Structure 3.1.1 — Periods, Groups, and Blocks of the Periodic Table

The periodic table is a systematic arrangement of elements based on their atomic number, electron configurations, and recurring chemical properties. It is organized into three major features:

• Periods — horizontal rows
• Groups — vertical columns
• Blocks — sections classified by the atomic orbital type being filled (s, p, d, or f)

1. Periods

• There are 7 periods in the periodic table, corresponding to the number of occupied electron shells (energy levels).
• As you move from left to right across a period:
  • Atomic number increases
  • Electron configuration changes systematically
  • Elements show a gradual change in physical and chemical properties

2. Groups

• There are 18 groups, numbered 1 to 18.
• Elements in the same group have:
  • The same number of valence electrons
  • Similar chemical properties
• Examples:
  • Group 1: Alkali metals (very reactive)
  • Group 17: Halogens (highly reactive non-metals)
  • Group 18: Noble gases (very stable, inert gases)

3. Blocks

• The table is also divided into four blocks, based on the type of orbital that receives the last electron:
• s-block: Groups 1 and 2, and helium — filling s orbitals
• p-block: Groups 13 to 18 — filling p orbitals
• d-block: Transition metals (Groups 3 to 12) — filling d orbitals
• f-block: Lanthanides and actinides (separate rows at the bottom) — filling f orbitals

 Positions of Metals, Metalloids, and Non-Metals in the Periodic Table

The periodic table clearly reflects differences in physical and chemical properties between three main classes of elements: metals, metalloids, and non-metals. Their positions allow predictions of reactivity, bonding behavior, and conductivity.

1. Metals

• Metals are found on the left-hand side and extend to the center of the periodic table.
• They include:
  • s-block elements: Groups 1 and 2
  • d-block elements: Transition metals (Groups 3–12)
  • Some elements in Groups 13–16 (e.g. Al, Sn, Pb)
  • All lanthanides and actinides in the f-block
• Key features: Good conductors of heat/electricity, malleable, ductile, typically solid at room temperature (except Hg)

2. Non-Metals

• Non-metals are located on the upper right-hand corner of the periodic table (except hydrogen).
• They include:
  • Group 14–16: C, N, O, P, S, Se, etc.
  • Group 17: Halogens
  • Group 18: Noble gases
  • Hydrogen: Placed with Group 1, but is a non-metal
• Key features: Poor conductors, brittle when solid, form acidic oxides, gain electrons in reactions

3. Metalloids (Semimetals)

• Metalloids are found along the zig-zag step-line (stair-step) between metals and non-metals.
• They include:
  • Boron (B)
  • Silicon (Si)
  • Germanium (Ge)
  • Arsenic (As)
  • Antimony (Sb)
  • Tellurium (Te)
  • Astatine (At)
• Key features: Show intermediate behavior, often semiconductors, chemically behave like metals or non-metals depending on context

The Four Blocks of the Periodic Table: s, p, d, and f

The periodic table is divided into four blocks based on which sublevel (s, p, d, or f) is being filled with electrons in the outermost (valence) shell of the elements. These blocks reflect electron configurations and chemical behavior.

1. s-block

• Location: Groups 1 and 2 (alkali and alkaline earth metals), plus hydrogen and helium
• Valence electrons: Enter the s sublevel
• Max electrons in s-sublevel: 2
• General electron configuration: ns¹ to ns²
• Examples: H (1s¹), Be (1s² 2s²), Mg (1s² 2s² 2p⁶ 3s²)

2. p-block

• Location: Groups 13 to 18 (right side of the table)
• Valence electrons: Enter the p sublevel
• Max electrons in p-sublevel: 6
• General electron configuration: ns²np¹ to ns²np⁶
• Examples: C (1s² 2s² 2p²), O (1s² 2s² 2p⁴), Cl (1s² 2s² 2p⁶ 3s² 3p⁵)

3. d-block

• Location: Transition metals (Groups 3 to 12, center of table)
• Valence electrons: Enter the d sublevel (after the s sublevel of the next shell has been filled)
• Max electrons in d-sublevel: 10
• General electron configuration: (n–1)d¹ to (n–1)d¹⁰ ns⁰–²
• Examples: Fe (1s² 2s²…3d⁶ 4s²), Cu (3d¹⁰ 4s¹)

4. f-block

• Location: Below the main table — includes lanthanides and actinides
• Valence electrons: Enter the f sublevel
• Max electrons in f-sublevel: 14
• General electron configuration: (n–2)f¹ to (n–2)f¹⁴
• Examples: Ce (4f¹ 5d¹ 6s²), U (5f³ 6d¹ 7s²)

Special Cases in Block Assignment

While most elements are clearly placed into blocks based on the sublevel their outermost electrons occupy, a few elements represent special cases due to exceptions in electron configuration or convention. These are important for understanding block classification in IBDP Chemistry.

• 1. Hydrogen (H):
  • Electron configuration: 1s¹ → s-block
  • However, it also shares some chemical properties with Group 17 halogens.
  • Classification: s-block by electron configuration.
• 2. Helium (He):
  • Electron configuration: 1s² → suggests it belongs to the s-block
  • But it is placed with Group 18 due to its chemical inertness (noble gas).
  • Classification: s-block by configuration, p-block by position.
• 3. Chromium (Cr) and Copper (Cu):
  • These have irregular electron configurations due to stability of half-filled/full-filled d orbitals:
  • Cr: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²)
  • Cu: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²)
  • Classification: Both are still considered d-block elements.
• 4. Lanthanides and Actinides:
  • These are part of Periods 6 and 7, and are shown separately below the main table.
  • Though their electrons fill the f-orbitals, the exact starting point can be inconsistent (e.g., La and Ac often start the series but are not f-block by configuration).
  • Classification: Generally considered f-block.