CBSE Class 10 Chemistry Periodic Classification of Elements MCQs Set 22

Answer: (a) Number of valence electrons

Explanation:
1. Valence electrons increase by one for each element across a period.
2. Atomic size and electropositive character decrease across the period.

Teacher's Note:
1. Left to right across a period, nuclear pull increases.
2. So size and metallic character decrease.

Answer: (c) Al

Explanation:
1. Al has configuration 2, 8, 3.
2. Cs has 1, Ca has 2 and S has 6 valence electrons.

Teacher's Note:
1. Aluminium is in Group 13.
2. Group 13 elements have three valence electrons.

Answer: (c) Mg

Explanation:
1. Li, Na and K have one valence electron and form \( \ce{M+} \) ions.
2. Mg has two valence electrons and forms \( \ce{Mg^2+} \).

Teacher's Note:
1. Group 1 gives +1 ions and Group 2 gives +2 ions.
2. Mg is in Group 2.

Answer: (d) Ca, Cs

Explanation:
1. Ca is in Group 2 and Cs is in Group 1, so they have 2 and 1 valence electrons.
2. The other pairs are in the same group.

Teacher's Note:
1. Chemical similarity needs the same group.
2. Different groups mean different valence electrons.

Answer: (c) \( \ce{M-} \)

Explanation:
1. Atomic number 9 gives configuration 2, 7 (fluorine).
2. It gains one electron to complete the octet and forms the \( \ce{M-} \) ion.

Teacher's Note:
1. Seven valence electrons means one electron is gained.
2. Halogens form -1 ions.

Answer: (c) Li

Explanation:
1. Atomic size decreases from left to right in a period.
2. Li is the first element of period 2, so it is the largest.

Teacher's Note:
1. More nuclear charge pulls the same shells closer.
2. Noble gases are not compared in this trend.

Answer: (d) Mg

Explanation:
1. Mg, Al, Si and P are in the third period.
2. Radius decreases from left to right, so Mg, the leftmost, is the largest.

Teacher's Note:
1. Compare elements of one period by position.
2. The left side has the largest atoms.

Answer: (c) Cl

Explanation:
1. Size decreases across period 3 from Na to Cl.
2. Cl is the smallest of the options (noble gas argon is measured differently).

Teacher's Note:
1. Chlorine has the highest nuclear pull among the listed atoms.
2. Atomic radius is smallest for halogens in a period.

Answer: (a) F, O, C, Cl, Br

Explanation:
1. In period 2 the radius decreases from C to O to F, so F < O < C.
2. Cl is in period 3 and Br in period 4, so they are larger, with Cl < Br.
3. The order is F < O < C < Cl < Br.

Teacher's Note:
1. First compare periods, then compare positions within a period.
2. More shells mean a larger atom.

Answer: (b) the lowest ionisation energy

Explanation:
1. Alkali metals have the largest size and the weakest hold on the outer electron in their period.
2. So their ionisation energy is the lowest.

Teacher's Note:
1. Easy loss of one electron makes alkali metals highly reactive.
2. Halogens have the highest values of electron affinity and electronegativity.

Answer: (c) Halogens

Explanation:
1. Halogens need one electron to complete the octet and have the highest electron affinity.
2. They therefore form \( \ce{X-} \) ions most readily.

Teacher's Note:
1. Gaining one electron is easy for halogens.
2. Alkali metals form cations.

Answer: (a) O < Se < S

Explanation:
1. Electron affinity values (kJ per mol released) are O 141, Se 195 and S 200.
2. Oxygen is smaller and its crowded 2p subshell repels the new electron, so it is lowest.
3. The ascending order is O < Se < S.

Teacher's Note:
1. The first element of a group often has an unusual value of electron affinity.
2. Oxygen is lower than sulphur for this reason.

Answer: (d) Nitrogen and phosphorus

Explanation:
1. N and P are both in Group 15 with 5 valence electrons.
2. The other pairs are in different groups.

Teacher's Note:
1. Similar behaviour needs the same group.
2. Boron is in Group 13 and germanium in Group 14.

Answer: (d) K > Mg > Al > B

Explanation:
1. Metallic character increases down a group and decreases across a period.
2. K (Group 1, period 4) is the most metallic, then Mg, then Al in period 3, and B at the top of Group 13 is the least.

Teacher's Note:
1. Combine the group trend and the period trend.
2. K > Mg > Al > B.

Answer: (b) decreases from left to right and increases down the group

Explanation:
1. Across a period nuclear charge increases, so metallic character falls.
2. Down a group atoms become larger, so metallic character rises.

Teacher's Note:
1. Metals are at the left and bottom of the table.
2. Non-metals are at the top right.

Answer: (a) ionisation energy

Explanation:
1. More shells put the outer electrons farther from the nucleus.
2. The ionisation energy therefore decreases down the group.

Teacher's Note:
1. Larger atoms lose electrons more easily.
2. Boiling point and density of noble gases increase down the group.

Answer: (d) Iodine

Explanation:
1. Non-metallic character decreases down Group 17.
2. Iodine is the lowest of the four halogens, so it has the least non-metallic character.

Teacher's Note:
1. Fluorine is the most non-metallic element.
2. Iodine is the biggest halogen listed here.

Answer: (a) Because they react with water to form alkalis

Explanation:
1. These metals react with water to give strongly basic hydroxides called alkalis.
2. For example, \( \ce{2Na + 2H2O -> 2NaOH + H2} \).

Teacher's Note:
1. The name comes from the alkali formed with water.
2. They form basic oxides and are highly reactive.

Answer: (b) \( \ce{Ba(OH)2} \)

Explanation:
1. Basic strength of hydroxides of Group 2 increases down the group.
2. Ba is the lowest of the four, so \( \ce{Ba(OH)2} \) is the most basic.

Teacher's Note:
1. More metallic character gives a more basic hydroxide.
2. \( \ce{Be(OH)2} \) is amphoteric.

Answer: (d) Sulphur

Explanation:
1. Non-metals form acidic oxides.
2. Sulphur gives \( \ce{SO2} \) and \( \ce{SO3} \), which form acids with water.
3. Na and Mg oxides are basic and Al oxide is amphoteric.

Teacher's Note:
1. Metal oxides are basic or amphoteric, non-metal oxides are acidic.
2. Sulphur is a non-metal.