Atomic Structure and Properties

Atomic Number and Mass Number

Understanding atomic structure is fundamental to chemistry. Each element is defined by its atomic number, which is the number of protons in its nucleus. The mass number is the sum of protons and neutrons.

• Atomic Number (Z): The number of protons in the nucleus. For sodium (Na), Z = 11.

• Mass Number (A): The total number of protons and neutrons. For sodium with 11 protons and 12 neutrons: .

• Element Identification: The atomic number uniquely identifies an element. Sodium (Na) always has 11 protons.

• Isotopes: Atoms of the same element with different numbers of neutrons (and thus different mass numbers).

Chemical Bonding

Ionic vs. Covalent Bonds

Chemical bonds form when atoms interact 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 that attract each other. Typically occurs between metals and non-metals (e.g., NaCl).

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Usually occurs between non-metals (e.g., H2O).

• Electron Configuration: Atoms bond to achieve a full valence shell, often following the octet rule (8 electrons in the outer shell).

• Example: Sodium (Na) donates an electron to chlorine (Cl) to form NaCl (ionic bond). Hydrogen and oxygen share electrons to form H2O (covalent bond).

The Stability of Noble Gases

Noble gases (Group 18) are chemically inert because their valence shells are full. This makes them stable and unlikely to form bonds under normal conditions.

• Full Valence Shell: Helium has 2 electrons; other noble gases have 8 electrons in their outermost shell.

• Low Reactivity: Their stability explains why noble gases rarely participate in chemical reactions.

Chemical Reactions and Equations

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: Coefficients are used to ensure the same number of each type of atom appears on both sides of the equation.

• Steps to Balance:

  1. Count atoms of each element on both sides.

  2. Add coefficients to balance atoms.

  3. Check your work to ensure mass is conserved.

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 H+ concentration). Example: pH 2 is a strong acid.

• Neutral Solution: pH 7 (pure water).

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

• Color Indicators: pH indicators change color depending on the acidity or basicity of a solution (red/orange for acids, green for neutral, blue/purple for bases).

Summary Table: Types of Chemical Bonds