Chapter 2 (PART THREE)

Carbon and hydrogen atoms form covalent bonds, in which the electrons are shared equally.
Some molecules contain polar covalent bonds, in which the electrons are not shared equally, but reside closer to one of the atoms in the bond.
These molecules with unequal charge distribution are called polar molecules.
Polar molecules contain equal numbers of protons and electrons, so they are electrically neutral.
A hydrogen bond forms from the relatively weak attraction between the slightly positive “H” end of a polar molecule and the slightly negative “N” or “O” end of a nearby polar molecule.
In water, the O nucleus pulls electrons more strongly than the H nuclei, giving O a partial negative charge and H a partial positive charge, which facilitates hydrogen bonding between water molecules.
Summary:
- Covalent bonds: electrons shared.
- Polar covalent bonds: unequal sharing.
- Polar molecules: overall neutral but with partial charges.
- Hydrogen bonds: weak attractions between H and N/O of neighboring polar molecules.

A polar molecule has a covalent bond in which the electrons are not shared equally.
A non-polar molecule has a covalent bond in which the electrons are equally shared.

A hydrogen bond forms from a slightly positive H end of a polar molecule being attracted to the slightly negative N or O end of another polar molecule.
In H2O, the O nucleus pulls electrons more strongly than the H nuclei because O has more positively charged protons, creating a partial negative charge on O and partial positive on H, enabling hydrogen bonding between water molecules.
The strength of hydrogen bonds is weaker than covalent bonds but critically influences properties of water and biomolecules.

A molecule is formed when two or more atoms form a chemical bond.
If atoms of the same element bond, a molecule of that element is formed (e.g., O_2 is an oxygen molecule).
If atoms of different elements combine, the molecule is called a compound.
Characteristics of compounds:
- Compounds always have definite types and numbers of atoms.
- Example: two atoms of H and one atom of O can bind to form water, H_2O; since it contains different types of atoms, it is classified as a compound.
Summary:
- Molecule = two or more atoms bonded.
- Homonuclear molecule = same element (e.g., O_2).
- Heteronuclear molecule = different elements (e.g., H_2O).

Hydrogen molecules: H_2
Oxygen molecule: O_2
Water molecules: H_2O
These examples illustrate how atoms combine to form simple molecules and compounds with definite compositions.

A molecular formula represents the numbers and types of atoms in a molecule:
- A formula uses symbols for each element.
- Glucose contains 6 C atoms, 12 H atoms, and 6 O atoms.
- Water contains 2 H atoms and 1 O atom.
Structural formulas (different representations) illustrate how atoms are joined and arranged in molecules.

Each atom forms a specific number of covalent bonds based on the number of electrons in its outermost shell:
- H atoms form single bonds, since they each have 1 electron to share.
- O atoms form double bonds, since they can share the 2 electrons in their outermost shell.
Structural formulas use lines to illustrate bonds: single lines for single bonds, double lines for double bonds, etc.
Three-dimensional molecular models show spatial relationships between atoms.

Structural/formula representations include:
- H–H or H_2
- O=O or O_2
- H–O–H for water, H_2O
- Linear models like CO_2 ( ext{carbon dioxide})

3D models provide a sense of the spatial arrangement of atoms within a molecule, beyond what 2D formulas show.
These models help explain molecular geometry, bond angles, and shapes that influence reactivity and properties.

A chemical reaction occurs as bonds are formed or broken between atoms, ions, or molecules.
Reactants are substances that are changed by the reaction; products are the substances formed.
Synthesis reactions: two or more atoms or molecules bond together to form a more complex product.
- General form: A + B
  ightarrow AB
Decomposition reactions: larger molecules are broken into smaller ones by breaking chemical bonds.
- General form: AB
  ightarrow A + B

Exchange reactions (e.g., acid-base reactions) occur as parts of molecules switch places, by breaking and forming new bonds.
Reversible reactions are those in which the products can change back into the reactants; they are symbolized by double arrows:
Catalysts influence the speed of chemical reactions without being used up in the process.
- In biological systems, catalysts are called enzymes.
Practical note: Reversibility and catalysis are key to metabolic regulation and energy flow in organisms.