9.1 Properties of Group 1 Elements (Alkali Metals)
Group 1 elements have ns¹ configuration in their outermost shells. They are called alkali metals (except hydrogen). They are highly reactive due to easy removal of the single valence electron.
KEY NOTES: Alkali Metals
ELECTRONIC CONFIGURATION
• Outer shell: ns¹
• Easily lose 1 electron → M⁺ ion
• Very low ionization energy
• Hydrogen is exception (non-metal gas)
REACTIVITY TREND
• Increases down the group
• Reason: Atomic size increases
• Easier to lose electron down group
• Reactivity order: Li < Na < K < Rb < Cs
REACTION WITH WATER
• General: 2M + 2H₂O → 2MOH + H₂↑
• Li: Reacts steadily
• Na: Reacts vigorously
• K: Reacts violently
• Rb: Sinks, reacts violently
• Cs: Explodes on contact
PHYSICAL PROPERTIES
• Softness increases down group
• Melting point decreases down group
• Density generally increases
• Good conductors of heat & electricity
Table 9.1: Physical Properties of Alkali Metals
| Metal | Li | Na | K | Rb | Cs |
|---|---|---|---|---|---|
| Melting Point (°C) | 180 | 98 | 64 | 39 | 28 |
| Density (g/cm³) | 0.53 | 0.97 | 0.86 | 1.53 | 1.87 |
Observation: Melting point decreases down group while density generally increases.
Interesting Information!
Cesium (Cs) explodes on contact with water, possibly shattering the container. Rubidium (Rb) sinks in water while other alkali metals float.
Lithium (Li) behaves differently from other alkali metals in some reactions due to its small size and high charge density.
Memory Trick:
“Li Na K Rb Cs Fr” → “Little Naughty Kids Rub Cat’s Fur”
Reactivity: “Lowest at top, highest at bottom” (Group 1 trend opposite to Group 17)
9.2 Properties of Group 17 Elements (Halogens)
Group 17 elements have ns²np⁵ configuration. They are highly electronegative non-metals that exist as diatomic molecules (X₂). They are called halogens meaning “salt-forming elements”.
KEY NOTES: Halogens
ELECTRONIC CONFIGURATION
• Outer shell: ns²np⁵
• Need 1 electron to complete octet
• Form X⁻ ions (anions)
• Highly electronegative
REACTIVITY TREND
• Decreases down the group
• Reason: Atomic size increases
• Harder to gain electron down group
• Reactivity order: F > Cl > Br > I
PHYSICAL STATES & COLORS
• F₂: Pale yellow gas
• Cl₂: Yellowish-green gas
• Br₂: Red-brown liquid (fuming)
• I₂: Shiny grey solid (purple vapors)
OXIDIZING POWER
• Strong oxidizing agents
• Oxidizing power: F₂ > Cl₂ > Br₂ > I₂
• Displacement reactions occur
• Cl₂ + 2NaBr → 2NaCl + Br₂
Atomic Size
Increases ↓
F < Cl < Br < I
Reactivity
Decreases ↓
F > Cl > Br > I
Melting Point
Increases ↓
F₂ < Cl₂ < Br₂ < I₂
Oxidizing Power
Decreases ↓
F₂ > Cl₂ > Br₂ > I₂
Hydrogen Halides: Thermal Stability
Thermal stability decreases down the group:
HF > HCl > HBr > HI
Reason: Bond length increases as halogen size increases, making H-X bond weaker.
• HF: Strong bond (small F atom)
• HI: Weak bond (large I atom)
• HI decomposes at room temperature more readily than HCl.
Interesting Information!
Chlorine is used to sterilize swimming pool water. It kills bacteria and other microorganisms.
Fluorine is the most reactive halogen and one of the most reactive elements known.
9.3 Properties of Transition Elements
Transition elements (Groups 3-12) are d-block metals with partially filled d-orbitals. They show unique properties like variable oxidation states, colored compounds, and catalytic activity.
