Explain why the atomic radius and the ionic radius both increase steadily on descending Group 1 from Li to Cs.

Example · Example 6

Q. Explain why the atomic radius and the ionic radius both increase steadily on descending Group 1 from Li to Cs.

West Bengal WbchseTextbookSubjectiveImportance★★★★★est

23% · 6/26 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Atomic radius is governed by two competing effects as atomic number increases: an increasing nuclear charge, which by itself would pull the electron cloud in and shrink the atom, and an increasing principal quantum number of the outermost electron (plus more inner shielding electrons), which by itself would push the electron cloud out and enlarge the atom.

Within a single group, the second effect dominates completely. Descending from lithium ( $n=2$ ) to sodium ( $n=3$ ) to potassium ( $n=4$ ) to rubidium ( $n=5$ ) to caesium ( $n=6$ ), the valence electron is placed in a shell of successively higher principal quantum number at every step, and each such step also adds a complete new inner shell of electrons that shields the valence electron even more from the (only modestly increasing) nuclear charge. The net effect is that both the atomic radius and, correspondingly, the ionic radius of $\text{M}^+$ increase substantially and steadily at every step down the group: $\text{Li} < \text{Na} < \text{K} < \text{Rb} < \text{Cs}$ . …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2023Set annual21 mark
$Q. Alkali metals import characteristic colour to the flame. (True/False)$
›Reveal solutionSolution
True — alkali metals impart characteristic colours to the flame because their single loosely-bound valence electron is easily excited by flame heat and emits visible light when it falls back to the ground state.

Alkali metals (Group 1: Li, Na, K, Rb, Cs) have a very low ionisation enthalpy, so their outermost (ns^1) electron is easily excited to a higher energy level by the thermal energy of a flame. When this electron falls back to its ground state, it emits energy in the form of visible light of a characteristic wavelength (colour), which is specific to each element:

Lithium: crimson red …
Start your 14-day free trial to see the full solution →
CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following is the strongest base ?$
(a) LiOH
(b) NaOH
(c) KOH
(d) CsOH
›Reveal solutionSolution

CsOH is the strongest base of the set.

For alkali metal hydroxides, basic strength increases down the group: LiOH < NaOH < KOH < RbOH < CsOH. Going down, the metal atom becomes larger and more electropositive, the M-OH bond becomes more ionic, and the hydroxide releases OH- more readily. Caesium, the most electropositive of the listed metals, therefore gives t …

Start your 14-day free trial to see the full solution →
CBSE 2022Set ANNUAL1 markMCQ
$Q. The fire producing by Alkali metals can be extinguished by$
(a) By CCl4
(b) By sand
(c) By water
(d) By kerosene
›Reveal solutionSolution
An alkali-metal fire is put out with dry sand — option (b).

From NCERT Class 11 Chemistry (s-Block Elements): alkali metals are highly reactive.

Water: 2Na + 2H₂O → 2NaOH + H₂ (explosive) — cannot be used.

CO₂ / CCl₄: hot alkali metals react with these too, so they are unsafe. …
Start your 14-day free trial to see the full solution →
CBSE 2018Set annual21 markMCQ
$Q. Super oxide is formed by:$
(a) Na
(b) K
(c) Li
(d) Mg
›Reveal solutionSolution
Going down group 1, the type of oxide formed on burning in excess air changes: Li -> normal oxide, Na -> peroxide, K (and below) -> superoxide.

When alkali metals are burnt in excess air/oxygen, they form different types of binary oxygen compounds depending on the metal's position in the group:

Lithium forms the normal oxide, Li2O.

Sodium forms the peroxide, Na2O2 (containing the peroxide ion, O2^2-).

Potassium, Rubidium, and Caesium form the superoxide, MO2 (e.g., KO2), containing the superoxide ion, O2^-. …
Start your 14-day free trial to see the full solution →

←Previous questionExample 5 — Arrange Li, Na, K, Rb and Cs in order of increasing first ionization e…→Next questionExample 7 — For the same period, is the first ionization enthalpy of a Group 2 ele…

Q.Explain why the atomic radius and the ionic radius both increase steadily on descending Group 1 from Li to Cs.

Concept understanding — Alkali Metals — General Characteristics

Q.Explain why the atomic radius and the ionic radius both increase steadily on descending Group 1 from Li to Cs.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days