Development of Atomic Model

Development of Atomic Model

460 BC: Democritus

  • Proposed the concept of atoms as tiny, indivisible spheres.

  • Introduced the idea that atoms could not be broken down further.

1804: John Dalton

  • Agreed with Democritus' idea of atoms as tiny, hard structures.

  • Proposed that each element is composed of different types of atoms, which are distinct and combine in fixed ratios to form compounds.

1897: J.J. Thomson

  • Discovered electrons, which led to revisions of Dalton's model.

  • Suggested a new model where atoms consist of a positively charged 'cloud' with negatively charged electrons ('Plum Pudding Model').

1909: Ernest Rutherford

  • Conducted the Rutherford Scattering Experiment, which involved shooting alpha particles at a thin gold foil.

  • Conclusions from the experiment:

    • Conclusion 1: Most alpha particles passed straight through, suggesting atoms are mainly empty space.

    • Conclusion 2: A small number of particles were deflected backwards , indicating the presence of a dense, positively charged nucleus.

    • Conclusion 3: Further movement of alpha particles at large angles hinted at a concentrated mass within the atom.

1913: Niels Bohr

  • Developed the Bohr Model that suggested electrons orbited the nucleus at specific energy levels (or shells) and could jump between levels.

  • Atoms remain stable because electrons in these shells do not spiral into

1933: Rutherford and James Chadwick

  • Chadwick proved the existence of neutrons, completing the understanding of protons, neutrons, and electrons in the atomic structure.

  • Illustrated that the atomic nucleus contains protons (positive charge) and neutrons (neutral) with electrons in shells around the nucleus.

Current Model of the Atom

Structure of the Atom

  • Nucleus: Contains protons and neutrons.

    • Positive charge due to protons.

    • Very dense.

  • Electrons: Carry a negative charge and occupy energy levels or shells around the nucleus.

  • Atoms have a significant amount of empty space between the nucleus and electrons.

Behaviour of Electrons

  • Electrons can gain energy by absorbing electromagnetic (EM) radiation, moving to higher energy levels further from the nucleus.

  • When releasing energy, they move to lower levels and emit EM radiation.

Isotopes

  • Isotopes are defined as two elements having the same number of protons and electrons but differing in the number of neutrons.

  • Isotopes of an element have unique mass numbers (total protons + neutrons).

Radioactive Decay

Types of Radioactive Decay

1. Alpha Decay

  • Alpha Particles: alpha particle emitted from nucleus

  • Composed of 2 protons and 2 neutrons.

  • Highly ionising but low penetration ability; can be stopped by paper.

  • Often used in smoke detectors due to their ability to ionize air and create a current.

2. Beta Decay

  • Beta Particles: High-speed electrons emitted from the nucleus.

  • Medium ionisation and penetration capabilities; can be stopped by thin sheets of aluminium.

  • In beta decay, a neutron is transformed into a proton, emitting a beta particle.

3. Gamma Decay

  • Gamma Rays: High-energy electromagnetic waves emitted from a nucleus.

  • Have no mass and can penetrate effectively; most penetrating type of radiation.

  • Stopped only by thick sheets of lead.

Summary of Radiation Characteristics

  • Alpha Radiation:

    • Symbol: \alpha

    • Composition: 2 protons, 2 neutrons

    • Low penetration, highly ionizing.

  • Beta Radiation:

    • Symbol: \beta

    • Composition: 0 protons, 0 neutrons

    • Moderate penetration, medium ionizing.

  • Gamma Radiation:

    • Symbol: \gamma

    • Composition: 0 protons, 0 neutrons

    • Highest penetration, least ionizing.

Nuclear Decay Equations

Alpha Decay Equation

A \rightarrow A-4 \text{ , } Z \rightarrow Z-2 + \text{He}^{2+} $$

  • Example:

  • ^{226}{88} \text{Ra} \rightarrow ^{222}{86} \text{Rn} + \text{He}^{2+}

Beta Decay Equation

A \rightarrow A \text{ , } Z \rightarrow Z+1

  • Example:

  • ^{14}{6} \text{C} \rightarrow ^{14}{7} \text{N} + \beta

Gamma Decay Equation

A \rightarrow A \text{ , } Z \rightarrow Z

  • Example:

  • ^{234}{90} \text{Th} \rightarrow ^{234}{90} \text{Th} + \gamma $$

Radioactive Decay Characteristics

Decay Characteristics

  • Radioactive decay is spontaneous and cannot be controlled; it is impossible to predict which nucleus will decay next.

  • Half-life: The time taken for half of the radioactive nuclei in a sample to decay.

  • Activity is measured in Becquerels (Bq) where 1 Bq = 1 decay per second.

  • Short half-life isotopes present high initial activity, rapidly decaying and potentially leading to dangerous exposure to radiation.

  • Long half-life isotopes decay slowly, maintaining lower activity levels over extended periods, posing a risk due to prolonged exposure.

Background Radiation

Sources of Background Radiation

  • Naturally occurring radioactive substances (rocks, soil, food).

  • Cosmic rays originating from outer space.

  • Human activities (nuclear explosions, medical treatment).

Radiation Exposure and Protection

  • To mitigate the effects of radiation exposure, strategies include:

    • Keeping radioactive sources in lead-lined boxes.

    • Using physical barriers and maintaining distance from radioactive materials.

    • Wearing protective gear (gloves, gowns) when handling radioactive substances.

Contamination vs. Irradiation

  • Contamination occurs when radioactive materials make contact and attach to an object or person.

  • Dangerous because it can introduce radioactive particles internally.

  • Methods to reduce contamination risks:

    • Use of gloves and protective suits.

  • Irradiation refers to exposure to radiation without direct contact with radioactive substances, such as radiation from external sources.

Effects of Radiation

Biological Effects

  • High doses of radiation can kill cells, while lower doses may cause changes leading to uncontrolled cell growth or cancer.

  • Symptoms of radiation exposure can include vomiting and fatigue, commonly termed radiation sickness.

Applications of Radioactivity

Medical Uses

  • Radioactive isotopes can be ingested to track bodily functions (e.g., Iodine-123 for thyroid imaging).

  • Radiotherapy: High doses of radiation are used to target and destroy cancer cells while trying to minimize damage to surrounding healthy tissues.

  • Gamma rays are typically employed due to their penetration depth.

Nuclear Reactions

Nuclear Fission

  • Definition: The process of splitting a nucleus into smaller nuclei, releasing energy.

  • A neutron collides with a nucleus,make nuclei unstable, splits into two , neutrons are released & process restarts.

  • Chain Reaction: Some released neutrons can initiate further fission events, making it possible to sustain a reaction and release large amounts of energy.

  • Control of nuclear fission reactions can be achieved by using absorbent materials that capture neutrons.

Nuclear Fusion

  • Definition: The joining or combining of smaller nuclei to form a larger, heavier nucleus, such as fusing hydrogen nuclei to create helium.

  • Fusion releases significantly more energy than fission.

  • Challenges of achieving fusion include the need for high temperatures and pressures to overcome e the repulsion