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 move in energy levels around the nucleus.

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

For an atom to be stable, its outermost energy level must be filled according to its capacity.

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

Elements are pure substances that cannot be broken down into simpler substances. There are about 90 naturally occurring elements, all listed on the periodic table.

The atomic number is the number of protons in an atom and also equals the number of electrons in a neutral atom. Atomic bsp;Number=Number bsp;of bsp;Protons

The atomic mass is the sum of protons and neutrons in the nucleus. Atomic bsp;Mass=Number bsp;of bsp;Protons+Number bsp;of bsp;Neutrons

Neutrons can be calculated as: Number bsp;of bsp;Neutrons=Atomic bsp;Mass-Atomic bsp;Number

Ions are atoms with a net charge due to loss or gain of electrons. Cations are positively charged (lost electrons), anions are negatively charged (gained electrons). The number of protons does not change in ions.

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

Compounds are substances formed when two or more different elements bond together (e.g., NaCl, H2O). 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., H2O); if shared equally, the bond is nonpolar (e.g., H2).

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

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

Water is a polar molecule with unique properties: cohesion (attraction between water molecules), adhesion (attraction to other substances), high specific heat capacity, evaporative cooling, and excellent solvent abilities.

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+) versus hydroxide ions (OH-) in a solution. The pH scale ranges 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: 6CO2+6H2O→C6H12O6+6O2