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 and non-metals (e.g., sodium chloride, NaCl).

• 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-metals (e.g., water, H2O).

• Key Difference: Ionic bonds involve electron transfer; covalent bonds involve electron sharing.

• Example: Sodium (Na) and chlorine (Cl) form an ionic bond in table salt, while hydrogen (H) and oxygen (O) form covalent bonds in water.

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 (e.g., sodium has 11 protons, so Z = 11).

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

• Example: An atom with 11 protons and 12 neutrons is sodium (Na), with a mass number of 23.

Periodic Properties

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.

• Full Valence Shell: Most noble gases have eight electrons in their outermost shell (helium has two), satisfying the octet rule.

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

• 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 followed: 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.

Acids, Bases, and the pH Scale

Identifying Acids and Bases

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

• Acidic Solutions: pH less than 7 (0–6), high concentration of H+ ions. Strong acids have pH values close to 0–2.

• Neutral Solution: pH of 7, equal concentrations of H+ and OH− ions (e.g., pure water).

• Basic (Alkaline) Solutions: pH greater than 7 (8–14), high concentration of OH− ions.

• Example: A solution with pH 2 is a strong acid; a solution with pH 13 is a strong base.