Chemistry Basics Quiz

Chapter 2: Water and Carbon: The Chemical Basis of Life

• Life on Earth is primarily based on chemical evolution
• Chemical evolution is the leading explanation for the origin of life
• Complex carbon-containing substances formed
• Replicating molecules emerged, leading to biological evolution
• Natural selection took over, eventually fulfilling all five characteristics of life

Link Between Chemistry and Biology

• Chemical evolution is the process of increasingly complex carbon-containing substances forming
• This resulted in molecules that could replicate themselves
• The switch marked the transition from chemical to biological evolution
• The original molecules multiplied and descendants developed metabolic functions and membranes
• This fulfilled the five main characteristics of life

Atoms, Ions, and Molecules: The Building Blocks of Chemical Evolution

• 96% of all matter in organisms is composed of four elements: hydrogen, carbon, nitrogen, and oxygen
• Simple substances evolved into complex structures in living cells
• Understanding the physical structures of the atoms (hydrogen, carbon, nitrogen, oxygen) is essential
• The structures of simple molecules (water, carbon dioxide) played fundamental roles in chemical evolution

Basic Atomic Structure

• Atoms have a nucleus consisting of protons and neutrons
• Protons have a positive charge (+1)
• Neutrons have no charge
• Orbiting electrons carry a negative charge (-1)
• Atoms exist in a state of charge balance and are electrically neutral when the number of protons equals the number of electrons

Understanding Elements

• Elements consist of only one type of atom.
• Atomic number is the characteristic number of protons in an atom's nucleus. It is written as a subscript left of the element's symbol.
• Mass number is the sum of protons and neutrons in an atom. It is also the atom's mass number.

Atoms are Tiny, so How Do We Weigh Them?

• Dalton (Da) is the unit of measurement for the mass of a proton and a neutron.
• The mass of an electron is negligible
• An atom's mass is essentially equal to its mass number
• Isotopes of an element have different numbers of neutrons but the same number of protons
• The atomic weight of an element is the average mass of the naturally occurring isotopes, weighted by their relative abundance

Weight of Molecules in Grams

• A mole is 6.022 x 10²³ atoms or molecules.
• Mass of one mole of an atom (or molecule) equals its atomic (or molecular) weight in grams.
• Molecular weight is the sum of the atomic weights of all atoms in a molecule

The Atomic Structure of the First 18 Elements

• Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur make up over 99% of the atoms in any organism.
• The elements highlighted in blue in the chart are the most abundant elements found in organisms.

Atomic Structure and Electron Shells

• Electrons orbit the nucleus in specific regions called orbitals.
• Orbitals combine to form electron shells
• Electron shells are numbered (e.g., 1, 2, 3...), with lower numbers closer to the nucleus
• Each electron shell holds a specific number of electrons
• Electrons fill the inner shells first, then the outer shells.
• Valence electrons are the electrons in the outermost shell

Covalent Bonding Hold Molecules Together by Sharing Electrons

• Sharing electrons in a covalent bond allows atoms to become more stable by completing valence shells
• If two hydrogen atoms share electrons, for instance, their outer shells are filled and thus become stable.

Electrons in Covalent Bonds Are Not Always Shared Equally (Polar bonds)

• Covalent bonds can be nonpolar (electrons shared equally) or polar (electrons shared unequally).
• Polar bonds arise from differences in electronegativity between atoms in a covalent bond;
• Water is an example of a molecule with polar covalent bonds, creating partial charges on the atoms

Nonpolar vs Polar Covalent Bonds

• Nonpolar covalent bonds involve equal electron sharing, producing no charge.
• Polar covalent bonds involve unequal electron sharing, producing partial positive and negative charges on the atoms.
• The difference in electron negativity determines the polarity or non polarity of the bond

Electronegativity: Strength with which atoms pull electrons toward themselves

• Atom's electronegativity is determined by the number of protons and the distance of the valence shell from the nucleus.
• Electronegativity increases as you move up and to the right on the periodic table.

Polar Bonds Produce Partial Charges on Atoms

• Polar covalent bonds occur when atoms with differing electronegativities share electrons unequally.
• In a water molecule (H₂O), oxygen is more electronegative than hydrogen, resulting in a partial negative charge on oxygen and partial positive charges on hydrogen.
• These partial charges are symbolized using delta symbols (δ).

Ionic Bonding

• Ionic bonds form when one atom transfers an electron to another, resulting in a full valence shell for both atoms.
• The electron's transfer produces ions.
• Cations are positively charged ions formed when an atom loses an electron
• Anions are negatively charged ions formed when an atom gains an electron

The Electron-Sharing Continuum

• Polar covalent bonds involve unequal sharing of electrons.
• Ionic bonds involve complete electron transfer
• Covalent bonds with equal sharing of electrons are nonpolar.
• Examples were listed including Hydrogen, Methane, Ammonia, Water and Sodium Chloride

Unpaired Electrons in the Valence Shell Can Participate in Double and Triple Covalent Bonds

• The number of unpaired electrons in an atom's valence shell determines the number of bonds that atom can form.

