Atoms and Subatomic Particles

• An atom is the smallest unit of an element, consisting of a nucleus (containing 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 has a maximum electron capacity:

  • 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.

• Atomic number = number of protons (and electrons in a neutral atom).

• Atomic mass = number of protons + neutrons. Neutrons=Atomic Mass−Atomic Number

Ions and Isotopes

• Ions are atoms with a net charge due to loss or gain of electrons (protons do not change).

• Isotopes are 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 bonded together (e.g., NaCl, H2O).

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

• Covalent bond: Atoms share electrons.

  • Polar covalent bond: Electrons shared unequally (e.g., H2O).

  • Nonpolar covalent bond: Electrons shared equally (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, leading to hydrogen bonding between molecules.

• Cohesion: Attraction between water molecules (responsible for surface tension).

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

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

• Evaporative cooling: Water removes heat as it evaporates (e.g., sweating).

• Versatile solvent: Water dissolves many substances due to its polarity.

Solutions and pH

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

• pH measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-) in a solution.

  • pH scale: 0 (acidic) to 14 (basic), 7 is neutral.

  • Acids increase H+ concentration; bases increase OH- concentration.

Chemical Equations

• Represent chemical reactions, showing reactants and products.

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

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