Atomic Structure & Bonding

Ionic vs. Covalent Bonding

Atoms form chemical bonds to achieve greater stability, typically 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. This usually 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. This typically 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. 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 $A = 11 + 12 = 23$.

• Example: An atom with 11 protons and 12 neutrons is sodium-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 bonds with other elements.

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

• Chemical Inertness: Their stable electron configuration 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 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 coefficients in front of $H_2$ and $H_2O$, 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. The scale ranges from 0 to 14.

• Acidic Solutions: pH less than 7 (high $[H^+]$), often red/orange on pH indicators.

• Neutral Solutions: pH equal to 7 (pure water), green on pH indicators.

• Basic (Alkaline) Solutions: pH greater than 7 (low $[H^+]$), blue/purple on pH indicators.

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