Molecule's Shape Often Dictates its Behavior

• The shape of molecules is determined by the geometry of their bonds.
• Repulsive forces between electrons in molecules have been determined
• The shapes of methane and water molecules have been explained.

Molecules Can Be Represented Several Ways

• Molecular formulas, structural formulas, ball-and-stick models, and space-filling models represent molecules in various ways.

Properties of Water and the Early Oceans

• Water is an excellent solvent due to its polar nature, allowing substances to dissolve and interact readily
• Solute dissolves into solvent to create a solution
• Certain substances react more strongly when they're dissolved in water or other solvents.
• The properties of water correlate directly with its structure.

A Water Can Interact with other Water Molecules

• Water is a polar molecule with partial positive charges on hydrogen and a partial negative charge on oxygen.
• Partial charges on hydrogen attract the partial negative charges on oxygen in another water molecule
• These interactions are called hydrogen bonds.

Water Is an Efficient Solvent

• Hydrogen bonds form between water molecules and polar solutes
• Hydrophilic molecules interact with water, allowing them to dissolve

Not everything dissolves in water

• Hydrophobic molecules do not interact with water, and therefore do not dissolve.
• Hydrophobic substances interact with each other due to hydrophobic interactions which improve the stability of clustered hydrophobic molecules.
• This is an example of why clustering often occurs with non-polar molecules

Cohesion and Adhesion

• Cohesion describes attraction between like molecules.
• Adhesion describes attraction between unlike molecules.
• Water molecules demonstrate strong cohesive forces due to hydrogen bonding.
• In plant systems, water molecules adhere to surfaces, like those found in plant roots and stems.

Surface Tension

• Surface tension is a phenomenon due to the cohesive forces in the surface layer of a liquid
• Cohesive forces at the surface of water molecules create resistance to increasing surface area
• Water molecules demonstrates elastic properties at its surface

Water Is Denser as a Liquid than as a Solid

• Most substances become denser as they solidify
• Water expands when it freezes
• Frozen water forms an open crystal structure, reducing density
• This is why ice floats on water

Water Has a High Capacity for Absorbing Energy (Specific Heat)

• Specific heat is the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C
• Water has a remarkably high specific heat due to extensive hydrogen bonding
• Many hydrogen bonds must be broken to increase water's temperature

The Role of Water in Acid-Base Chemical Reactions

• Chemical reactions involve the breaking and formation of chemical bonds.
• Water molecules dissociate into hydrogen ions (H⁺) and hydroxide ions (OH^-)
• Chemical equilibrium describes a reaction where the forward and reverse reaction rates are equal

Measuring the Concentration of Protons

• A mole (mol) is a way to measure large quantities of molecules (6.022 x 10²³)
• Molecular weight is the sum of atomic weights of atoms in a molecule
• Molarity is the number of moles of solute in one liter of solution (M).

The Role of Water in Acid-Base Chemical Reactions

• Acids are substance that release protons (H⁺)
• Bases are substances that accept protons (H⁺)
• Water acts as both an acid and a base in chemical reactions

The pH of a Solution Reveals Whether It Is Acidic or Basic

• pH measures the concentration of hydrogen ions (H⁺) on a logarithmic scale
• Acids have lower pH (less than 7)
• Bases have higher pH (greater than 7)
• pH of water is 7 and a neutral pH
• Buffers maintain a relatively constant pH in living organisms

Chemical Reactions

• Chemical reactions involve breaking and forming chemical bonds.
• Chemical reactions can be balanced to maintain equal numbers of atoms on both sides of the reaction
• Exothermic reactions releases heat energy
• Endothermic reactions require energy to proceed

What Makes a Chemical Reaction Spontaneous?

• Spontaneous reactions proceed without continuous external energy input
• Products of spontaneous reactions often have lower potential energy and increased entropy than reactants.
• Many factors influence if a reaction will proceed spontaneously

Stanley Miller's Spark-Discharge Experiment in 1953

• Simulating early Earth conditions, the experiment demonstrated that simple molecules could produce complex molecules (such as amino acids).
• Heat and electrical charges in the experiment produced complex organic compounds
• The experiment suggested early Earth conditions could create molecules essential for life.

Life is Carbon-Based

• Carbon is a central element in life's molecules.
• Carbon atoms can form long chains or rings, forming the structural foundation for complex organic compounds like octane and glucose.

Functional Groups Define the Chemical Behavior of Organic Molecules

• Functional groups are specific groups of atoms attached to a carbon skeleton that give a molecule its unique properties.
• Examples of functional groups include hydroxyl, carboxyl, carbonyl, amino, sulfhydryl, and phosphate groups.
• Each type of functional group has distinctive chemical characteristics.

Small Organic Molecules Can Assemble into Large Molecules

• This is necessary to build complex molecules of life.
• Monomers combine to form polymers via condensation reactions, producing water
• The reverse reaction (hydrolysis) breaks apart polymers

End of Chapter Questions and Review

• Questions 1, 2, 3, 5, 8, 12, 16 are provided for further review and practice
• A review chapter summarizes the concepts in the main chapter.