KEY NOTES: Transition Metals
ELECTRONIC CONFIGURATION
• General: [Noble gas] (n-1)d¹⁻¹⁰ ns¹⁻²
• Incomplete d-subshells
• Can lose electrons from both s and d orbitals
CHARACTERISTIC PROPERTIES
• Variable oxidation states
• Form colored compounds
• Catalytic properties
• High melting & boiling points
• Malleable & ductile
• High densities
CATALYTIC APPLICATIONS
• Iron (Fe): Haber process (NH₃ production)
• Vanadium pentoxide (V₂O₅): Contact process (H₂SO₄ production)
• Platinum (Pt): Catalytic converters
• Nickel (Ni): Hydrogenation of oils
COLORED COMPOUNDS
• Due to d-d electron transitions
• Examples:
– CuSO₄: Blue
– KMnO₄: Purple
– K₂Cr₂O₇: Orange
– NiCl₂: Green
Industrial Catalysts
Haber Process: N₂ + 3H₂ ⇌ 2NH₃ (Fe catalyst)
Contact Process: 2SO₂ + O₂ → 2SO₃ (V₂O₅ catalyst)
Catalytic Converters: Pt, Pd, Rh convert harmful exhaust gases to less harmful ones
Hydrogenation: Unsaturated oils → Margarine (Ni catalyst)
Memory Trick:
“Transition Metals = Variable Colors & Catalysts”
Catalysts: “Fe for NH₃, V for H₂SO₄, Pt for cars, Ni for margarine”
9.4 Properties of Noble Gases
Group 18 elements (noble gases) have complete outer electron shells (ns²np⁶ except He: 1s²). They are monoatomic gases with very low boiling points and minimal chemical reactivity.
KEY NOTES: Noble Gases
ELECTRONIC CONFIGURATION
• He: 1s²
• Others: ns²np⁶ (complete octet)
• Stable configuration
• Very high ionization energy
PHYSICAL PROPERTIES
• Monoatomic gases
• Colorless, odorless
• Very low boiling points
• Boiling point increases down group
• He has lowest boiling point (-269°C)
CHEMICAL PROPERTIES
• Very low reactivity
• “Inert gases” (mostly unreactive)
• Form compounds only under extreme conditions
• Xe forms some compounds (XeF₂, XeF₄, XeF₆)
USES
• He: Balloons, cooling superconductors
• Ne: Neon signs
• Ar: Welding, incandescent bulbs
• Kr, Xe: Specialized lighting
Group 1
Alkali Metals
ns¹, Very Reactive
Group 17
Halogens
ns²np⁵, Reactive
Groups 3-12
Transition Metals
Catalysts, Colored
Group 18
Noble Gases
Inert, ns²np⁶
Interesting Information!
Helium (He) has the lowest boiling point of any element (-269°C). It remains liquid even at absolute zero under normal pressure.
Radon (Rn) is radioactive and can accumulate in buildings, posing health risks.
Chapter 9 Solved Exercises
(i) Which halogen will have the least reactivity with alkaline earth metals?
Show ExplanationExplanation: Halogen reactivity decreases down the group: F > Cl > Br > I
• Fluorine is most reactive
• Iodine is least reactive with alkaline earth metals
• This is because atomic size increases down group, making it harder to gain electrons
(ii) Which compound do you expect to be coloured?
Show ExplanationExplanation: Transition metal compounds are often colored due to d-d electron transitions.
• KCl, BaCl₂, AlCl₃: Main group metal compounds → colorless
• NiCl₂: Nickel(II) chloride (transition metal) → green color
• Other colored transition metal compounds: CuSO₄ (blue), KMnO₄ (purple), K₂Cr₂O₇ (orange)
(v) Which halogen acid is unstable at room temperature?
Show ExplanationExplanation: Thermal stability of hydrogen halides decreases down the group: HF > HCl > HBr > HI
• HF: Most stable (strong H-F bond)
• HI: Least stable, decomposes at room temperature
• Reason: Bond length increases with halogen size → weaker bond
(vii) Which group elements are the most reactive elements?
Show ExplanationExplanation: Group 1 (alkali metals) are the most reactive metals.
• Low ionization energy → easy to lose electron
• Reactivity increases down the group
• Cesium (Cs) is most reactive metal
• Fluorine (Group 17) is most reactive non-metal
i. Why does it become easier to cut an alkali metal when we move from top to bottom in group 1?
Answer:
As we move down group 1:
1. Atomic size increases → weaker metallic bonding
2. Interatomic forces decrease
3. Metals become softer
4. Lithium is hardest, Cesium is softest alkali metal
This is why it becomes easier to cut alkali metals down the group.
ii. Predict the reactivity of potassium towards halogens.
