Atomic Structure, Bonding, and Chemical Properties

Ionic vs. Covalent Bonding

Chemical bonds are the forces that hold atoms together in compounds. 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 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.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Usually occurs between non-metal atoms. Both atoms achieve a stable electron configuration by sharing electrons.

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

Atomic Properties of Sodium

Each element is defined by its atomic structure, specifically the number of protons, neutrons, and electrons it contains.

• Atomic Number: The number of protons in the nucleus of an atom. For sodium, the atomic number is 11.

• Mass Number: The sum of protons and neutrons in the nucleus. For sodium with 11 protons and 12 neutrons: $11 + 12 = 23$.

• Example: Sodium (Na) has 11 protons, 12 neutrons, and (in a neutral atom) 11 electrons.

The Stability of Noble Gases

Noble gases (Group 18 of the periodic table) are known for 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.

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

• Inertness: Because their valence shells are full, noble gases rarely gain, lose, or share electrons.

• Example: Helium (He), neon (Ne), and argon (Ar) are all noble gases that are chemically stable.

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: $H_2 + O_2 \rightarrow H_2O$

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

• Explanation: By placing a coefficient of 2 in front of $H_2$ and $H_2O$, there are now 4 hydrogen atoms and 2 oxygen atoms on both sides of the equation.

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 (H+).

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

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

• Basic (Alkaline) Solutions: pH greater than 7 (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.