Understanding Atomic Structure

Atomic Structure

• A positively charged nucleus contains neutrons and protons, and is surrounded by negatively charged electrons.
• Atoms have a radius of approximately 1 × 10-10 meters.
• The nucleus is 10,000 times smaller than the atom.
• Most of the atom's mass is concentrated in the nucleus.

Subatomic Particles

• Protons have a relative mass of 1 and a relative charge of +1.
• Neutrons have a relative mass of 1 and a relative charge of 0.
• Electrons have a relative mass of approximately 0 (0.0005) and a relative charge of -1.

Electron Arrangement

• Electrons exist at different distances from the nucleus, occupying different energy levels.
• Electron arrangements can change through interactions with electromagnetic (EM) radiation.

Isotopes and Elements

• Atoms of the same element have the same number of protons.
• Neutral atoms have equal numbers of electrons and protons.
• Isotopes are atoms of the same element with different masses, meaning they have the same number of protons but different numbers of neutrons.
• Isotopes include Carbon-12, Carbon-13 and Carbon-14
• Isotopes and Elements can be represented with the following formula: Ax±nZA where:
• X is the element symbol
• A is the mass number (neutrons + protons)
• Z is the proton number
• N is the charge

Charge

• In a neutral atom, the number of electrons equals the number of protons, resulting in no overall charge.
• If there are more electrons than protons: negative charge.
• If there are fewer electrons than protons: positive charge.
• The number of protons defines the element and does not change for a particular element.

Atoms and EM Radiation

• When electrons change orbits (move closer or further from the nucleus).
• When electrons move to a higher orbit (further from the nucleus).
• The atom absorbs EM radiation.
• When electrons fall to a lower orbit (closer to the nucleus).
• The atom emits EM radiation.
• If an electron gains enough energy, it can leave the atom and form a positive ion.

Evolution of the Atomic Model

• 1800: Dalton proposed that all matter consists of indivisible, tiny spheres called atoms.

• 1897: J.J. Thomson discovered the electron, leading to the Plum Pudding Model

• Plum Pudding Model: A sphere of positive charge with negative electrons dispersed throughout to cancel out the charge

• 1911: Rutherford realized that most of the atom is empty space.

• Gold Foil Experiment:

  • Most alpha particles went straight through the gold foil, therefore most of the atom is empty space
  • Some alpha particles were slightly deflected, therefore the nucleus must be charged to deflect the positive alpha particles
  • Few alpha particles were deflected by >90°, therefore the nucleus contained most of the mass

• 1913: Rutherford’s Model shows a positively charged nucleus at the center of the atom, with negative electrons existing in a cloud around the nucleus

• 1913: Bohr proposed that the electrons existed in fixed ‘orbitals’, otherwise the electrons in the cloud close to the nucleus would get attracted, and cause the atom to collapse.

• Later on:

• The positive charge of the nucleus could be subdivided into smaller particles, each with the same amount of charge, which became known as the proton.

• 20 years after the 'nucleus' was an accepted scientific idea, James Chadwick provided evidence to prove neutrons existed

Radiation

• Some atomic nuclei are unstable and undergo radioactive decay to become more stable, emitting radiation. This is a random process.
• Activity measures the rate at which unstable nuclei decay in a source, measured in Becquerel (Bq). A sample with high activity has a fast decay rate.
• Count-rate is the number of decays recorded by a detector per second (e.g., using a Geiger-Muller tube).

Forms of Decay

• Alpha (α) Decay: Consists of a helium nucleus. Highly ionizing and weakly penetrating. Can be stopped by ~5cm of air.
• Beta Minus (β) Decay: Consists of an electron. Medium ionizing and medium penetration ability. Stopped by ~50cm of air, or a sheet of paper.
• Gamma (γ) Decay: Electromagnetic radiation. Low ionizing and highly penetrating. Requires very far in air or a few cm of lead to be stopped.
• Neutrons

Nuclear Equations

• Used to represent radioactive decay.
• Alpha Particle: 42He
• Beta Particle: 0−1e
• The emission of nuclear radiation can change the mass and/or charge of the nucleus.

