chapter two structure of matter

Objectives of the Chapter

• Understand the historical development of atomic theory.

• Identify and describe the atomic structure and the role of electron shells.

• Comprehend radioactivity and the properties of alpha and beta particles.

• Differentiate between particulate and electromagnetic ionizing radiation.

Outline of Key Topics

• History of the Atom

  • Centuries of Discovery

  • Greek Atom Theory

  • Dalton’s Atomic Theory

  • Thomson's Plum Pudding Model

  • Rutherford’s Nuclear Model

  • Bohr’s Model

• Atomic Structure

  • Fundamental Particles

  • Electron Arrangements

  • Electron Binding Energy

• Radioactivity

  • Types of Ionizing Radiation

  • Properties of Alpha and Beta Particles

  • Radioactive Half-life

The Development of Atomic Theory

• Greek Atom: Proposed by early philosophers, where matter was believed to be made of four elements (earth, water, air, fire) combined with essences (wet, dry, hot, cold).

• John Dalton (1808): Introduced the idea that each element has identical atoms differing from those of other elements, leading to atomic theory based on atomic mass.

• J.J. Thomson (1897): Discovered electrons and proposed the Plum Pudding Model, describing atoms as a mix of positive charge with embedded electrons.

• Ernest Rutherford (1911): Conducted gold foil experiment, leading to the nuclear model of the atom, which has a small, dense positively charged nucleus and electrons surrounding it.

• Niels Bohr (1913): Enhanced Rutherford's model by introducing quantized electron orbits or shells around the nucleus, forming the Bohr model, which describes the atom like a small solar system.

Atomic Structure

• Fundamental Particles

  • Protons: Positively charged, located in the nucleus, ~1 amu.

  • Neutrons: Neutrally charged, also in the nucleus, ~1 amu.

  • Electrons: Negatively charged, located in orbitals around the nucleus, negligible mass (~0.000549 amu).

• Electron Arrangement

  • Electrons occupy specific energy shells (K, L, M, N)

  • Maximum electrons per shell given by the formula: 2n², where n is the shell number.

• Electron Binding Energy

  • The amount of energy required to remove an electron from the atom. K-shell electrons are tightly bound, more energy is required to ionize them compared to those in outer shells.

Radioactivity

• Types of Ionizing Radiation

  • Particulate Radiation:

    • Alpha Particles: Composed of 2 protons and 2 neutrons; not significantly penetrating but highly ionizing; emitted from heavy radioactive nuclei.

    • Beta Particles: Electrons or positrons emitted from nuclei; lighter than alpha particles and can penetrate deeper into tissue.

  • Electromagnetic Radiation:

    • Includes X-rays and gamma rays; photon-based, no mass or charge, can penetrate various materials depending on energy.

• Radioactive Half-life

  • Time required for half of the radioactive substance to decay; unique for each isotope; utilized in various applications like radiometric dating.

Summary of Key Concepts

• Atoms are the smallest unit of elements and compounds are formed by atoms combining through either ionic (electrons transferred) or covalent (electrons shared) bonds.

• Nucleons (protons and neutrons) are found within the nucleus; electrons orbit the nucleus in defined shells or energy levels.

• Nuclear stability is dictated by the balance of protons and neutrons, leading to concepts of isotopes, isobars, and isotones.

• Radioactivity describes the decay of unstable atomic nuclei, releasing particles and energy, characterized primarily through beta and alpha emissions.

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