements with equal total nucleons.
• Isotones = same number of neutrons N, different Z and A.

• Isotopes have identical chemical properties but slightly different physical properties (because chemistry depends on electrons, which are equal in number).
• Hydrogen has three isotopes: protium (¹H), deuterium (²H) and tritium (³H).
• Uranium's important isotopes are U-235 and U-238 — U-235 is used as nuclear fuel.
• Carbon-14 (¹⁴C) is a radioactive isotope used in carbon dating of fossils.
• Cobalt-60 is used in cancer (radiotherapy) treatment; iodine-131 is used to treat the thyroid gland.

Some heavy, unstable nuclei spontaneously break down and shoot out radiation — a phenomenon called radioactivity.

• Marie Curie and Pierre Curie discovered the radioactive elements polonium and radium.
• Marie Curie coined the term "radioactivity" and won two Nobel Prizes (Physics and Chemistry).
• Radioactivity is the spontaneous emission of radiation from the unstable nucleus of an atom.
• It is a nuclear property, not affected by temperature, pressure or chemical state.
• Elements with atomic number greater than 82 (above lead) are naturally radioactive.
• The SI unit of radioactivity is the becquerel (Bq); the older unit is the curie (Ci).

A radioactive nucleus can emit three distinct kinds of radiation, which differ in their nature, charge, mass and how far they can penetrate matter.

• Beta (β) rays are fast-moving electrons, negatively charged and very light.
• Gamma (γ) rays are high-energy electromagnetic waves, with no charge and no mass.

• Gamma rays are the most penetrating and chargeless; alpha rays are the least penetrating.
• Alpha rays have the highest ionising (and least penetrating) power; gamma rays the opposite.
• Penetrating power order: γ > β > α.
• Ionising power order: α > β > γ.
• Gamma rays travel at the speed of light and are not deflected by electric or magnetic fields.
• During alpha emission Z decreases by 2 and A decreases by 4; during beta emission Z increases by 1 while A stays the same.

A radioactive sample does not decay all at once — it fades away at a fixed rate measured by its half-life.

• After one half-life, half the sample remains; after two half-lives, one-quarter remains, and so on.
• The half-life is constant for a given element and is independent of temperature, pressure or chemical combination.
• Half-life cannot be changed by any physical or chemical process.

• The half-life of Carbon-14 is about 5730 years, which is why it is used to date ancient fossils and remains.
• The half-life of Uranium-238 is about 4.5 billion years.
• Carbon dating, based on ¹⁴C half-life, estimates the age of dead plants and animals.

A heavy nucleus can be split into lighter pieces, releasing a huge amount of energy — this is nuclear fission, the basis of atom bombs and nuclear reactors.

• It is triggered when a slow neutron strikes a heavy nucleus like Uranium-235.
• The fuel used is U-235 (and also Plutonium-239).
• Each fission releases 2–3 more neutrons, which split more nuclei — a chain reaction.
• An uncontrolled chain reaction causes the explosion of an atom bomb.
• A controlled chain reaction is used in a nuclear reactor to generate electricity.
• Discovered by Otto Hahn and Fritz Strassmann (explained by Lise Meitner & Otto Frisch).

• Fuel: Uranium-235 or Plutonium-239.
• Moderator: slows down neutrons — heavy water (D₂O) or graphite are used.
• Control rods: absorb extra neutrons to control the reaction — made of cadmium or boron.
• Coolant: carries away the heat produced (e.g. water, liquid sodium).
• The atom bomb dropped on Hiroshima used uranium; the one on Nagasaki used plutonium.

When very light nuclei join together, they release even more energy than fission — this is nuclear fusion, the process that powers the Sun and the stars.

• It requires extremely high temperature and pressure (millions of degrees) to occur.
• The Sun's (and all stars') energy comes from nuclear fusion of hydrogen into helium.
• The hydrogen bomb is based on uncontrolled nuclear fusion and is far more powerful than the atom bomb.
• Fusion releases more energy per unit mass than fission and produces little radioactive waste.

