Formation and Synthesis of Heavier Elements

Learning Objectives

• Give evidence for and describe the formation of heavier elements during star formation and evolution.

• Explain how the concept of atomic number led to the synthesis of new elements in the laboratory.

• Appreciate the importance of the discovery and understanding of atomic number, which led to the synthesis of new elements in the laboratory.

How Did the Universe Form?

• Theories/beliefs about the formation of the universe:

  • Big Bang Theory

  • Creationism

Big Bang Theory

• Occurred 13.8 billion years ago.

• The universe started with a small singularity.

• Stretched the fabric of spacetime, sending superheated matter in all directions.

• As it expanded, matter cooled and aggregated:

  • Forming atoms, then elements, then stars, galaxies, and ultimately everything we know and see today.

Key Aspects of the Big Bang Theory

• Proposes the universe was once extremely compact/dense and hot.

• The Big Bang was NOT an explosion carrying matter outward from a point. It refers to the rapid expansion of space itself.

• Explains the formation of different elements involving nuclear reactions, including fusion, fission, and radioactive decay.

Inside the Atom: Meet the Particle Players

• Three main subatomic particles in an atom:

  • Protons

  • Neutrons

  • Electrons

Periodic Table of Elements

• Key components to identify:

  • Atomic number

  • Symbol

  • Name

  • Abridged standard atomic weight

Memory Lane

• Mass number (A) = Number of protons (Z) + Number of neutrons (N)

  • $A = Z + N$

Nucleosynthesis

• The process of creating new atomic nuclei from preexisting nucleons (protons and neutrons).

Big Bang Nucleosynthesis

• Within about 3 minutes after the Big Bang, conditions cooled enough for protons and neutrons to form hydrogen nuclei.

• Some of these nuclei combined to form helium, though in much smaller quantities (just a few percent).

• After about 20 minutes, Big Bang nucleosynthesis ended, and no further nuclei could form.

• Produced no elements heavier than lithium

Formation of Light Elements in the Big Bang

• The universe began from a tiny, extremely dense, and hot energy that expanded (about 13.8 billion years ago).

• Energy condensed into quarks, which combined to form protons and neutrons as the universe continued to cool.

• After one second, the protons and neutrons existed.

• For about 1,000 seconds, the lightest elements could form everywhere in the universe.

• At the end of this period of nucleosynthesis, hydrogen (1 proton) made up close to 75% of the ordinary matter in the universe by mass.

Composition of Light Elements

• Hydrogen: P (Proton)

• Deuterium: pn (Deuteron)

• Helium: PP, Pn, np

• Tritium: P, n

• Lithium-6: n

• Lithium-7: P, n

Stars

• Immense, luminous spheres of extremely hot gases held together by gravity.

• They pass through various stages known as the life cycle of a star.

Stellar Evolution

• The process by which a star changes through time; comparable to a human life cycle.

• Mass depends upon the amount of stellar material available in the nebula from which it forms.

  • The more massive a star, the shorter its life span because the hydrogen supply is used up much quicker, due to the higher core temperatures.

  • Stars that burn for longer tend to be much colder.

• Solar mass: $1.989 \times 10^{30} kg$

Life Cycle of a Star Stages

• Giant Gas Cloud/Nebula (gravitational collapse)

• Protostar

• T-Tauri Phase

• Main Sequence

• Red Giant

• Planetary Nebula

• White Dwarf

• Black Dwarf

• Red Supergiant

• Supernova

• Neutron Star

• Black Hole

Stellar Nucleosynthesis

• The process by which elements are created within stars by combining protons and neutrons together from the nuclei of elements (H and He).

• Elements from Beryllium to Iron.

Star Formation Theory

• Stars are formed when a dense region of a molecular cloud collapses.

Nebula

• Giant cloud of gas and dust; this stage is the start of their life cycle.

Protostar

• Result of the gravitational collapse [contraction] of a nebula.

• Due to the increase in temperature [reaching about 10 million K], a nuclear reaction occurred [hydrogen fusion].

Main Sequence Star

• Formed when the protostar contracts, glows, and attains its gravitational equilibrium or becomes stable.

• 90% of a star’s life is spent in this stage.

