PHYSICAL SCIENCE THIRD QUARTER EXAMINATION REVIEWER
NUCLEOSYNTHESIS
Nucleosynthesis is the creation of new atomic nuclei, the centers of atoms that are made up of protons and neutrons.
Nucleosynthesis first occurred within a few minutes of the Big Bang. At that time, a quark-gluon plasma, a soup of
particles known as quarks and gluons, condensed into protons and neutrons.
3 TYPES OF NUCLEOSYNTHESIS
Synthesis of the naturally occurring elements and their isotopes present in the Solar System solids may be divided into
three broad segments: primordial nucleosynthesis (H, He), energetic particle (cosmic ray) interactions (Li, Be, B), and
stellar nucleosynthesis (C and heavier elements).
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Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) is the production
of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as
a nucleus) during the early phases of the Universe. (see ppt slides)
There are two important characteristics of Big Bang nucleosynthesis (BBN): It lasted for only about
seventeen minutes (during the period from 3 to about 20 minutes from the beginning of space
expansion after that, the temperature and density of the universe fell below that which is required for
nuclear fusion, with 108K minimum temperature to fuse H to He. Stars formed this was are main
sequence stars.
Stellar nucleosynthesis is the process by which elements are created within stars by combining the
protons and neutrons together from the nuclei of lighter elements. All of the atoms in the universe
began as hydrogen. Fusion inside stars transforms hydrogen into helium, heat, and radiation.
Supernovae are so powerful they create new atomic nuclei. As a massive star collapses, it produces a
shockwave that can induce fusion reactions in the star's outer shell. These fusion reactions create new
atomic nuclei in a process called nucleosynthesis.
Note: Review the step-by-step process of BBN and Stellar Nucleosynthesis and energetic particle interactions (tri-alpha
process etc) as per the slides or u may search via YouTube for more explanations.
Early History of the Atom
Matter is composed of indivisible building blocks. This idea was recorded as early as the 5th century BCE by Leucippus
and Democritus. The Greeks called these particles atomos, meaning indivisible, and the modern word “atom” is derived
from this term. Democritus proposed that different types and combinations of these particles were responsible for the
various forms of matter. However, these ideas were largely ignored at the time, as most philosophers favored the
Aristotelian perspective. These early atomists theorized that the two fundamental and oppositely characterized
constituents of the natural world are indivisible bodies—atoms—and void. The latter is described simply as nothing, or
as the negation of being, but all these are theories using debate and reason and no scientific experiment was done to
cement the ideas. This was also the time when protoscience, an unscientific or pseudoscientific field of study which later
becomes a science (e.g. astrology becoming astronomy and alchemy becoming chemistry) abounds thereby concepts
from early atomists provided as a bridge to the modern science we now know to be.
The concept of the atom was revisited and elaborated upon by many scientists and philosophers, including Galileo,
Newton, Boyle, and Lavoisier. In 1661 Boyle presented a discussion of atoms in his The Sceptical Chymist where Boyle
used the term “corpuscle” to describe the microscopic material particles, and their clusters, of which he believed the
material world was composed and related it to the atoms. However, the English chemist and meteorologist John Dalton
is credited with the first modern atomic theory, as explained in his A New System of Chemical Philosophy.
https://youtu.be/NSAgLvKOPLQ  YouTube video for clarification the models
Law of Multiple Proportions- The law of multiple proportions states that elements combine in small whole number
ratios to form compounds.
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The law of multiple proportions is a rule of stoichiometry.
John Dalton formulated the law of multiple proportions as part of his theory that atoms formed the
basic indivisible building block of matter.
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The law of multiple proportions says that when elements form compounds, the proportions of the
elements in those chemical compounds can be expressed in small, whole-number ratios.
The law of multiple proportions is an extension of the law of definite composition, which states that
compounds will consist of defined ratios of elements.
law of multiple proportions: A law stating that if two elements form a compound, then the ratio of the
mass of the second element and the mass of the first element will be small, whole-number ratios.
atom: The smallest possible amount of matter that still retains its identity as a chemical element, now
known to consist of a nucleus surrounded by electrons.
Law of Definite Proportions/Composition- The law of definite composition states that chemical compounds are
composed of a fixed ratio of elements as determined by mass.
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The law of definite composition was proposed by Joseph Proust based on his observations on the
composition of chemical compounds.
Proust proposed that a compound is always composed of the same proportions of elements by mass.
Though initially controversial, the law of definite composition was supported by Dalton’s atomic theory.
The law of definite composition was proposed by Joseph Proust based on his observations on the
composition of chemical compounds.
Proust proposed that a compound is always composed of the same proportions of elements by mass.
Though initially controversial, the law of definite composition was supported by Dalton’s atomic theory.
Law of Conservation of Mass- The law of conservation of mass states that mass in an isolated system is neither created
nor destroyed.
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The law of conservation of mass states that mass in an isolated system is neither created nor destroyed
by chemical reactions or physical transformations.
According to the law of conservation of mass, the mass of the products in a chemical reaction must
equal the mass of the reactants.
