Atomic Structure and Nuclear Physics Milestones

Early Atomic Theories and Discoveries

Early Theories

• Greek philosophers believed everything was made by combining a few simple, undecomposable substances: the four elements: water, air, fire, and earth.

Dalton's Atomic Theory

• Matter is made up of extremely small, indivisible, and indestructible atoms.
• All atoms of a specific chemical element are identical but differ from atoms of other elements.
• Chemical compounds are formed by the union of atoms of different elements in the same, fixed proportions.

Fundamental Definitions

• Chemical Element: A substance that cannot be decomposed into simpler substances.
• Compound: A substance that can be decomposed into simpler substances.
• Electrolysis: A process used to decompose a pure substance using an electrical current.

Atomic Models

Thomson's Model

• Atoms are composed of negatively charged electrons distributed throughout a sphere of positive charge, making the atom electrically neutral overall.

Rutherford's Model

• Atoms are composed of protons, neutrons, and electrons.
• The number of protons and electrons is equal, resulting in electrical neutrality.
• Protons and neutrons are concentrated in a very small nucleus.
• Electrons revolve around the nucleus in a larger electronic cloud (or shell).

Atomic Characteristics

Atomic Number (Z)

• The number of protons in an atom's nucleus.

Mass Number (A)

• The total number of particles in the nucleus of an atom (the sum of protons and neutrons).

Number of Neutrons (N)

• $N = A - Z$.

Isotopes

• Atoms that have the same atomic number but different mass numbers.

Relative Atomic Mass

• The weighted average of the masses of the natural isotopes of an element.

Radioactivity and Nuclear Processes

Radioactivity

• Some isotopes (radioisotopes) exhibit radioactivity because their nuclei are unstable and must disintegrate.

Types of Radiation

• Alpha Radiation ($\alpha$): Helium nuclei, formed by 2 protons and 2 neutrons.
• Beta Radiation ($\beta$): High-speed electrons emitted.
• Gamma Radiation ($\gamma$): Radiation similar to light or X-rays, but with much higher energy.

Half-Life (Semidesintegration Period)

• The time it takes for half of the nuclei of a radioactive isotope sample to disintegrate.

Nuclear Fission

• The process where uranium atoms, when bombarded with neutrons, break apart, creating a chain reaction.

Risks of Fission

• Nuclear reactions can accidentally get out of control.
• Radioactive waste is generated and must be carefully isolated from the environment.

Nuclear Fusion

• The process where small nuclei join together to form a nucleus with greater mass, releasing a large amount of energy.