The Chemical Basis of Life (Chemical Bonds)

Matter: Occupies space and has mass.
States of Matter: Found in solid, liquid, and gas states.
- Examples: Rocks (solid), Water (liquid), Gas (gaseous).

Element: A substance that cannot be broken down into other substances.
- Periodic Table: Consists of 92 naturally occurring elements.
- Examples of Elements: Gold (Au), Copper (Cu), Platinum (Pt), Silver (Ag), Carbon (C), Oxygen (O).
Compound: Substances composed of two or more different elements.
- Examples: Table Salt (NaCl), Water (H₂O), Sugar (C₁₂H₂₂O₁₁).
- More common in nature than pure elements.

Structure: Elements organized in vertical columns and horizontal rows.
Groups: Elements in a vertical column have similar chemical properties due to the same number of valence electrons.
- Notable Groups: Alkali metals, Alkaline earth metals, Halogens, Noble gases.
Periods: Each horizontal row represents elements with the same total number of electron shells and ordered by increasing atomic number.

118 Different Elements: Matter is composed of these elements.
Elements Constituting Most Organisms: Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N) account for 96% of body weight.
Definition of Element: A substance that consists of only one kind of atom.

Atom: The smallest particle that retains chemical properties.
Subatomic Particles:
- Protons: Positively charged, defines the atomic number.
- Neutrons: No charge, contribute to mass number (mass = protons + neutrons).
- Example: Carbon has 6 protons and 6 neutrons, so the mass number is 12.
- Electrons: Negatively charged, equal in number to protons, resulting in a net electrical charge of 0.
Atomic Properties:
- Atomic Number: Number of protons in the nucleus.
- Atomic Mass: Sum of protons and neutrons in the nucleus.

Electron Distribution: Electrons occupy different energy levels or shells around the nucleus.
- First Shell: Can hold 2 electrons.
- Second Shell: Can hold up to 8 electrons.
Reactivity: Atoms with incomplete outer shells are more reactive, often gaining, losing, or sharing electrons to fill outer layers.

Potential Energy: Energy based on the electron's position relative to the nucleus.
- Moving electrons closer releases energy; moving them further requires energy input.
Chemical Behavior: Determined by the distribution of electrons; electrons can capture or release energy, forming bonds between atoms.

Ion Definition: Atoms that have gained or lost electrons, resulting in a charge imbalance.
- Cations: Positively charged ions formed by losing electrons.
- Example: Sodium (Na) → Na⁺ (11 protons, 10 electrons).
- Anions: Negatively charged ions formed by gaining electrons.
- Example: Chloride (Cl) → Cl⁻ (17 protons, 18 electrons).

Definition: Atoms held together by energy, forming a cohesive structure.
Types of Chemical Bonds:
- Ionic Bonds: Formed by the electrical attraction between oppositely charged ions.
- Covalent Bonds: Involve the sharing of electron pairs between atoms.
- Types:
  - Nonpolar Covalent Bonds: Electrons are shared equally (e.g., H₂, O₂).
  - Polar Covalent Bonds: Unequal sharing due to differing electronegativities (e.g., H₂O).
- Hydrogen Bonds: Weak bonds formed between slightly charged regions of polar molecules.

Covalent Bonds:
- Involve the sharing of electrons; can be single or double bonds based on the number of shared pairs.
- Stronger than ionic bonds.
Nonpolar vs. Polar Covalent Bonds:
- Nonpolar: Equal sharing (e.g., CH bonds).
- Polar: Unequal sharing with partial charges (e.g., water).
Ionic Bonds:
- Formed when one atom donates electrons to another. Sodium chloride (NaCl) serves as a common example of an ionic bond.

Definition: Making and breaking of chemical bonds.
- Reactants: Starting molecules in a chemical reaction.
- Products: Final molecules resulting from a reaction.
Reversibility: All chemical reactions can be reversed.
Example of Biological Reaction: Photosynthesis
- Chemical Equation:
 $6 CO2 + 6 H2O —→ C6H{12}O6 + 6 O2$