The law of conservation of mass is useful for a number of calculations and can be used to solve for
unknown masses, such the amount of gas consumed or produced during a reaction.
law of conservation of mass: A law that states that mass cannot be created or destroyed; it is merely
rearranged.
product: A chemical substance formed as a result of a chemical reaction.
reactant: Any of the participants present at the start of a chemical reaction. Also, a molecule before it
undergoes a chemical change.
VERY IMPORTANT!!!
Study the Lewis Dot Structure, Octet Rule and VSEPR Concept via the YouTube videos I have sent in the GC, since the
3D models via video are so much better than 2D diagrams on a document.
Polar vs Non-Polar – Polar molecules occur when there is an electronegativity difference between the bonded atoms.
Nonpolar molecules occur when electrons are shared equal between atoms of a diatomic molecule or when polar bonds
in a larger molecule cancel each other out.
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Depending on the relative electronegativities of the two atoms sharing electrons, there may be partial transfer
of electron density from one atom to the other. When the electronegativities are not equal, electrons are not
shared equally and partial ionic charges develop.
The greater the electronegativity difference, the more ionic the bond is. Bonds that are partly ionic are called
polar covalent bonds.
Nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the
two atoms are equal.
Nonpolar Covalent Bond
A bond between 2 nonmetal atoms that have the same electronegativity and therefore have equal sharing of the
bonding electron pair
Example: In H-H each H atom has an electronegativity value of 2.1, therefore the covalent bond between them is
considered nonpolar
Polar Covalent Bond
A bond between 2 nonmetal atoms that have different electronegativities and therefore have unequal sharing of the
bonding electron pair
Example: In H-Cl, the electronegativity of the Cl atom is 3.0, while that of the H atom is 2.1
The result is a bond where the electron pair is displaced toward the more electronegative atom. This atom then obtains
a partial-negative charge while the less electronegative atom has a partial-positive charge. This separation of charge or
bond dipole can be illustrated using an arrow with the arrowhead directed toward the more electronegative atom.
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Within a molecule each polar bond has a bond dipole
A polar molecule always contains polar bonds, but some molecules with polar bonds are nonpolar.
Polar Molecule
A molecule in which the bond dipoles present do not cancel each other out and thus results in a molecular dipole.(see
below). Cancellation depends on the shape of the molecule or Stereochemistry and the orientation of the polar bonds.
Molecular Dipole
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A result of the bond dipoles in a molecule.
Bond dipoles may or may not cancel out thereby producing either molecules that are nonpolar, if they cancel, or
polar, if they do not cancel
Examples:
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CO2 is a linear molecule with 2 bond dipoles that are equal and oppositely directed therefore the bond polarities
cancel and the molecule is nonpolar.
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HCN is a linear molecule with 2 bond dipoles that are in the same direction and are not equal therefore the bond
polarities do not cancel and the molecule is polar
Covalent Bond vs. Ionic Bond- There are primarily two forms of bonding that an atom can participate in: Covalent and
Ionic. Covalent bonding involves the sharing of electrons between two or more atoms. Ionic bonds form when two or
more ions come together and are held together by charge differences.
The definition of ionic bond, is a bond between atoms where electrons are (mostly) transferred from one atom to
another. We say mostly, because there is always some sharing of electrons between atoms, but in Ionic bonds, the
sharing is very unequal. The less equal the sharing of the electrons, the more ionic character the bond has.
Ionic bonds occur between a metal and a non-metal. Unlike covalent bonds, ionic bonds transfer their valence electrons
between atoms. In ionic bonding, the electronegativity difference between non-metals and metals exceeds 1.7. The
metal atom transfers its electrons to the non-metal atom. Therefore, the metal atom becomes a positively charged
cation and the non-metal atom becomes a negatively charged anion. Consequently, ionic bonds create two charged ions,
the metal always donates its electron, and the non-metal always accepts the electron. An example of an ionic bond is
the bond in sodium chloride, which is salt. Sodium’s valence electron is transferred to the outer electron shell of
chloride.
Molecules with ionic bonds form ionic compounds. Molecules with covalent bonds form covalent compounds. Covalent
compounds often melt at lower temperatures, because their covalent bonds are easier to break. We hope you
understand ionic vs covalent bonds and compounds a little better now.
Does NaCl has Ionic or Covalent Bonds?
NaCl, sodium chloride or table salt, is the “classic” example of an ionic compound. Sodium is a metal, and chlorine is a
non-metal. It has ionic bonds, has a crystalline structure. In solution, it separates into ions in solution.
Properties of Ionic Compounds
Differences between Compounds with Covalent and Ionic Bonds
The definition of an ionic compound, is a chemical compound composed of ions held together by electrostatic forces –
basically held together by ionic bonds. They are formed by neatly packed ions of opposite charge. The compound is
neutral, but it consists of positively and negatively charged cations and anions. Let’s look at some differences between
ionic and covalent bonds and compounds.
1. Ionic bonds generally tend to transfer electrons, covalent bonds share them more easily
2. Ionic compounds generally tend to have higher melting and boiling points, covalent compounds have lower
melting & boiling points
3. Ionic compounds tend to have more polar molecules, covalent compounds less so
4. Organic compounds tend to have covalent bonds
5. Ionic compounds are usually between a metal and a non-metal. Non-metal with a non-metal compounds are
covalent.
