Atomic Structure & Bonding

Ionic vs. Covalent Bonding

Chemical bonds are the forces that hold atoms together in compounds. The two primary 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. 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 its basic nuclear structure.

• Atomic Number (Z): The number of protons in the nucleus of an atom. It uniquely identifies the element. For example, sodium (Na) has 11 protons, so its atomic number is 11.

• 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 .

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

Periodic Properties

The Stability of Noble Gases

Noble gases (Group 18 of the periodic table) 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 chemical bonds under normal conditions.

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

• Chemical Inertness: Because their valence shells are full, noble gases rarely gain, lose, or share electrons, making them very stable.

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

Chemical Reactions

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 a coefficient of 2 in front of and , there are now 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 is used to determine the acidity or basicity of a solution, based on the concentration of hydrogen ions ().

• Acidic Solutions: pH less than 7. Strong acids have pH values close to 0.

• Neutral Solutions: pH equal to 7. Pure water is an example.

• Basic (Alkaline) Solutions: pH greater than 7. Strong bases have pH values close to 14.

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