Atomic Structure P4

Model of Atom: Radius

• Atoms are entirely small - Radius of 1 x 10⁻¹⁰ m. • Radius of a nucleus is 1 x 10⁻¹⁴ m (10000 x smaller than an atom).

Subatomic Particles properties

• Proton: +1 charge, 1 mass, located In the Nucleus. • Neutron: 0 charge, 1 mass, located In the Nucleus. • Electron: -1 charge, Almost 0 (0.0005) mass, located in Electron Shells (surrounding Nucleus).

Why atoms have 0 charge

• Atoms are neutral as they have the same amount of protons (+1) and electrons (-1), so they cancel out to have 0 charge.

Mass number vs Atomic number

• Mass number: Total number of protons and neutrons. • Atomic number: Number of protons (and electrons).

Energy Levels behavior

• Electrons absorb a specific amount of electromagnetic radiation (energy) and are excited to a higher energy level (further from the nucleus). • When the electrons fall back down to a lower energy level they emit the electromagnetic radiation back.

Isotopes definition

• Different forms of the same element with the same number of protons but different number of neutrons.

Ionisation definition

• The process of losing electrons (positively charged) or gaining electrons (negatively charged).

History of the atom: John Dalton

• John Dalton (1800s) Solid Sphere Model: Everything is made of solid sphere atoms that cannot be divided or broken and all atoms of specific elements are identical.

History of the atom: J.J. Thomson

• J.J. Thomson (1904) Plum Pudding Model: A sphere of positive charge with electrons embedded within it.

History of the atom: Rutherford

• Rutherford (1911) Nuclear Model: Most of the mass in a tiny nucleus containing positive charge surrounded by orbiting electrons.

History of the atom: Niels Bohr

Niels Bohr (1913) Bohr Model: Electrons are in fixed specific paths called shells at specific distances from nucleus.

History of the atom: James Chadwick

• James Chadwick (1932): Discovered the neutron.

Rutherford's Gold Foil experiment

• The nuclear model was concluded as alpha particles (positively charged) were aimed at a thin sheet of gold foil. A detector screen was placed around the foil to see where the particles landed.

Rutherford's experiment findings

• Most of the alpha particles pass through undeflected = Atom is mostly empty space.

• Some of the alpha particles were deflected through a small angle = Nucleus has positive charge that repelled them.

• A few of the alpha particles deflected through a large angle (more than 90°) = Nucleus is tiny and contains most of the atoms mass.

Radioactive Decay definition

• A random process where an unstable nucleus emits radiation (e.g. alpha, beta or gamma) to become more stable.

Measuring Decay: Activity & Count rate

• Activity: the rate at which a source of unstable nuclei decays, measured in Bq.

• Count rate: the number of radiation counts measured by a detector per second using a Geiger Muller detector. (Must subtract background radiation to find an accurate activity).

Half-life definitions

• Definition 1: The time it takes for the number of radioactive nuclei in an isotope to halve. • Definition 2: The time it takes for activity (and count rate) to halve.

Alpha Decay mechanism & equation

• This process happens when the nucleus is too large and is unstable due to a strong electrostatic repulsion between many protons.

The nucleus randomly dumps together 2 protons and 2 neutrons. This forms an alpha particle (⁴₂He) that is immediately released.

Releasing this reduces the size of the nucleus, making it more stable. • General equation: ᴬℤ X → ᴬ⁻⁴ℤ₋₂ Y + ⁴₂He

Alpha Radiation properties

• High Ionising power: as they are large and highly charged (+2) so they interact very strongly by removing electrons from atoms they pass by.

• Short range in air (5cm): Their high ionising power means they lose energy very quickly, so alpha particles can only travel short distances before being stopped.

• Absorbed by paper: Their high ionising power means they lose energy very quickly, so thin materials stop them. • Attracted towards negative plate in electric field.

Uses of Alpha

• Smoke detector: as alpha particles have high ionising power causing current to flow and stops when smoke enters. Its short range is so its easily contained in the device. (long half-life so it doesn't need replaced).

Beta Decay mechanism & equation

• This process happens when a nucleus has too many neutrons compared to protons, making it unstable.

A neutron inside the unstable nucleus randomly transforms into a proton and electron. The new proton remains in the nucleus. The electron (beta particle) is released at high speed.

