Atoms and Subatomic Particles

• An atom is the smallest unit of an element, consisting of a nucleus (protons and neutrons) and electrons orbiting in energy levels.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle outside the nucleus.

Energy Levels and Electron Configuration

• Electrons occupy energy levels (shells) around the nucleus.

• Each energy level holds a specific maximum number of electrons:

  • First level: 2 electrons

  • Second level: 8 electrons

  • Third level: 18 electrons

• Stability is achieved when all occupied energy levels are filled.

Elements and the Periodic Table

• An element is a pure substance that cannot be broken down into simpler substances.

• There are about 90 naturally occurring elements; only 25 are essential for life.

• Major elements in living organisms: Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O).

Atomic Number and Atomic Mass

• Atomic number: Number of protons in an atom; also equals the number of electrons in a neutral atom.

• Atomic mass: Sum of protons and neutrons.

• Number of neutrons = Atomic mass - Atomic number

• n=A−Z where n = neutrons, A = atomic mass, Z = atomic number.

Ions and Isotopes

• Ions: Atoms with a net charge due to loss or gain of electrons.

  • Cation: Positive ion (lost electrons).

  • Anion: Negative ion (gained electrons).

• Isotopes: Atoms of the same element with different numbers of neutrons (e.g., Carbon-12, Carbon-13, Carbon-14).

Compounds, Molecules, and Chemical Bonds

• Compound: Substance formed from two or more different elements chemically bonded (e.g., NaCl, H2O).

• Molecule: Group of atoms held together by covalent bonds (e.g., O2).

• Covalent bond: Atoms share electrons.

  • Polar covalent: Unequal sharing (e.g., H2O).

  • Nonpolar covalent: Equal sharing (e.g., H2).

• Ionic bond: Transfer of electrons between atoms, forming oppositely charged ions (e.g., Na+ + Cl- → NaCl).

• Hydrogen bond: Weak attraction between a hydrogen atom (in a polar molecule) and another electronegative atom (important in water and biological molecules).

Properties of Water

• Water is a polar molecule due to uneven charge distribution (oxygen is slightly negative, hydrogens are slightly positive).

• Hydrogen bonding leads to:

  • Cohesion: Attraction between water molecules.

  • Adhesion: Attraction between water and other substances (causes meniscus, capillary action).

  • High specific heat capacity: Water absorbs large amounts of heat before changing temperature.

  • Evaporative cooling: Loss of heat as water evaporates (e.g., sweating).

  • Versatile solvent: Dissolves many substances due to polarity.

Solutions and pH

• Solution: Homogeneous mixture of solute (substance dissolved) and solvent (substance doing the dissolving; water is the universal solvent).

• pH: Measures concentration of hydrogen ions (H+) vs. hydroxide ions (OH-).

• pH scale: 0 (acidic, more H+) to 14 (basic, more OH-); 7 is neutral (pure water).

• pH=−log([H+])

Chemical Equations

• Represent chemical reactions: Reactants → Products.

• Chemical equation example: 6CO2+6H2O→C6H12O6+6O2

• Coefficients indicate the number of molecules; subscripts indicate the number of atoms in a molecule.