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:

• First level: 2 electrons

• Second level: 8 electrons

• Third level: 18 electrons

Atoms are stable when their outermost energy level is full.

Electron configuration shows the arrangement of electrons in energy levels; stability depends on filled outer shells.

An element is a pure substance 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 mass is the sum of protons and neutrons in the nucleus: Atomic mass=Number of protons+Number of neutrons

To find the number of neutrons: Neutrons=Atomic mass−Atomic number

Ions are atoms with a net charge due to loss or gain of electrons; protons do 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 (e.g., O2).

Covalent bonds involve sharing of electrons between atoms; can be polar (unequal sharing, as in H2O) or nonpolar (equal sharing, as in H2).

Ionic bonds form when atoms transfer electrons, resulting in oppositely charged ions that attract (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 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 being 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+) versus hydroxide ions (OH-) in a solution; the scale ranges from 0 (acidic) to 14 (basic), with 7 being neutral.

• Acids have more H+ ions (pH < 7), bases have more OH- ions (pH > 7).

Chemical equations represent reactions, showing reactants and products. Coefficients indicate the number of molecules; subscripts show the number of atoms in each molecule. Example: C6H12O6+6O2→6CO2+6H2O+energy