1. The modern periodic table is arranged by increasing:
- a) Atomic mass
- b) Atomic number ✅
- c) Electronegativity
- d) Chemical reactivity
Chapter Overview
This chapter explains how elements are classified into different families and how their properties vary periodically across the modern periodic table. Key topics: historical development, modern periodic law, periodic trends (atomic radius, ionic radius, ionization energy, electron affinity, electronegativity), and periodicity across periods and groups.
1. Historical Development
- Dobereiner's Triads: Early grouping of elements in triads based on similar properties.
- Newlands' Law of Octaves: Noticed repetition every eighth element (works for light elements).
- Mendeleev's Periodic Table: Organized by increasing atomic mass and predicted properties of undiscovered elements — left gaps for them.
- Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers (Moseley).
2. Modern Periodic Table
The modern (long-form) periodic table is arranged by increasing atomic number. Periods are horizontal rows; groups (families) are vertical columns. Important groups: Alkali metals (Group 1), Alkaline earth metals (Group 2), Transition metals (d-block), Halogens (Group 17), Noble gases (Group 18).
3. Important Periodic Trends
- Atomic radius: Generally decreases across a period (left → right) due to increased nuclear charge; increases down a group due to added shells.
- Ionic radius: Cations are smaller than parent atoms; anions are larger. Within isoelectronic series, radius decreases with increasing nuclear charge.
- Ionization energy (IE): Energy required to remove an electron. IE increases across a period and decreases down a group. Exceptions occur (e.g., between Be→B, N→O) due to subshell configurations.
- Electron affinity (EA): Energy change when an electron is added. More negative across a period (generally); variations exist for noble gases and filled subshells.
- Electronegativity: Tendency of an atom to attract bonding electrons. Increases across a period and decreases down a group (Pauling scale commonly used).
- Metallic character: Decreases across a period and increases down a group.
4. Periodicity in Properties — Quick Summary
Across a Period (Left → Right)
- Atomic radius: ↓
- Ionization energy: ↑
- Electron affinity: (more negative) generally ↑
- Electronegativity: ↑
- Metallic character: ↓
Down a Group (Top → Bottom)
- Atomic radius: ↑
- Ionization energy: ↓
- Electron affinity: generally ↓ or less negative
- Electronegativity: ↓
- Metallic character: ↑
5. Applications & Significance
Periodic trends are essential for predicting chemical behavior — reactivity, bond type, oxidation states, and properties of compounds. Mendeleev's predictions validated the periodic approach and guided discovery of new elements.
