Atomic Structure and Properties

Atomic Number and Mass Number

The atomic structure of an element is defined by its number of protons, neutrons, and electrons. The atomic number is the number of protons in the nucleus and uniquely identifies the element. The mass number is the sum of protons and neutrons in the nucleus.

• Atomic Number: For sodium, the atomic number is 11 (11 protons).

• Mass Number: For sodium with 11 protons and 12 neutrons, the mass number is .

• Example: Sodium (Na) has 11 protons, 12 neutrons, and typically 11 electrons.

Chemical Bonding and Molecular Shapes

Ionic vs. Covalent Bonding

Chemical bonds are formed to achieve stable electron configurations. The two primary types are ionic and covalent bonds.

• Ionic Bonds: Formed when one atom transfers electrons to another, resulting in oppositely charged ions. Typically occurs between metals and non-metals.

• Covalent Bonds: Formed when two atoms share electron pairs. Usually occurs between non-metals.

• Example: Sodium chloride (NaCl) is an ionic compound; water (H2O) is a covalent compound.

Stability of Noble Gases

Noble gases (Group 18) are chemically stable due to their full valence electron shells. This makes them largely unreactive and unlikely to form bonds.

• Key Point: Noble gases have 8 valence electrons (except Helium, which has 2).

• Example: Argon, Neon, and Helium are noble gases that rarely participate in chemical reactions.

Matter, Measurements, Reactions, and Stoichiometry

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.

• Unbalanced Equation:

• Balanced Equation:

• Explanation: Balancing ensures equal numbers of each atom on both sides of the equation.

Acid-Base Equilibria and Aqueous Equilibria

Identifying Acids and Bases

The pH scale measures the concentration of hydrogen ions in a solution, indicating whether it is acidic, neutral, or basic.

• Acidic: pH 0–6 (strong acids have lower pH values, e.g., pH 2).

• Neutral: pH 7 (pure water).

• Basic (Alkaline): pH 8–14.

• Color Indicators: Acids are red/orange, neutral is green, bases are blue/purple.

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

Summary Table: Types of Chemical Bonds