Q.Why is Na less reactive than K ?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Periodic Trend Metallic Character
What "Metallic Character" Actually Means
Imagine you have a piece of copper wire and a lump of charcoal. The copper is shiny, you can hammer it into a thin sheet, and it conducts electricity. The charcoal is dull, brittle, and does not conduct electricity well. That difference — the set of properties that make a metal "metallic" — is what we call metallic character.
Metallic character is not a single number you can measure directly. It is a qualitative trend that describes how strongly an element behaves like a metal. The more metallic an element is, the more it shows these traits:
- Shiny (lustrous) appearance
- High electrical and thermal conductivity
- Malleability (can be hammered into sheets) and ductility (can be drawn into wires)
- Tendency to lose electrons and form positive ions (cations)
The last point is the key chemical reason behind the trend. Metals are electron-losers. Non-metals are electron-gainers.
The Periodic Trend: The Precise Statement
Metallic character decreases from left to right across a period, and increases from top to bottom down a group.
Let's break that into two parts.
Across a Period (Left to Right)
Take Period 3: Na → Mg → Al → Si → P → S → Cl → Ar.
Sodium (Na) is a highly reactive metal — it loses its one valence electron very easily. As you move right, the elements become less willing to lose electrons. Magnesium loses two electrons but holds them a bit tighter. Aluminium still behaves like a metal but is less reactive. Silicon is a metalloid — it has some metallic and some non-metallic properties. Phosphorus, sulfur, chlorine, and argon are clearly non-metals.
Why? The nuclear charge (number of protons) increases across the period, pulling the electrons in tighter. The valence electrons are held more strongly, so the atom is less willing to give them away. Losing electrons becomes harder → metallic character decreases.
Down a Group (Top to Bottom)
Take Group 1: Li → Na → K → Rb → Cs → Fr.
Lithium is a metal, but it is relatively hard and has a high melting point for a metal. Caesium is so metallic that it melts in your hand and explodes on contact with water. The metallic character increases dramatically as you go down.
Why? The atomic radius increases down the group. The valence electron is farther from the nucleus and is shielded by more inner electron shells. The nucleus holds it much more loosely. Losing that electron becomes very easy → metallic character increases.
The same logic applies to all groups. Even in Group 14, carbon (top) is a non-metal, silicon and germanium are metalloids, and tin and lead (bottom) are metals. The trend is consistent.
The One Reason Behind Both Trends
Both trends come down to a single idea: how easily an atom can lose an electron.
| Direction | Change in electron loss ease | Effect on metallic character |
|---|---|---|
| Left → Right | Harder (higher ionization energy) | Decreases |
| Top → Bottom | Easier (lower ionization energy) | Increases |
If you ever forget the trend, remember: Metals are electron-losers. The easier it is to lose an electron, the more metallic the element. Ionization energy (the energy needed to remove an electron) is your best friend here — lower ionization energy = higher metallic character.
A Quick Visual Summary
| Period | Left side | Middle | Right side |
|--------|-----------|--------|------------| …
K lies below Na in group 1, so it has a larger atomic radius and lower ionisation enthalpy, meaning it loses its valence electron more easily. …
K's larger atomic size (one shell more than Na) gives it a lower ionisation enthalpy, so it loses its outer electron more easily and is more reactive than Na.
Sodium and potassium are both group 1 (alkali) metals, reacting by losing their single valence electron to form M+ ions. Reactivity in this group depends on how easily that electron can be removed (ionisation enthalpy):
- K (period 4) has one more electron shell than Na (period 3), so its valence electron is farther from the nucleus and experiences more shielding from inner electrons.
- This makes K's ionisation enthalpy lower than Na's (K: ~419 kJ/mol vs Na: ~496 kJ/mol), so K loses its electron more readily. …
Showing the 12 most recent of 15 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.The elements in the right hand side in periodic table are(a) Metals(b) Non-metals(c) Metalloids(d) Transition elements
›Reveal solutionSolution
The right-hand side of the periodic table contains the non-metals.
Metallic character decreases across a period from left to right. Metals occupy the left and centre, metalloids run along the diagonal 'staircase', and non-metal …
- CBSE 2026Set ANNUAL1 markMCQQ.Calculate the number of sigma(σ) and pi (π) bonds present in benzene (C6H6).(a) a) 6σ and 3π(b) b) 3σ and 6π(c) c) 3σ and 12π(d) d) 12σ and 3π
›Reveal solutionSolution
[!TLDR]
d) 12σ and 3π
Why
Benzene has 6 C-C sigma bonds + 6 C-H sigma bonds = 12 sigma bonds, and 3 pi bonds from the 3 alternating do …
- CBSE 2026Set ANNUAL1 markQ.Unsaturated hydrocarbons contain carbon-carbon ________ bonds.
›Reveal solutionSolution
[!TLDR]
double bond
Method
Unsaturated hydrocarbons (alkenes/alkynes) contain carbon-carbon multiple bonds; from the given word bank …
- CBSE 2025Set ANNUAL1 markQ.The most electropositive element is ______.
›Reveal solutionSolution
Caesium (Cs) is conventionally cited as the most electropositive stable element (electropositivity increases down a group, decreases across a period).
Electropositive character (metallic character / tendency to lose electrons and form cations) increases on going down a group and decreases on going from left to right across a period, because of decreasing ionisation enthalpy down a group. Caesium, a Group 1 (alkali metal) element near the bottom-left of the periodic table, has one of the lowest ionisation enthalpies among stable elements, making it the most electropositive commonly …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following statements is not true for alkali metals?(a) These are largest in size in their respective periods.(b) These belong to Group-1 of periodic table.(c) These are strongest metals in their respective periods.(d) Ionization enthalpy of these increases down the group.