50 MCQs — Practice (Answers highlighted)
2. Which scientist is credited with the long-form periodic table (based on atomic number)?
- a) Newlands
- b) Mendeleev
- c) Moseley ✅
- d) Dobereiner
3. Which group contains the noble gases?
- a) Group 1
- b) Group 16
- c) Group 17
- d) Group 18 ✅
4. Atomic radius generally ______ across a period.
- a) Increases
- b) Decreases ✅
- c) Remains same
- d) Varies randomly
5. Ionization energy is the energy required to:
- a) Add an electron
- b) Remove an electron ✅
- c) Break a bond
- d) Form an ion pair
6. Electronegativity tends to _____ down a group.
- a) Increase
- b) Decrease ✅
- c) Remain constant
- d) Oscillate
7. Which element has highest electronegativity (Pauling scale)?
- a) Fluorine ✅
- b) Oxygen
- c) Chlorine
- d) Nitrogen
8. Which of these is NOT a transition metal?
- a) Iron
- b) Copper
- c) Sodium ✅
- d) Titanium
9. Among Li, Na, K — which has the largest atomic radius?
- a) Li
- b) Na
- c) K ✅
- d) All equal
10. Electron affinity measures:
- a) Tendency to lose electron
- b) Tendency to gain electron ✅
- c) Ionization energy
- d) Electronegativity
11. Which group contains halogens?
- a) Group 14
- b) Group 17 ✅
- c) Group 2
- d) Group 18
12. Metallic character _____ across a period.
- a) Increases
- b) Decreases ✅
- c) Remains same
- d) Fluctuates
13. Which pair is isoelectronic?
- a) Na+ and Ne
- b) Na+ and Ne ✅
- c) Na and Ne
- d) Na+ and K+
14. Which element has the smallest atomic radius?
- a) Cs
- b) He ✅
- c) Fr
- d) Rb
15. First ionization energy generally increases because:
- a) Atomic size increases
- b) Effective nuclear charge increases ✅
- c) Electron shielding increases
- d) Number of shells increases
16. Which element will have higher ionization energy: O or S?
- a) S
- b) O ✅
- c) Equal
- d) Depends on isotope
17. Newlands proposed law of octaves based on:
- a) Atomic numbers
- b) Atomic masses (repetition every 8 elements) ✅
- c) Electronegativity
- d) Valency
18. Which element has higher electron affinity?
- a) Na
- b) Cl ✅
- c) K
- d) He
19. Which element is most metallic?
- a) Li
- b) Na
- c) Cs ✅
- d) Fr
20. Which group contains alkaline earth metals?
- a) Group 1
- b) Group 2 ✅
- c) Group 17
- d) Group 18
21. Which of these elements has highest first ionization energy?
- a) Na
- b) Ne ✅
- c) Mg
- d) Al
22. The d-block elements are commonly called:
- a) p-block elements
- b) Transition metals ✅
- c) Inner transition metals
- d) Noble gases
23. Which of these has lowest electronegativity?
- a) F
- b) Cs ✅
- c) O
- d) Cl
24. Among these, which has largest ionic radius?
- a) Na+
- b) Mg2+
- c) O2- ✅
- d) F-
25. Which property shows a general increase from left to right in a period?
- a) Atomic radius
- b) Ionization energy ✅
- c) Metallic character
- d) Ionic radius
26. The lanthanides are placed in which block?
- a) s-block
- b) p-block
- c) f-block ✅
- d) d-block
27. Which of the following has the highest electron affinity?
- a) Noble gases
- b) Halogens ✅
- c) Alkali metals
- d) Alkaline earth metals
28. Which element is a metalloid?
- a) Sodium
- b) Magnesium
- c) Argon
- d) Silicon ✅
29. The term 'period' in periodic table refers to:
- a) Vertical column
- b) Horizontal row ✅
- c) Group of elements
- d) Block of elements
30. Which of the following elements is diatomic?
- a) Neon
- b) Chlorine (Cl2) ✅
- c) Argon
- d) Sodium
31. Which element has the highest atomic radius in period 3?
- a) Cl
- b) S
- c) Na ✅
- d) Ar
32. Elements in the same group have:
- a) Similar valence electron configuration ✅
- b) Same atomic radius
- c) Same electronegativity
- d) Identical chemical properties
33. Which of the following shows decrease in atomic size?
- a) Down a group
- b) Across a period (left → right) ✅
- c) Both
- d) None
34. Which of these has the highest metallic character?
- a) O
- b) N
- c) K ✅
- d) F
35. The effective nuclear charge felt by electrons generally _____ across a period.
- a) Decreases
- b) Increases ✅
- c) Remains same
- d) Oscillates
36. Which element has more metallic character: Ca or Sr?
- a) Ca
- b) Sr ✅
- c) Equal
- d) Depends on allotrope
37. Which of the following is true for noble gases?
- a) Highly reactive
- b) High electronegativity
- c) Very low chemical reactivity ✅
- d) Form many stable ions
38. Which property is usually more negative across a period?
- a) Ionization energy
- b) Electron affinity (more negative) ✅
- c) Atomic radius
- d) Metallic character
39. Which of these elements is an alkali metal?
- a) Potassium (K) ✅
- b) Calcium
- c) Iron
- d) Helium
40. Which has larger atomic radius: S or Se?
- a) Se ✅
- b) S
- c) Equal
- d) Depends on allotrope
41. Which of these elements commonly shows +2 oxidation state?
- a) Na
- b) Ca ✅
- c) Cl
- d) F
42. Which element is placed at top of Group 17?
- a) Fluorine (F) ✅
- b) Chlorine
- c) Bromine
- d) Iodine
43. Which statement about alkali metals is true?
- a) They are poor conductors
- b) They are highly reactive and soft ✅
- c) They are noble gases
- d) They are non-metals
44. Ionization energy and atomic radius are related how?
- a) Larger radius → Higher IE
- b) Larger radius → Lower IE ✅
- c) No relation
- d) Depends on electronegativity
45. Which of these has the highest melting point?
- a) Noble gases
- b) Transition metals ✅
- c) Alkali metals
- d) Halogens
46. Which element has more negative electron affinity: Cl or S?
- a) Cl ✅
- b) S
- c) Equal
- d) Depends on isotope
47. Which block contains the noble gases?
- a) d-block
- b) p-block ✅
- c) s-block
- d) f-block
48. The periodicity of properties is mainly due to:
- a) Regular variation in atomic number and electronic configuration ✅
- b) Random distribution of elements
- c) External pressure
- d) Temperature only
49. Which element is likely to form an anion?
- a) Na
- b) Cl ✅
- c) K
- d) Ca
50. Which of the following properties shows periodicity?
- a) Atomic radius, ionization energy, electronegativity ✅
- b) Color only
- c) Taste only
- d) Melting point only
Quick Revision Tips
- Memorize group names and important elements (F, O, Cl, Na, K, etc.).
- Practice trends by comparing elements in same period and group.
- Remember exceptions: IE anomalies (Be→B, N→O) and electron affinity trends.