Einstein showed that mass and energy are two forms of the same thing, and a tiny loss of mass in the nucleus releases an immense amount of energy.

• Where E = energy, m = mass converted, and c = speed of light (3 × 10⁸ m/s).
• Mass and energy are interconvertible — mass can be converted into energy and vice versa.
• Because c² is a huge number, even a tiny mass gives an enormous amount of energy.
• In fission and fusion, a small "mass defect" disappears and reappears as released energy, exactly as E = mc² predicts.
• This equation is the basis of nuclear energy, the atom bomb and the energy of stars.

When light of high enough frequency falls on a metal, it knocks electrons out of it — a phenomenon Einstein explained, winning him the Nobel Prize.

• The emitted electrons are called photoelectrons.
• It was explained by Albert Einstein in 1905, using the idea that light travels in packets called photons.
• Einstein won the Nobel Prize (1921) for explaining the photoelectric effect, not for relativity.
• Emission occurs only if the light's frequency is above a minimum value called the threshold frequency.
• Below the threshold frequency, no electrons are emitted, no matter how bright the light.
• The energy of a photon is E = hν (where h = Planck's constant, ν = frequency of light).
• Increasing the intensity (brightness) increases the number of electrons, not their energy.
• Increasing the frequency increases the energy of the emitted electrons.

Light behaves in two different ways depending on the experiment, behaving sometimes like a wave and sometimes like a stream of particles.

• Phenomena like interference, diffraction and polarisation prove the wave nature of light.
• The photoelectric effect and Compton effect prove the particle (photon) nature of light.
• Light travels as tiny energy packets called photons (quanta of energy).
• Louis de Broglie proposed that matter (like electrons) also has a wave nature — the matter waves (de Broglie waves).
• De Broglie wavelength λ = h/mv (where h = Planck's constant, m = mass, v = velocity).

X-rays are a high-energy, invisible radiation that can pass through soft tissue, making them invaluable in medicine and industry.

• Roentgen won the first-ever Nobel Prize in Physics (1901) for this discovery.
• X-rays are high-energy electromagnetic waves with a very short wavelength.
• They are not deflected by electric or magnetic fields (they are chargeless).
• X-rays travel in straight lines at the speed of light and can pass through flesh but not through bone or metal.
• X-rays are produced when fast-moving electrons strike a metal (target) such as tungsten.

• Medical: imaging bones, detecting fractures and diagnosing diseases (X-ray photographs).
• Cancer treatment (radiotherapy) using high-energy X-rays.
• Security: scanning luggage at airports and detecting hidden objects.
• Industry: detecting cracks and flaws inside metals and welds.
• Science: studying crystal structures (X-ray crystallography).

A laser produces an intense, narrow, single-colour beam of light in which all the waves march perfectly in step.

• Laser light is monochromatic (single wavelength / one colour).
• Laser light is coherent (all waves are in the same phase) and highly directional (a very narrow beam).
• Laser light is very intense and can travel long distances without spreading.
• Lasers are used in surgery, eye operations, CD/DVD players, barcode scanners, printing and cutting metals.
• Lasers are used in holography, fibre-optic communication and measuring large distances (e.g. Earth to Moon).

Semiconductors are materials whose ability to conduct electricity lies between that of conductors and insulators, and they form the heart of all modern electronics.

• Silicon (Si) and Germanium (Ge) are the most common semiconductors.
• The conductivity of a semiconductor increases with temperature (unlike metals, whose conductivity falls).
• Adding small amounts of impurity to improve conductivity is called doping.
• N-type semiconductor has extra electrons (negative carriers); P-type has "holes" (positive carriers).
• Semiconductors are used to make diodes, transistors, LEDs, solar cells, ICs and microchips.
• The transistor, made of semiconductors, replaced the bulky vacuum tube and made modern electronics possible.