Physci Handout: Formation of Heavier Elements

The Formation of Heavier Elements during Star Formation and Evolution Elements heavier than beryllium are formed through stellar nucleosynthesis. Stellar nucleosynthesis is the process by which elements are formed within stars. The abundances of these elements change as the stars evolve. Evolution...

The Formation of Heavier Elements during Star Formation and Evolution Elements heavier than beryllium are formed through stellar nucleosynthesis. Stellar nucleosynthesis is the process by which elements are formed within stars. The abundances of these elements change as the stars evolve. Evolution of Stars Evolution of Stars Star Formation Theory - Stars form from the collapse of dense regions in molecular clouds. - Fragments of the collapsing cloud contract to form a protostar. Protostar Phase - Strong gravitational forces cause the protostar to contract, increasing temperature. - At around 10 million K, nuclear reactions initiate. Main Sequence Star - Nuclear reactions release positrons and neutrinos, increasing pressure and halting contraction. - Gravitational equilibrium is reached, marking the transition to a main sequence star. Hydrogen Fusion - In the main sequence phase, hydrogen fuses into helium through the proton-proton chain. Red Giant Phase - Once most hydrogen is converted to helium, fusion stops and core pressure decreases. - Gravity compresses the star, initiating helium and hydrogen burning. - Helium is converted to carbon in the core, while hydrogen fuses into helium in the surrounding shell, transforming the star into a red giant. Element formation Low mass stars  Element formation for High mass stars The Indivisible Atom - Democritus of Abdera (460 - 370 B.C.) and his teacher Leucippus of Miletus (c.500 B.C.) were Greek scholars who believed that matter could be divided into tiny particles until such point where it can no longer be divided anymore. They became the first proponents of the atomic theory. Their early ideas on atoms are summarized below. - All matter is made up of tiny, indivisible particles called atoms, which come from the Greek word atomos meaning uncuttable. The atoms are indestructible, impenetrable, and unchangeable. - The atoms make up the universe as they are continuously moving in a "void" that surrounds them, repelling each other when they collide, or combining into clusters. - Atoms are completely solid which means that there is no void or empty space inside that will make them prone to disintegration or destruction. - Atoms are homogeneous in nature. They have no internal structures. - Atoms come in different shapes and sizes. - These proposed ideas about atoms were supported by some Greek philosophers but were strongly opposed by others especially Aristotle. - John Dalton described the atom as spherical. - Joseph John Thomson discovered the electron. - Ernest Rutherford proposed that the electrons orbit around the nucleus. He, together with his students, discovered the proton. - Niels Bohr proposed that electrons orbit around the nucleus in set energy levels. - James Chadwick discovered the neutron. - Niels Bohr proposed that the electrons orbit around the nucleus in set energy levels. - In the quantum mechanical model, the nucleus is surrounded by a cloud of electrons called orbitals. - The nuclear model states that the nucleus is small, dense, and located at the center of the atom. It contains protons and neutrons. - The nucleus is positively charged. It contains nearly all the mass of the atom. The electrons orbit around it. - The nuclear model has been deduced from the experiment done by Rutherford. Polarity means having dipoles, a positive and a negative end. Based on polarity, molecules can be polar or nonpolar. Polar molecules have dipoles. Their dipole moments do not add up to zero (or do not cancel out). Water and carbon monoxide are examples of polar molecules. Nonpolar molecules do not have positive or negative ends. Their dipole moments add up to zero (they cancel out). Carbon tetrachloride and methane are examples of nonpolar molecules. Generally, you can tell if a molecule is polar or nonpolar based on: - its structure or shape - the polarity of the individual bonds present in the molecule Steps in Determining the Polarity of a Molecule 1\. Draw the correct Lewis structure and molecular geometry of the molecule. 2\. Identify the polarity of each bond present in the molecule. A bond is polar when the atoms in the bond have different electronegativity. Recall that electronegativity is the measure of the tendency of an atom to attract a bonding pair of electrons. (You may use the periodic table to determine the electronegativity values of the atoms.) 3\. Draw the dipole moment vectors for polar bonds. The dipole moment vector points to the more electronegative atom.  4\. Determine the sum of the dipole moment vectors. If the dipole moments cancel out each other, the molecule is nonpolar; otherwise, it is polar. - Polar molecules have stronger attractive forces compared to nonpolar molecules. - In general, polar molecules have higher boiling and melting points compared to nonpolar ones. - "Like dissolves like." Polar solutes dissolve in polar solvents while nonpolar solutes dissolve in nonpolar solvents. Intermolecular forces are the attractive forces between molecules. The three types of IMFA are London dispersion forces, dipole-dipole forces, and hydrogen bonding. - The properties of molecules depend on the type and strength of their intermolecular forces of attraction. - "Like dissolves like." When the solute and the solvent both exhibit same intermolecular forces of attraction, they form a solution. - When comparing properties, stronger intermolecular forces result in higher boiling and melting points, higher viscosity, higher surface tension, and lower vapor pressure. - Increasing strengths of IMFA: London dispersion forces, Dipole-dipole forces, H-bondin Biomolecules are large organic compounds that are important to life's processes. They are generally classified into four major groups -- proteins, carbohydrates, lipids, and nucleic acids. - Proteins are biomolecules composed of amino acid units. The sequence of amino acids determines the protein's shape and function. In the human body, proteins hasten chemical reactions, transport substances, and provide structural support. - Carbohydrates are molecules that are composed of carbon, hydrogen, and oxygen. Their functions are to store energy and serve as the framework of cellular structures. - Lipids are large, nonpolar biomolecules mainly composed of carbon, hydrogen, and oxygen. They function as reserved sources of energy and protective coating of organisms. According to the collision theory, the rate of reaction is directly proportional to the number of collisions between the reactants. - An effective collision is characterized by reactants colliding with proper orientation and enough energy to surpass the activation energy. - The activation energy or energy barrier is the energy needed to be surpassed by the reactants so that they will be transformed into products. - There are three factors that affect the rate of the reaction: 1) concentration, 2)temperature, and 3) particle size, 4) increasing pressure, 5) use of catalyst - Increasing the concentration or the temperature of the reaction leads to an increase in reaction rate. On the other hand, decreasing the particle size increases the reaction rate. - A catalyst increases the rate of the reaction by lowering the activation energy of a reaction. - A homogeneous catalyst exists in the same phase as the reaction it catalyzes. - A heterogeneous catalyst exists in a different phase as the reaction it catalyzes. - Enzymes are homogeneous, highly specific, and efficient biological catalysts. - Energy sources may be renewable or nonrenewable. - Renewable energy sources are those that do not get depleted. - Nonrenewable energy sources are finite, so they will get depleted over time. - Common structures in all power plants are the steam- or vapour-driven turbines which spin generators to produce electricity.