Answer:
Potassium (K) is a highly reactive alkali metal (Group 1). It reacts vigorously with halogens:
• 2K + Cl₂ → 2KCl (violent reaction)
• 2K + Br₂ → 2KBr (vigorous reaction)
• 2K + I₂ → 2KI (reaction occurs)
Reactivity decreases with halogen size: Cl₂ > Br₂ > I₂
Potassium reacts most vigorously with fluorine, but fluorine handling is dangerous.
iv. Why does iodine exist in the solid state at room temperature?
Answer:
Iodine exists as solid at room temperature because:
1. Large atomic size: Iodine atoms are large → stronger London dispersion forces
2. Molecular weight: I₂ has higher molecular weight (254 g/mol) than other halogens
3. Intermolecular forces: Stronger van der Waals forces between I₂ molecules
4. Comparison: F₂, Cl₂ are gases; Br₂ is liquid; I₂ is solid at room temperature
5. Sublimation: Iodine sublimes (solid → vapor) when heated, producing purple vapors
i. Which noble gas should have the lowest boiling point and why?
Answer:
Helium (He) has the lowest boiling point (-269°C or 4K).
Reasons:
1. Smallest atomic size: Weakest London dispersion forces
2. Lightest noble gas: Atomic mass = 4 g/mol
3. Electronic configuration: 1s² (only 2 electrons)
4. Trend: Boiling point increases down group: He < Ne < Ar < Kr < Xe < Rn
5. Unique property: Helium remains liquid even at absolute zero under normal pressure
iv. Name any three elements in the periodic table which exist as liquids.
Answer:
Three elements that exist as liquids at room temperature:
1. Mercury (Hg): Transition metal, liquid metal, used in thermometers
2. Bromine (Br): Halogen, red-brown fuming liquid
3. Gallium (Ga): Melts at 30°C, can melt in hand
Additional: Cesium (Cs) and Rubidium (Rb) melt slightly above room temperature (28°C and 39°C respectively).
v. Why are transition elements different from normal elements?
Answer:
Transition elements differ from main group elements in several ways:
1. Electronic configuration: Incomplete d-orbitals [(n-1)d¹⁻¹⁰]
2. Variable oxidation states: Can lose different numbers of electrons
3. Colored compounds: Due to d-d electron transitions
4. Catalytic properties: Many are good catalysts
5. Magnetic properties: Many are paramagnetic
6. Formation of complexes: Coordinate compounds with ligands
7. High density, melting point, strength: Generally harder and stronger
i. Explain the role of catalytic converter in an automobile.
Answer:
A catalytic converter is a device in automobile exhaust systems that:
1. Purpose: Converts harmful exhaust gases into less harmful ones
2. Catalysts used: Platinum (Pt), Palladium (Pd), Rhodium (Rh)
3. Reactions catalyzed:
• Oxidation: CO → CO₂
• Oxidation: Unburned hydrocarbons → CO₂ + H₂O
• Reduction: NOx → N₂ + O₂
4. Structure: Ceramic honeycomb structure coated with catalyst
5. Importance: Reduces air pollution, required by law in most countries
ii. Why do the chemical reactivities of alkali metals increase down the group whereas they decrease down the group in case of halogens?
Answer:
Alkali Metals (Group 1):
• Reactivity increases down the group
• Reason: Atomic size increases → easier to lose outermost electron
• Ionization energy decreases down group
• Trend: Li < Na < K < Rb < Cs (increasing reactivity)
Halogens (Group 17):
• Reactivity decreases down the group
• Reason: Atomic size increases → harder to gain electron
• Electron affinity decreases down group
• Trend: F > Cl > Br > I (decreasing reactivity)
Summary: Alkali metals lose electrons (easier with larger size), halogens gain electrons (harder with larger size).
v. Why hydrogen bromide is thermally unstable as compared to hydrogen chloride?
Answer:
Hydrogen bromide (HBr) is less thermally stable than hydrogen chloride (HCl) because:
1. Bond strength: H-Br bond is weaker than H-Cl bond
2. Bond length: Br atom is larger than Cl → longer H-Br bond → weaker bond
3. Thermal stability trend: HF > HCl > HBr > HI
4. Decomposition: HBr decomposes at lower temperatures than HCl
5. Reason: As halogen size increases down group, bond with hydrogen becomes weaker due to poorer overlap of orbitals