2.2 Chemistry Notes: The Periodic Table and Atomic Structure

A Predictive Model

• Electron Configuration and Properties:

  • An element's chemical and physical properties are determined by its electron configuration, which includes the arrangement of valence electrons around the nucleus.

  • The periodic table structure helps predict these properties, especially within chemical families like alkali metals, alkaline earth metals, halogens, noble gases, and transition metals, by recognizing patterns in valence electrons.

• Periodic Table Blocks (s, p, d, f):

  • The periodic table is divided into four blocks (s, p, d, f) which help visualize and predict an element’s reactivity and chemical properties.

    • s-block: Includes Groups 1 and 2. Elements have 1 or 2 electrons in their outer s subshell.

    • p-block: Includes Groups 13–18. Elements have p subshell electrons.

    • d-block: Includes transition metals (Groups 3–12), with filled or partially filled d subshells.

    • f-block: Includes lanthanides and actinides, where f subshells are being filled.

Group-Specific Reactivity and Properties

• Alkali Metals (Group 1):

  • Have one electron in the outer s subshell.

  • Highly reactive due to having only one valence electron.

• Alkaline Earth Metals (Group 2):

  • Have two electrons in the outer s subshell.

  • Less reactive than alkali metals due to a filled s orbital.

• Halogens (Group 17):

  • One electron short of a full octet.

  • Very reactive, especially in forming compounds.

• Noble Gases (Group 18):

  • Have a full valence shell.

  • Relatively nonreactive due to stable electron configuration.

• Transition Metals (Groups 3–12):

  • Contain filled or partially filled d subshells.

  • Generally less reactive than Groups 1 and 2 elements.

The Periodic Table as a Predictive Model (Continued)

• Predicting Electron Configurations:

  • The periodic table can predict electron configurations based on an element’s position, reflecting systematic changes across periods.

• Core and Valence Electrons:

  • Example: Sodium (Na)

    • Atomic number: 11

    • Configuration: [Ne] 3s¹ (10 core electrons and 1 valence electron)

  • Core electrons are those in the inner energy levels, while valence electrons are in the outermost energy level and participate in chemical reactions.

• Developing Models:

  • Chlorine (Cl) Example:

    • Use chlorine’s position to model its electron configuration, including protons in the nucleus, core electrons, and valence electrons.

Coulomb’s Law

• Interactions of Charged Particles:

  • Coulomb’s Law describes the force (F) between two charged particles, depending on the charge (q) of each particle and the distance (d) between them.

  • Formula: F=keq1q2d2F = k_e \frac{q_1 q_2}{d^2}F=ke​d2q1​q2​​

    • The force is directly proportional to the product of the charges and inversely proportional to the square of the distance.

• Implications of Coulomb’s Law:

  • Larger charges result in stronger attractions or repulsions.

  • Greater distances between charges weaken the force.

The Shielding Effect and Effective Nuclear Charge

• Shielding Effect:

  • Core electrons shield valence electrons from the full attractive force of the nucleus.

  • The repulsive forces between electrons reduce the effective nuclear charge experienced by each electron.

• Effective Nuclear Charge (Zeff):

  • Formula: Zeff=Z−SZ_{\text{eff}} = Z - SZeff​=Z−S

    • ZZZ: Nuclear charge (number of protons).

    • SSS: Shielding constant (approximated by the number of core electrons).

• Trends in Zeff:

  • Increases from left to right across a period because the nuclear charge increases while the number of core electrons remains constant.

  • Example of increasing Zeff:

    • Sodium (Na): Zeff=11−10=+1Z_{\text{eff}} = 11 - 10 = +1Zeff​=11−10=+1

    • Magnesium (Mg): Zeff=12−10=+2Z_{\text{eff}} = 12 - 10 = +2Zeff​=12−10=+2

    • Aluminum (Al): Zeff=13−10=+3Z_{\text{eff}} = 13 - 10 = +3Zeff​=13−10=+3

• Periodic Trends:

  • Zeff increases across a period and decreases down a group.

• Comparative Models:

  • Comparing elements like silicon and germanium, which are in the same group, can show how Zeff changes due to differences in shielding and nuclear charge.