• In the core of a main sequence star, hydrogen fuses into helium through the proton-proton chain.

• The gravitational force of a main sequence star forces hydrogen and helium to fuse, resulting in the burning of the two primordial elements.

Red Giant

• The star is unable to generate heat when it runs out of hydrogen in its core, leading to its contraction and expansion.

• It cools down and glows red.

• Formed when helium is converted to carbon [triple-alpha process] at the core while hydrogen is converted into helium surrounding the core.

Planetary Nebula

• A region of glowing shell of cosmic gas, plasma, and dust formed from the cast-off outer layers of a dying star that ejects into space after running out of fuel.

White Dwarf

• Red giant star becomes exhausted of nuclear fuel.

  • The outer material is blown off into space, leaving the inert Carbon.

• The remnant of a planetary nebula.

• Extremely dense – about the size of Earth but with the mass of the Sun.

Black Dwarf

• The remnant of the white dwarf that has cooled down and no longer emits light and heat.

• This process takes such a long time that no black dwarfs have formed since the beginning of the Universe, so they are strictly theoretical.

Red Supergiant Star

• Massive stars cool and expand faster than low-mass stars and will turn into a red supergiant star.

• The process of nuclear fusion in the core carries on until iron is formed.

• No further fusion can occur at this stage, as fusing iron consumes energy rather than releasing it.

Supernova

• Happens when the core can no longer produce the needed energy to resist gravitational force, leading to its explosion and the release of large amounts of energy.

• Fusion of iron can finally occur, and all heavier elements.

Supernova Nucleosynthesis

• Elements heavier than Iron are formed during a supernova event.

• Elements from Cobalt to Uranium.

Neutron Star

• If the collapsing core is of 1.4-3 solar masses, it forms a Neutron Star.

• Highly dense, heavy, and trim body comprised of neutrally charged neutrons.

• The densest and heaviest objects in the Universe.

Black Hole

• Stellar cores of more than 3 solar masses: the force of gravity is so strong that the collapse is unstoppable.

• The gravitational force is so strong that nothing can escape it, not even light.

Nuclear Reactions

• Nuclear reaction is a process in which a nucleus either combines with another nucleus (through nuclear fusion) or splits into smaller nuclei (through nuclear fission).

• These processes involve the emission of energetic particles of an atom, a phenomenon known as radioactivity.

• The radioactive particles may be elements, electrons, protons, and neutrons, among others.

Atomic Number and Synthesis of New Elements

• Nuclear reaction:

  • Alpha Emission: A particle with two protons and two neutrons is emitted, resulting in a lighter new element.

  • Example: $\begin{array}{l}_{92}^{238}U \rightarrow _{90}^{234}Th + _{2}^{4}He\end{array}$

  • Beta Emission: A neutron becomes a proton, and an electron will be ejected, resulting in a new element with the same mass.

    • Example: $\begin{array}{l}_{53}^{131}I \rightarrow _{54}^{131}Xe + _{-1}^{0}e\end{array}$

  • Gamma Emission: A gamma ray will be emitted when a radioactive nuclide leaves a nucleus in an excited state.

    • Example: $\begin{array}{l}_{56}^{137}Ba \rightarrow _{56}^{137}Ba + \gamma-photon\end{array}$

Alpha Particle

• Helium Nucleus

• 2 protons, 2 neutrons

• Relative Charge: +2

• Relative Mass: 4

• $\begin{array}{l}_{2}^{4}He\end{array}$

• $\begin{array}{l}_{88}^{222}Ra \rightarrow _{86}^{218}Rn + _{2}^{4}He\end{array}$

Beta(-) Particle

• Basically the same as an electron

• Has a mass of zero

• Charge of -1

• An excess neutron transforms into a proton and an electron. The proton stays in the nucleus, and the electron is ejected energetically.

Beta plus (+) Decay/Positron Emission

• A proton is converted to a neutron, and the process creates a positron (the antimatter form of an electron) and an electron neutrino

• Anti-particle of electron

• Has a +1 charge

• $\begin{array}{l}_{+1}^{0}\beta, _{1}^{0}\beta\end{array}$

Proton-Proton Chain Reaction in Main Sequence Star

• Nuclear fusion; an average star gets its energy and converts Hydrogen into Helium

• It starts with a proton and a neutron fused to form deuterium.

