Origin of Atoms

Origin of Atoms

• The universe began approximately 13.8 billion years ago.

• Initial formation of hydrogen, helium, and lithium atoms occurred within the first 300,000 years after the Big Bang.

• Clumping of matter eventually led to the formation of stars, which through nuclear fusion created heavier elements.

Nuclear Reactions Overview

• Transition from discussing chemical reactions to nuclear reactions.

• Chemical reactions involve electrons: they rearrange atoms without changing the nuclei, hence no creation or destruction of atoms.

• In contrast, nuclear reactions involve changes in the nucleus, leading to changes in the atomic number of elements.

• Nuclear reactions typically occur under high temperatures or energies, distinguishing them from chemical reactions.

Atomic Structure

• An element is defined by the number of protons in its nucleus (atomic number).

• Notation of nuclear reactions:

  • Atomic number (number of protons) as a subscript on the left.

  • Mass number (number of protons + neutrons) as a superscript on the left.

• Protons and neutrons are collectively known as nucleons.

• Example: Potassium has an atomic number of 19.

• Nitrogen-12 has 7 protons and 5 neutrons (atomic number is 7).

Isotopes

• Isotopes are variations of the same element with differing numbers of neutrons.

• Example: Carbon-12 vs Carbon-13.

  • Carbon-12: 6 protons, 6 neutrons.

  • Carbon-13: 6 protons, 7 neutrons.

Types of Nuclear Reactions

• Fusion: Joining of two light nuclei to form a heavier nucleus (e.g., what happens in stars).

• Fission: The splitting of a heavier nucleus into lighter nuclei.

• Radioactive Decay: Process by which an unstable nucleus emits particles (alpha, beta, or gamma).

Fusion Process

• High temperatures can facilitate fusion:

  • Example: Deuterium (D) and Tritium (T) are isotopes of hydrogen that can fuse.

  • Result of fusion: formation of a helium nucleus, a neutron, and a significant amount of energy.

• Nuclear equation representation of the fusion reaction captures conservation of nucleons and protons, ignoring electrons.

Energy and Plasma

• Nuclear reactions release substantial energy, which is harnessed for nuclear power.

• Understanding of plasma:

  • Plasma is a state of matter containing unbound positive and negative particles.

  • It responds strongly to electromagnetic fields; consists primarily of charged particles.

  • Outnumbering the other states, plasma is especially common in stars where nuclear reactions primarily occur.

Forces in Nuclear Reactions

• As nuclei approach, they repel due to electrostatic force (both positively charged).

• Overcoming this repulsion requires significant energy, increasing potential energy.

• Upon getting very close (nuclear distance), the strong nuclear force can be activated:

  • The strongest fundamental force that operates at short distances, holding the nucleus together.

  • Strong nuclear force acts attractively to bind nucleons, allowing fusion to occur if the kinetic energy is sufficient to overcome the repulsive electrostatic forces.