Radioactive Decay

• Alpha Decay: AXY → A−4Z−2 Y + 42He

• Alpha decay causes both the mass and charge of the nucleus to decrease

• Beta Decay: AXY → Az+1Y + 0−1e

• Beta decay does not cause the mass of the nucleus to change, but does cause the charge of the nucleus to increase.

• Gamma Decay:

• Does not cause the mass or charge to change.

Half-Life

• The half-life of an isotope is the time taken for half the nuclei in a sample to decay or the time taken for the activity or count rate of a sample to decay by half.
• It cannot be predicted when any one nucleus will decay, but the half-life is a constant that enables the activity of a very large number of nuclei to be predicted during the decay.
• For example if 80 atoms falls to 20 over 10mins, the half-life of the atoms is 5mins.

Half-Life Properties

• Short Half-Life: source poses less of a risk, it is very radioactive initially, but quickly dies down, so presents less of a long-term risk.
• Long Half-Life: source remains weakly radioactive for a long period of time.
  • For example Americium, which has a half-life of 432 years and is an alpha emitter, is used in smoke alarms

Net Decline

• Can be calculated by halving the initial number of nuclei a specific number of times Net Decline = (initial number − number after X half lives) / initial number

Contamination

• Lasts for a long period of time. The source of the radiation is transferred to an object.
• Radioactive contamination is the unwanted presence of radioactive atoms on other materials, such as radioactive dust settling on skin
• Contamination is hazardous through the decaying of the contaminated atoms releasing radiation

Irradiation:

• Lasts only for a short period of time. The source emits radiation, which reaches the object.
• Exposes an object to nuclear radiation, but does not make it radioactive.
• Radioactive dust emitting beta radiation irradiates skin.
• Irradiation is used medically to kill bacteria on the surface of medical items, without making the medical tools themselves radioactive.

Scientific Reports

• Scientific reports need to be peer reviewed before publishing to prevent safety levels based on incorrect measurements causing people to die if the report is on the effects of radiation on humans.

Background Radiation (Physics only)

• Weak radiation that can be detected from natural/external sources: cosmic rays, radiation from underground rocks, nuclear fallout, and medical rays.
• The level of background radiation and radiation dose may be affected by occupation and/or location.
• Measurement of Radiation Dose, Sieverts (Sv).

Uses (Physics only)

• Tracers: Technetium is used as a medical tracer due to its half-life of 6hrs.

• decays into a safe isotope that can be excreted by the body

• is injected or swallowed will decay quickly but for long enough to be detected

• is a gamma emitter, so can pass through the body tissue without being absorbed

• Chemotherapy: Gamma emitters are used to emit gamma rays, are directed onto cancerous cells, which absorb the energy and die, controlling the disease.

• Can be used to control any other unwanted tissue too

• It is hard to direct accurately, which may cause surrounding healthy cells may also be irradiated, and their destruction.

Nuclear Fission (Physics only)

• Nuclear fission is the splitting of a large and unstable nucleus (e.g., uranium or plutonium).
• Spontaneous fission is rare; commonly unstable nucleus must first absorb a neutron.
• When unstable nuclei absorbs a neutron it splits into two smaller nuclei, roughly equal in size, and then emits two or three neutrons and gamma rays.
• Energy is released by the fission reaction.
• Released neutrons may collide with other radioactive nuclei, making them absorb the neutron, which causes them to become unstable. This nucleus splits, releasing another neutron and produces more energy
• This chain reaction occurs as energy is being released and one split causes another. Neutrons need to be absorbed initially before it splits, because it needs a little extra energy before it can split and release lots of energy.
• After the nucleus splits, it leaves two smaller nuclei roughly equal in size who have kinetic energy
• If the chain reaction is not controlled, it will increase at an exponential rate – which is what happens in a nuclear weapon
• This process is used in nuclear fission with uranium nuclei

Nuclear Fusion (Physics only)

• This is when two small nuclei fuse to form a heavier nucleus, releasing energy.
• The sum of the masses of the two nuclei is more than the mass of the heavier nucleus.
• Some of the mass is converted into energy (released as radiation).
• The sun is a natural fusion reactor.
• Fusion would be a much more efficient way of producing energy compared to fission, however no design has been produced that could accomplish positive net energy on Earth.