6. Ionic compounds have ions in solution or in the molten state and conduct electricity
7. Ionic bonds are much stronger than covalent bonds
8. Ionic compounds tend to be a solid with a definite shape at room temperature, covalent compounds are usually
gases, liquids or soft solids
9. Ionic compounds often do not dissolve in organic solvents, while covalent compounds often do
Examples of Compounds with Ionic Bonds
Here are some ionic bond examples:
Sodium chloride, NaCl
Magnesium sulfate, MgSO4
Cesium fluoride, CeF
Strontium hydroxide, Sr(OH)2
Potassium Cyanide, KCN
Examples of Compounds with Covalent Bonds
Here are some covalent bond examples:
Water, H2O
Methane, CH4
Ammonia, NH3
Tin (IV) iodide, SnI4
Titanium (IV) chloride, TiCl4
https://youtu.be/PVL24HAesnc  Further information on Polar and Non-Polar
Electronegativity- Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The
Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and
values range down to cesium and francium which are the least electronegative at 0.7.
Figure 6.1.1 below shows the electronegativity values of the elements as proposed by one of
the most famous chemists of the twentieth century: Linus Pauling. In general,
electronegativity increases from left to right across a period in the periodic table and
decreases down a group. Thus, the nonmetals, which lie in the upper right, tend to have the
highest electronegativities, with fluorine the most electronegative element of all (EN = 4.0).
Metals tend to be less electronegative elements, and the group 1 metals have the lowest
electronegativities. Note that noble gases are excluded from this figure because these atoms
usually do not share electrons with others atoms since they have a full valence shell. (While
noble gas compounds such as XeO2 do exist, they can only be formed under extreme
conditions, and thus they do not fit neatly into the general model of electronegativity.)
Electronegativity and Bond Type
The absolute value of the difference in electronegativity (ΔEN) of two bonded atoms provides a rough
measure of the polarity to be expected in the bond and, thus, the bond type. When the difference is
very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or
ionic. The absolute values of the electronegativity differences between the atoms in the bonds H–H,
H–Cl, and Na–Cl are 0 (nonpolar), 0.9 (polar covalent), and 2.1 (ionic), respectively. The degree to
which electrons are shared between atoms varies from completely equal (pure covalent bonding) to
not at all (ionic bonding). Figure 6.1.26.1.2 shows the relationship between electronegativity
difference and bond type.
https://youtu.be/mEszSvBMd7Q Electronegativity video
Lone Pair- A lone pair refers to a pair of valence electrons that are not shared with another atom and is sometimes
called a non-bonding pair. ( they are not involved in sharing) Lone pairs are found in the outermost electron shell of
atoms.
Predicting the Shapes of Molecules
There is no direct relationship between the formula of a compound and the shape of its molecules.
The shapes of these molecules can be predicted from their Lewis structures, however, with a model
developed about 30 years ago, known as the valence-shell electron-pair repulsion (VSEPR)
theory.
The VSEPR theory assumes that each atom in a molecule will achieve a geometry that minimizes
the repulsion between electrons in the valence shell of that atom. The five compounds shown in the
figure below can be used to demonstrate how the VSEPR theory can be applied to simple
molecules.
There are only two places in the valence shell of the central atom in BeF2 where electrons can be
found. Repulsion between these pairs of electrons can be minimized by arranging them so that they
point in opposite directions. Thus, the VSEPR theory predicts that BeF2 should be
a linear molecule, with a 180o angle between the two Be-F bonds.
There are three places on the central atom in boron trifluoride (BF3) where valence electrons can be
found. Repulsion between these electrons can be minimized by arranging them toward the corners
of an equilateral triangle. The VSEPR theory therefore predicts a trigonal planar geometry for the
BF3 molecule, with a F-B-F bond angle of 120o.
BeF2 and BF3 are both two-dimensional molecules, in which the atoms lie in the same plane. If we
place the same restriction on methane (CH4), we would get a square-planar geometry in which the
H-C-H bond angle is 90o. If we let this system expand into three dimensions, however, we end up
with a tetrahedral molecule in which the H-C-H bond angle is 109o28'.
Repulsion between the five pairs of valence electrons on the phosphorus atom in PF5 can be
minimized by distributing these electrons toward the corners of a trigonal bipyramid. Three of the
positions in a trigonal bipyramid are labeled equatorial because they lie along the equator of the
molecule. The other two are axial because they lie along an axis perpendicular to the equatorial
plane. The angle between the three equatorial positions is 120o, while the angle between an axial
and an equatorial position is 90o.
There are six places on the central atom in SF6 where valence electrons can be found. The repulsion
between these electrons can be minimized by distributing them toward the corners of
an octahedron. The term octahedron literally means "eight sides," but it is the six corners, or
vertices, that interest us. To imagine the geometry of an SF6 molecule, locate fluorine atoms on
opposite sides of the sulfur atom along the X, Y, and Z axes of an XYZ coordinate system.