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.

• Protons have a positive charge, neutrons have no charge, and electrons have a negative charge.

Energy Levels and Electron Configuration:

• Electrons occupy specific energy levels around the nucleus; each level holds a set number of electrons (first: 2, second: 8, third: 18).

• Stable electron configurations are achieved when energy levels are filled according to the element's total electrons.

Elements and the Periodic Table:

• Elements are pure substances that cannot be broken down further; each has a unique atomic number (number of protons).

• Atomic mass is the sum of protons and neutrons.

• Only a subset of elements are essential for life, with C, H, N, and O making up most of human mass.

Isotopes and Ions:

• Isotopes are atoms of the same element with different numbers of neutrons.

• Ions are charged atoms formed by gaining or losing electrons; protons remain unchanged.

Chemical Bonds:

• Covalent Bonds: Atoms share electrons; can be polar (unequal sharing, e.g., H2O) or nonpolar (equal sharing, e.g., H2).

• Ionic Bonds: Formed when atoms transfer electrons, resulting in oppositely charged ions (e.g., Na+ + Cl- → NaCl).

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom, important in water and biological molecules.

Properties of Water:

• Water is polar, allowing it to form hydrogen bonds, leading to high cohesion, adhesion, high specific heat, and its role as a versatile solvent.

• Cohesion and adhesion explain phenomena like surface tension and capillary action.

• Water's high specific heat helps regulate temperature; evaporative cooling (e.g., sweating) is a result.

Solutions and pH:

• Solutions consist of a solute (substance dissolved) and a solvent (substance doing the dissolving, often water).

• pH measures the concentration of hydrogen ions (H+) versus hydroxide ions (OH-), ranging from 0 (acidic) to 14 (basic), with 7 being neutral.

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

Chemical Equations:

• Chemical reactions are represented by equations showing reactants and products.

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

• Example equation (photosynthesis): 6CO2+6H2O→C6H12O6+6O2