Atomic Structure & Bonding

Ionic vs. Covalent Bonding

Atoms form chemical bonds to achieve greater stability, often by attaining a full outer electron shell. The two primary types of chemical bonds are ionic and covalent bonds, which differ in how electrons are distributed between atoms.

• Ionic Bonds: Formed when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other. Typically occurs between metals (which lose electrons) and non-metals (which gain electrons).

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons, allowing both to achieve a stable electron configuration. Commonly occurs between non-metal atoms.

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

Atomic Properties

Atomic Number and Mass Number

Each element is defined by its atomic structure, specifically the number of protons and neutrons in its nucleus.

• Atomic Number (Z): The number of protons in the nucleus of an atom. Determines the element's identity. For sodium, Z = 11.

• Mass Number (A): The total number of protons and neutrons in the nucleus. For sodium with 11 protons and 12 neutrons, A = 23.

• Formula: where is the mass number, is the atomic number, and is the number of neutrons.

• Example: Sodium (Na): 11 protons, 12 neutrons → Atomic number = 11, Mass number = 23.

Periodic Properties of the Elements

The Stability of Noble Gases

Noble gases (Group 18) 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.

• Full Valence Shell: Most noble gases have 8 valence electrons (except Helium, which has 2), satisfying the octet rule.

• Chemical Inertness: Their stability means they rarely participate in chemical reactions under standard conditions.

• Example: Helium (He), Neon (Ne), and Argon (Ar) are common noble gases.

Chemical Reactions

Balancing Chemical Equations

Balancing chemical equations ensures 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 Example:

• Balanced Equation:

• Explanation: By adding coefficients, we ensure there are 4 hydrogen atoms and 2 oxygen atoms on both sides of the equation.

Acid and Base Equilibrium

Identifying Acids and Bases Using the pH Scale

The pH scale measures the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity. The scale ranges from 0 to 14.

• Acidic: pH 0–6 (high H+ concentration, red/orange color)

• Neutral: pH 7 (equal H+ and OH- concentrations, green color)

• Basic (Alkaline): pH 8–14 (low H+ concentration, blue/purple color)

• Example: A solution with pH 2 is a strong acid.