›Reveal solutionSolution
Alkali metals are the softest metals of their periods, not the strongest — statement (c) is false.
Checking each statement about Group-1 (alkali) metals:
- (a) They are largest in size in their respective periods — true, since they have the lowest nuclear charge and highest shielding for their period.
- (b) They belong to Group 1 — true, by definition.
- (c) They are the strongest metals in their respective periods — false. Because of their large atomic size and single loosely-held valence electron, the metallic bonding in alkali metals is weak, making them among the softest metals (many can be cut with a knife), not the strongest. …
- CBSE 2023Set ANNUAL1 markMCQQ.Considering the elements B, Al, Mg and K the correct order of their metallic character is :(a) B > Al > Mg > K(b) Al > Mg > B > K(c) Mg > Al > K > B(d) K > Mg > Al > B
›Reveal solutionSolution
Metallic character rises going down a group and falls going across a period, so K > Mg > Al > B.
Metallic character depends on how easily an atom loses its valence electron(s) — the lower the ionisation enthalpy, the more metallic the element.
- Across a period, metallic character decreases left to right as effective nuclear charge increases and electrons are held more tightly.
- Down a group, metallic character increases as atomic size increases and the valence electron is farther from the nucleus and more easily lost. …
- CBSE 2022Set ANNUAL1 markMCQQ.Arrange B, Al, Mg and K in correct order of their metallic character :(a) B > Al > Mg > K(b) Al > Mg > B > K(c) Mg > Al > K > B(d) K > Mg > Al > B
›Reveal solutionSolution
Metallic character rises down a group and falls across a period, so the order is K > Mg > Al > B.
Metallic character (the tendency to lose electrons and form cations) increases on moving down a group, because atomic size increases and the outer electrons are held less tightly. It decreases on moving left to right across a period, because effective nuclear charge increases and electrons are held more tightly.
- K is a period-4 alkali metal (group 1) — the most metallic element among the four.
- Mg (period 3, group 2) is more metallic than Al (period 3, group 13), since Al lies further right in the same period. …
- CBSE 2022Set ANNUAL1 markMCQQ.Which alkali metal having least melting point ?(a) Na(b) K(c) Rb(d) Cs
›Reveal solutionSolution
Alkali-metal melting points fall down the group; Cs, the heaviest metal listed, has the lowest melting point (~28.5°C).
Approximate melting points: Na ≈ 98°C, K ≈ 63°C, Rb ≈ 39°C, Cs ≈ 28.5°C. As atomic size increases down group 1, the metallic bonding (delocalised valence electron holding the metal cations together) becomes progressively weaker because the single valence electron is farther from the nucleus and less tightly bound …
- CBSE 2022Set ANNUAL1 markQ.Why is Na less reactive than K ?
›Reveal solutionSolution
K's larger atomic size (one shell more than Na) gives it a lower ionisation enthalpy, so it loses its outer electron more easily and is more reactive than Na.
Sodium and potassium are both group 1 (alkali) metals, reacting by losing their single valence electron to form M+ ions. Reactivity in this group depends on how easily that electron can be removed (ionisation enthalpy):
- K (period 4) has one more electron shell than Na (period 3), so its valence electron is farther from the nucleus and experiences more shielding from inner electrons.
- This makes K's ionisation enthalpy lower than Na's (K: ~419 kJ/mol vs Na: ~496 kJ/mol), so K loses its electron more readily. …
- CBSE 2022Set ANNUAL1 markMCQQ.Arrange these elements in increasing order of metallic character: Si, Al, Mg, Na, P.(a) P<Si<Al<Mg<Na(b) Si<Al<P<Mg<Na(c) Mg<Na<Si<P<Al(d) Na<Mg<Al<Si<P
›Reveal solutionSolution
Metallic character decreases left to right across a period, so among Na, Mg, Al, Si, P (period 3), the increasing order of metallic character is P < Si < Al < Mg < Na.
Across period 3, effective nuclear charge increases while atomic size decreases, so metallic character decreases from Na (leftmost, most metallic) to P (rightmost, least metallic among these five): Na > Mg > Al …
- CBSE 2021Set ANNUAL1 markMCQQ.Arrange B, C, N, F and Si in correct order of their non-metallic character.(a) B > C > Si > N > F(b) Si > C > B > N > F(c) F > N > C > B > Si(d) F > N > C > Si > B
›Reveal solutionSolution
Using electronegativity as the measure of non-metallic character: F (4.0) > N (3.0) > C (2.5) > B (2.0) > Si (1.9), giving option (c).
Step 1 — Recall the periodic trend: Non-metallic character increases left to right across a period (more protons pull electrons in more strongly) and decreases top to bottom down a group (electrons are farther from the nucleus).
Step 2 — Place the period-2 elements first: B, C, N, F are all period 2. Moving left to right, non-metallic character rises steadily: B < C < N < F.
…
- CBSE 2021Set ANNUAL1 markQ.Why are alkali metals not found in nature?
›Reveal solutionSolution
Alkali metals have the lowest ionization enthalpies of any group, so they lose their single valence electron very easily, react readily with almost everything around them (air, water, moisture), and are therefore always found combined as compounds, never as free elements.
Step 1 — Recall the periodic trend: Ionization enthalpy generally decreases down a group. Group 1 (alkali metals) sit at the far left of the periodic table and have the single largest atomic radii and lowest effective nuclear charge on the valence electron in their respective periods, giving them the lowest ionization enthalpies among all elements.
Step 2 — Consequence for reactivity: A low ionization enthalpy means the outermost electron is very easily removed, making alkali metals extremely strong reducing agents that react vigorously with oxygen, moisture, and even nitrogen in the air.
…
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