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: Involve the complete transfer of one or more electrons from a metal atom to a non-metal atom. This results in the formation of oppositely charged ions that are held together by electrostatic attraction. Example: Sodium chloride (NaCl).

• Covalent Bonds: Involve the sharing of electron pairs between two non-metal atoms. Both atoms achieve a stable electron configuration by sharing electrons. Example: Water (H2O).

• Key Difference: Ionic bonds result from electron transfer, while covalent bonds result from electron sharing.

Example: In NaCl, sodium donates an electron to chlorine, forming Na+ and Cl-. In H2O, each hydrogen shares an electron with oxygen.

Atomic Properties

Atomic Number and Mass Number

Each element is defined by its atomic number and mass number, which describe its nuclear composition.

• 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.

Example: Sodium (Na) has 11 protons and 12 neutrons: $A = 11 + 12 = 23$.

Periodic Properties

The Stability of Noble Gases

Noble gases (Group 18) are characterized by their full valence electron shells, making them highly stable and chemically inert under standard conditions.

• Full Valence Shell: Most noble gases have eight electrons in their outermost shell (except helium, which has two).

• Chemical Inactivity: Their stable electron configuration means they rarely form chemical bonds with other elements.

Example: Neon (Ne) and Argon (Ar) are noble gases that do not readily react with other substances.

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.

• Unbalanced Equation: $H_2 + O_2 \rightarrow H_2O$

• Balanced Equation: $2H_2 + O_2 \rightarrow 2H_2O$

• Explanation: The balanced equation has 4 hydrogen atoms and 2 oxygen atoms on both sides, maintaining mass conservation.

Example: When hydrogen gas reacts with oxygen gas to form water, coefficients are adjusted to balance the number of each atom.

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 0–6; high concentration of H+ ions. Strong acids have pH values close to 0.

• Neutral Solution: pH 7; pure water is neutral.

• Basic (Alkaline) Solutions: pH 8–14; low concentration of H+ ions, high concentration of OH- ions.

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