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 maximum number of electrons:

• First energy level: 2 electrons

• Second energy level: 8 electrons

• Third energy 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.

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. 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 (e.g., O2).

Covalent bonds involve the sharing of electrons between atoms.

• Polar covalent bonds: electrons are shared unequally, creating partial charges (e.g., H2O).

• Nonpolar covalent bonds: electrons are shared equally (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 water and biological molecules.

Water is a polar molecule with unique properties:

• Cohesion: water molecules stick to each other due to hydrogen bonding.

• Adhesion: water molecules stick to other substances, causing phenomena like capillary action.

• High specific heat capacity: water absorbs a lot of heat before changing temperature.

• Versatile solvent: dissolves many substances due to polarity.

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.

• pH scale ranges from 0 (acidic) to 14 (basic), with 7 being neutral.

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

Chemical equations represent reactions, showing reactants and products.

• Coefficients indicate the number of molecules or atoms involved.

• Subscripts show the number of atoms in a molecule.

• Example: 6CO2 + 6H2O  C6H12O6 + 6O2