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

Protons are positively charged particles in the nucleus; neutrons have no charge and are also in the nucleus; electrons are negatively charged and found in energy levels around the nucleus.

Energy levels (shells) are regions where electrons are likely to be found. Each level can hold a specific number of electrons: the first holds 2, the second 8, and the third 18.

An atom is stable when its outermost energy level is full.

Electron configuration describes the arrangement of electrons in an atom's energy levels, which determines chemical stability and reactivity.

Elements are pure substances that cannot be broken down into simpler substances. Each element is defined by its atomic number (number of protons).

The atomic mass of an element is the sum of its protons and neutrons: Atomic Mass=Number of Protons+Number of Neutrons

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

Ions are charged atoms formed by gaining or losing electrons. Cations are positive (lost electrons), anions are negative (gained electrons).

Compounds are substances formed when two or more different elements bond together. Molecules are groups of atoms held together by covalent bonds.

Covalent bonds involve the sharing of electrons between atoms. If electrons are shared unequally, the bond is polar (e.g., water); if shared equally, the bond is nonpolar (e.g., hydrogen gas).

Ionic bonds form when atoms transfer electrons, resulting in oppositely charged ions that attract each other (e.g., Na+ and Cl- form NaCl).

Hydrogen bonds are weak attractions between a hydrogen atom in one molecule and an electronegative atom in another, important in water and biological molecules.

Water is a polar molecule, leading to properties such as cohesion (attraction between water molecules), adhesion (attraction to other substances), high specific heat, and its role as a versatile solvent.

Solutions consist of a solute (substance dissolved) and a solvent (substance doing the dissolving); water is a universal solvent.

pH measures the concentration of hydrogen ions (H+) in a solution, ranging from 0 (acidic) to 14 (basic), with 7 being neutral. pH=-log([H^+])

Chemical equations represent reactions, showing reactants and products. Coefficients indicate the number of molecules; subscripts show the number of atoms in each molecule.

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