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CHAPTER 5

Periodic Classification of Elements

Trace the development of the periodic table and understand trends across periods and groups.

Topics in this chapter

  1. Need for classification
  2. Dobereiner’s triads
  3. Newlands’ law of octaves
  4. Mendeleev’s periodic table
  5. Modern periodic law
  6. Groups and periods
  7. Valency trend
  8. Atomic size trend
  9. Metallic and non-metallic character

Key learning points

✓ The modern periodic table is arranged by increasing atomic number.

✓ Elements in the same group have similar valence-electron configurations.

✓ Atomic size generally decreases across a period and increases down a group.

✓ Metallic character generally decreases across a period and increases down a group.

Multiple-choice questions

1. The modern periodic law is based on:
  1. Atomic mass
  2. Atomic number
  3. Density
  4. Valency only

Answer: B — Atomic number

2. Across a period, atomic size generally:
  1. Increases
  2. Decreases
  3. Remains constant
  4. First increases then decreases

Answer: B — Decreases

3. Elements in the same group generally have the same number of:
  1. Shells
  2. Neutrons
  3. Valence electrons
  4. Isotopes

Answer: C — Valence electrons

Very short-answer questions

  1. State the modern periodic law.
  2. How many periods are present in the modern periodic table?
  3. What happens to metallic character across a period?

Short-answer questions

  1. Compare Mendeleev’s periodic table with the modern periodic table.
  2. Explain trends in valency and atomic size across a period.

Long-answer question

  1. Describe the development of periodic classification and explain major periodic trends.