• General equation: ᴬℤ X → ᴬℤ₊₁ Y + ⁰₋₁e

Beta Radiation properties

• Medium ionising power: as they are smaller and lighter (-1 charge), so they interact less strongly than alpha particles, making it less effective at removing electrons from atoms they pass by.

• Medium range in air (1m): Their medium ionising power means they lose energy slower, so beta particles can travel further distances.

• Absorbed by aluminium: Their medium ionising power means they lose energy slower so thicker materials stop them.

• Attracted towards positive plate in electric field.

Uses of Beta

• Thickness Monitoring: as Beta particles have medium penetration they can test the thickness of metal sheets.

Gamma Radiation mechanism

• This process happens when a nucleus is left with too much energy (excited state) after an alpha or beta decay.

The nucleus randomly releases this energy as an electromagnetic wave to become more stable.

Mass doesn't change, Atomic number doesn't change.

Gamma Radiation properties

• Low ionising power: as they have no mass or charge, so they rarely interact with atoms.

• High range in air (Unlimited): Their low ionising power means they lose energy extremely slowly, so gamma rays travel to very long distances.

• Absorbed by thick concrete / lead: Their low ionising power means they lose energy slower so thicker materials stop them.

• Unreflected in electric field.

Uses of Gamma: Medical Tracers

• Gamma emitting isotopes are injected in the body to track internal processes (e.g. blood flow). High penetration allows gamma cameras or PET detectors to detect gamma rays outside the body.

Benefit: early and accurate diagnosis. Short half-life to reduce radiation exposure.

Risk: radioactive exposure (slight risk of cancer).

Uses of Gamma: Radiotherapy

• High-energy gamma rays are used to kill cancerous cells by directing them correctly to the right dosage. Their high penetration means they can reach deep tumors inside the body.

Benefit: Effective cancer treatment to kill the cancer cells / shrink tumors. Short half-life to reduce radiation exposure to protect surrounding healthy cells.

Risk: Can damage healthy cells surrounding the cancer cells / causes risk of new cancer.

Hazards of Radiation: Irradiation vs Contamination

• Irradiation: Being exposed to radiation from an external source (you don't become radioactive).

• Contamination: Radioactive materials gets onto or into a person (you do become radioactive as the unstable nuclei are present in/on you).

Reducing hazards of Irradiation and Contamination

• Reduce Irradiation: Time (minimise the time near the radioactive source), Distance (increase distance between you and source), Shielding (Use thick lead barrier to absorb radiation).

• Reduce Contamination: Wear protective clothing, Sealed containment suit.

Danger of Irradiation on types of radiation

• Alpha: Low danger as it can't penetrate the skin.

• Beta: Medium danger as it can penetrate skin and cause damage.

• Gamma: Very high danger as it can highly penetrate to deep organs and damage them.

Danger of Contamination on types of radiation

• Alpha: Very high danger as it has high ionising to cause damage in a very localized area. • Beta: Medium danger as it has medium ionising to cause less damage over a wider area. • Gamma: Low danger as it has low ionising power to cause no damage and pass straight out the body.

Background Radiation definition & sources

• Definition: Low-level radiation that is around us all the time. • Natural sources: cosmic rays, rocks, soil, radon gas. • Man-made sources: medical x-rays, nuclear explosions/waste.

Nuclear Fission mechanism

• The process of splitting a large, unstable nucleus into two nuclei, releasing energy.

A large, unstable nucleus absorbs a slow-moving neutron.

The nucleus becomes more unstable and splits into two daughter nuclei and releases 2 or 3 new neutrons.

A lot of energy is released (kinetic energy and gamma radiation). (Energy can be used to heat water and generate steam in a power station). Mass decreases as well.

Chain Reaction details

• The neutrons given off initiate events with other unstable nuclei, causing them to split as well in a continuous process. The reactor is controlled using control rods that absorb excess neutrons to allow the chain reaction to be controlled. If its uncontrolled, it leads to an explosion.

Nuclear Fusion mechanism

• The process of joining two small, light nuclei to form a single, larger nucleus.

High temperature and pressure are required to give the nuclei enough kinetic energy to overcome strong electrostatic repulsion of positive charge.

The two light nuclei collide together at high speed, allowing the strong nuclear force to join them to form a single and larger nucleus. Mass decreases as atoms are converted into this energy.

A larger amount of energy is released (kinetic and gamma radiation) which is energy used to power stars and suns. Fusion produces very little radioactive waste than fission.