• When one proton collides with deuterium, Helium-3 is formed.

• Two Helium-3 nuclei collide and will form Helium-4

• Massive Star: They undergo CNO (Carbon, Nitrogen, Oxygen) cycle to convert Hydrogen into Helium.

  • Carbon-12 fused with proton (H) emits a gamma-ray, producing nitrogen-13.

  • Nitrogen-13 is unstable and emits a beta particle, decaying to carbon-13 (beta plus decay).

  • Carbon-13 captures proton (H) = Nitrogen-14 via emission of a gamma-ray.

  • Nitrogen-14 + proton = Oxygen-15 by emitting a gamma-ray.

  • Oxygen-15 undergoes beta plus decay = Nitrogen-15.

  • Nitrogen-15 fused with proton gives off Helium nucleus (alpha particle) and ends up with Carbon-12.

  • The process repeats again.

  • 6 PROTON → neutron + positron

  • 7 PROTON → neutron + positron

Tri Alpha Process:

-Happens in red giant stars once they leave the stage of main sequence star.
-This is how three Helium-4 are converted into Carbon

• $\begin{array}{l}_{2}^{4}He + _{2}^{4}He + _{2}^{4}He \rightarrow _{6}^{12}C \end{array}$

Simple Helium Fusion

• $\begin{array}{l}_{2}^{4}He + _{2}^{4}He \rightarrow _{4}^{8}Be \end{array}$

  • $_{4}^{8}Be$ is unstable and breaks apart rapidly.

  • $\begin{array}{l}_{2}^{4}He + _{2}^{4}He + _{2}^{4}He \rightarrow _{6}^{12}C + \nu \end{array}$

Alpha Ladder/Process/Capture

• Red super giant stars convert helium into heavier elements until Iron.

• Fusing iron uses up energy.

• No energy output, the star can’t support its gravity.

• This causes the core to collapse, leading to a supernova explosion.
  -BETA PLUS DECAY = Proton → neutron + positron

Supernova Nucleosynthesis

-Elements heavier than Iron are formed during a supernova event.
-Elements from Cobalt to Uranium

Neutron Capture

• A neutron is added to a seed nucleus, creating a heavier isotope (stable or unstable).

If it's unstable, it often undergoes beta minus decay—a neutron turns into a proton, and the element changes into a new one.
Neutron capture can be slow or rapid.

S-Process or Slow Process

• Happens in older stars.

• Neutrons are captured slowly, allowing time for unstable isotopes to decay into stable ones before another neutron is added.

• Builds elements like strontium (Sr), barium (Ba), and lead (Pb).

R-Process or Rapid Process

• Happens in extreme environments with many neutrons, like: Supernovae and Neutron star mergers.

• Neutrons are captured very quickly, faster than the nucleus can decay.

• Produces very heavy, unstable nuclei that later decay into stable elements.

• Builds gold (Au), platinum (Pt), etc.

• This is what happens in a supernova forming heavier elements than Iron, with the process known as supernova nucleosynthesis.

Artificial Elements

• Most elements beyond uranium are not naturally formed in stars or supernovae; instead, they are created artificially in laboratories through nuclear reactions.

• Do not exist in nature (or exist in only trace, short-lived forms)

• Are created by bombarding heavy atoms with neutrons or lighter nuclei (like calcium).

• Are very unstable and radioactive, decaying in seconds or less.

Practical Application of Lab-Made Elements

• Plutonium-238

  • Used as a power source for space missions (radioisotope thermoelectric generators – RTGs) like the Voyager, Curiosity, and Perseverance rovers.

• Americium-241

  • Commonly used in smoke detectors and industrial gauging devices (like thickness gauges).

• Californium-252

  • Used in nuclear reactors and radiography; also in neutron moisture gauges and to start nuclear reactors (as a neutron source).

Synthetic/Artificial Elements

-Thanks to the concept of atomic number: Scientists knew exactly how to create new elements.
-It helped extend the periodic table beyond the naturally occurring elements (like Uranium, Z=92).