Formation of Elements, Atomic Structure, Chemical Reactions

Physical Science: Second Semester Study Notes

Subject Overview:

• Topics include the formation of elements, atomic structure, chemical reactions, models of the universe, and theories of light.

Formation of Light and Heavy Elements:

• The origin of naturally occurring elements falls into two phases: Big Bang/Primordial Nucleosynthesis and Stellar Nucleosynthesis.
• Nucleosynthesis creates new atomic nuclei from preexisting nucleons (protons and neutrons).
• Primordial or Big Bang Nucleosynthesis produces the "light elements" shortly after the Big Bang.

Nuclear Fusion:

• High energy and temperature in the universe cause neutrons and protons to combine via nuclear fusion.
• Through nuclear fusion, light elements like Hydrogen (H), Helium (He), and trace amounts of Lithium (Li) and Beryllium (Be) were formed.
• Isotopes produced during Big Bang nucleosynthesis were H-1, H-2, H-3, H-4, L-7.
• An Isotope is a form of an element with the same atomic number but a different atomic mass or mass number.

Heavier Elements:

• Heavy elements formed billions of years after stars appeared.
• The density inside a star is sufficient to sustain fusion long enough to synthesize heavy elements.
• Stars are hot and dense enough to burn hydrogen-1 (1H) into helium-4 (4He).
• Stellar Nucleosynthesis is forming heavy elements by fusing lighter nuclei inside stars.
• Nuclear synthetic pathways or nuclear fusions help produce heavy elements like the Carbon-Nitrogen-oxygen cycle, proton-proton fusion, and triple alpha process.
• Layers near star cores are hot enough to nucleosynthesize heavy elements like silicon and iron.

Elements Heavier than Iron:

• Elements heavier than iron cannot form via fusion because of the tremendous energy needed.
• Heavy elements form in a supernova, which is a massive star explosion.
• In a supernova, a neutron capture reaction results in the formation of heavy elements by adding neutrons to existing nuclei instead of fusing light nuclei
• Adding neutrons to a nucleus does not change an element, but instead produces a more massive isotope of the element.
• Elements heavier than iron need a massive amount of energy to form, therefore they are produced from a neutron capture reaction.
• A supernova involves the explosive death of a star.