Atoms and Subatomic Particles:

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

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

Atomic Number and Mass:

• The atomic number is the number of protons in an atom and determines the element.

• Atomic mass is the sum of protons and neutrons.

• Number of neutrons = atomic bsp;mass − atomic bsp;number

Electron Configuration and Energy Levels:

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

• The first energy level holds up to 2 electrons, the second up to 8, and the third up to 18.

• Stable electron configurations occur when energy levels are filled according to the element's number of electrons.

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).

Elements, Compounds, and Molecules:

• An element is a pure substance made of one type of atom.

• A compound is formed when two or more different elements bond together (e.g., NaCl, H2O).

• A molecule is a group of atoms held together by covalent bonds (e.g., O2).

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 that attract (e.g., Na+ + Cl- → NaCl).

• Hydrogen bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom in another (important in water and biological molecules).

Properties of Water:

• Water is polar, allowing it to form hydrogen bonds.

• Exhibits cohesion (attraction between water molecules) and adhesion (attraction to other substances), leading to phenomena like meniscus and capillary action.

• High specific heat capacity and acts as a versatile solvent.

Solutions and pH:

• A solution consists of a solute (substance dissolved) and a solvent (substance doing the dissolving; water is a common solvent).

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

Chemical Equations:

• Represent chemical reactions, showing reactants and products.

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

• Example: 6CO2 + 6H2O → 6O2 + 6H2O + energy