Atomic Structure and Bonding

Ionic vs. Covalent Bonding

Chemical bonds are the forces that hold atoms together in compounds. The two main types of chemical bonds are ionic bonds and covalent bonds, which differ in how electrons are distributed between atoms.

• Ionic Bonds: Formed when one atom transfers electrons to another, resulting in the formation of oppositely charged ions. This typically occurs between metals and non-metals. The metal loses electrons to become a positively charged cation, while the non-metal gains electrons to become a negatively charged anion. The electrostatic attraction between these ions forms the ionic bond.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. This usually occurs between non-metal atoms. The shared electrons allow each atom to achieve a stable electron configuration, often resembling that of noble gases.

• Example: Sodium chloride (NaCl) is an example of an ionic compound, while water (H2O) is an example of a covalent compound.

Atomic Properties

Atomic Number and Mass Number

Each element is defined by its atomic number and mass number, which describe the composition of its nucleus.

• Atomic Number (Z): The number of protons in the nucleus of an atom. It determines the identity of the element. For example, an atom with 11 protons is sodium (Na).

• Mass Number (A): The total number of protons and neutrons in the nucleus. For sodium with 11 protons and 12 neutrons, the mass number is .

Chemical Stability

The Stability of Noble Gases

Noble gases (Group 18 elements) are characterized by their lack of chemical reactivity. This is due to their full valence electron shells, which make them energetically stable and unlikely to form bonds with other elements.

• Helium (He): Has 2 electrons (full first shell).

• Other noble gases (Ne, Ar, etc.): Have 8 valence electrons (full outer shell).

• Result: Noble gases are generally inert and do not readily participate in chemical reactions.

Chemical Reactions and Equations

Balancing Chemical Equations

Balancing chemical equations ensures that the Law of Conservation of Mass is obeyed: atoms are neither created nor destroyed in a chemical reaction. Each side of the equation must have the same number of each type of atom.

• Unbalanced Equation:

• Balanced Equation:

• Explanation: By placing coefficients in front of the chemical formulas, the number of hydrogen and oxygen atoms is equal on both sides of the equation (4 H and 2 O on each side).

Acids, Bases, and the pH Scale

Identifying Acids and Bases

The pH scale is used to measure the acidity or basicity of a solution, based on the concentration of hydrogen ions (H+).

• Acids: Substances with a pH less than 7. Strong acids have pH values close to 0 (e.g., pH = 2).

• Bases (Alkaline substances): Substances with a pH greater than 7, up to 14.

• Neutral: A pH of 7 indicates a neutral solution (e.g., pure water).

• Color Indicators: pH indicators change color depending on the acidity or basicity of the solution (red/orange for acids, green for neutral, blue/purple for bases).