Covalent Bonds: Nonpolar and Polar

Nonpolar and Polar Covalent Bonds

• Electrons in a covalent bond are not always shared equally between atoms
• Electronegativity is the strength with which atoms pull electrons toward themselves
• Electronegativity varies among elements, with oxygen (O) > nitrogen (N) > sulfur (S), carbon (C), hydrogen (H), and phosphorus (P)

Ionic Bonding and Electron-Sharing Continuum

• In an ionic bond, an electron is completely transferred from one atom to another, resulting in the formation of ions
• Cations are formed when an atom loses an electron and becomes positively charged
• Anions are formed when an atom gains an electron and becomes negatively charged

Properties of Water

• Water is essential for life, making up 75% of a cell's composition
• Water is an excellent solvent due to its ability to form extensive hydrogen bonds with itself and with polar groups on nearby molecules
• Water exhibits cohesive and adhesive properties, and its solid form (ice) is less dense than its liquid form
• Water has a high thermal capacity, which is important for life's chemistry

Water as a Solvent

• Hydrogen bonds between water molecules and polar solutes allow ionic and polar molecules to dissolve in water
• Hydrophobic (nonpolar) molecules do not dissolve in water, but interact with each other through hydrophobic interactions
• van der Waals interactions increase the stability of clustered hydrophobic molecules

Cohesion, Adhesion, and Surface Tension

• Cohesion is the attraction between like molecules, while adhesion is the attraction between unlike molecules
• Water is cohesive due to hydrogen bonds, and adheres to surfaces with polar or charged components

Acid-Base Chemical Reactions

• Acids are substances that give up protons during chemical reactions, increasing the hydronium ion concentration (H3O+)
• Bases are substances that acquire protons during chemical reactions, decreasing the hydronium ion concentration
• pH is a logarithmic notation used to express the concentration of protons in a solution
• A pH of less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution

Carbon-Based Life

• Carbon is the basis of almost all molecules found in organisms, except for water
• Carbon forms four covalent bonds due to its four valence electrons
• Organic compounds contain carbon bonded to other elements, resulting in a limitless array of molecular shapes

Covalent Bonds and Electronegativity

• Atoms become more stable by forming covalent bonds, where electrons are shared between atoms.
• Electrons in covalent bonds are not always shared equally, and atoms can pull shared electrons towards their nuclei with varying strengths, known as electronegativity.
• Electronegativity is the strength with which atoms pull electrons towards themselves, and for biologically abundant elements, oxygen has the highest electronegativity, followed by nitrogen, sulfur, carbon, hydrogen, and phosphorus.

Ions and Ionic Bonds

• In an ionic bond, electrons are completely transferred from one atom to another, giving each atom a full valence shell.
• Ions are atoms or molecules that carry a charge, and can be either cations (positively charged) or anions (negatively charged).

Properties of Water

• Water is a vital component of cells, making up 75% of cell content.
• Water forms extensive hydrogen bonds with itself and with polar groups on nearby molecules, leading to several properties essential for life's chemistry.
• Water is an excellent solvent due to its high cohesion and adhesion properties.
• Water has a high thermal capacity, with a high specific heat and heat of vaporization.
• Hydrogen bonds between water molecules and polar solutes enable the dissolution of charged or polar molecules.

Hydrophilic and Hydrophobic Molecules

• Hydrophilic molecules, including ions and polar molecules, interact with water's partial charges, allowing them to stay in solution.
• Hydrophobic molecules, which are uncharged and nonpolar, do not dissolve in water and interact with each other through hydrophobic interactions.
• Van der Waals interactions increase the stability of clustered hydrophobic molecules.

Cohesion and Adhesion

• Cohesion is the attraction between like molecules, while adhesion is the attraction between unlike molecules.
• Water is cohesive due to its hydrogen bonds, and adheres to surfaces with polar or charged components.
• Unlike most substances, water expands as it freezes, forming a less dense solid than its liquid state.

Carbon and Organic Compounds

• Carbon is a crucial atom in organic compounds, forming four covalent bonds due to its four valence electrons.
• Organic compounds have a limitless array of molecular shapes, with different combinations of single and double bonds.
• Six functional groups are found in organic compounds: amino, carboxyl, carbonyl, hydroxyl, phosphate, and sulfhydryl groups.
• Each of these functional groups has specific properties, such as attracting or dropping protons, linking molecules, or being polar and hydrophilic.

Atomic Bonding and Compounds

• Atoms become more stable by forming covalent bonds.
• Electrons in covalent bonds are not always shared equally.
• Compounds are molecules in which atoms of different elements are bonded together.

Electronegativity

• Electronegativity is the strength with which atoms pull electrons toward themselves.
• For biologically abundant elements, the order of electronegativity is: O > N > S > C > H > P.

Ionic Bonds

• Unlike in a covalent bond, electron is completely transferred from one atom to another in an ionic bond.
• This transfer gives each atom a full valence shell.
• Ions are atoms or molecules that carry a charge.
• Cations are atoms that lose an electron and become positively charged.
• Anions are atoms that gain an electron and become negatively charged.

Properties of Water

• Water is a critical component of life, making up 75% of a cell.
• Water is an excellent solvent due to its ability to form extensive hydrogen bonds with itself and with polar groups on nearby molecules.
• Water has cohesive and adhesive properties, allowing it to stick to itself and other surfaces.
• Ice is less dense than liquid water, which makes it float.
• Water has a high thermal capacity, with a high specific heat and heat of vaporization.
• Hydrogen bonds can form between water molecules and polar solutes, allowing them to dissolve in water.

Hydrophilic and Hydrophobic Molecules

• Hydrophilic molecules are ions and polar molecules that stay in solution due to their interactions with water's partial charges.
• Hydrophobic molecules are uncharged and nonpolar compounds that do not dissolve in water.
• Hydrophobic molecules interact with each other through hydrophobic interactions, which increase the stability of clustered hydrophobic molecules.
• Van der Waals interactions also contribute to the stability of clustered hydrophobic molecules.

Cohesion and Adhesion

• Cohesion is the attraction between like molecules, while adhesion is the attraction between unlike molecules.
• Water is cohesive due to hydrogen bonds, which allows it to stick together.
• Water also adheres to surfaces with polar or charged components.

Carbon and Organic Compounds

• Carbon is a critical element in organisms, forming four covalent bonds due to its four valence electrons.
• Organic compounds are molecules that contain carbon bonded to other elements.
• These compounds can form a limitless array of molecular shapes with different combinations of single and double bonds.
• Six functional groups found in organic compounds are:
  • Amino groups, which attract protons and act as bases.
  • Carboxyl groups, which drop protons and act as acids.
  • Carbonyl groups, which have sites that link molecules into more complex compounds.
  • Hydroxyl groups, which are polar and hydrophilic.
  • Phosphate groups, which have two negative charges.
  • Sulfhydryl groups, which link together via disulfide bonds.