CBSE Class 10 Chemistry Periodic Classification of Elements MCQs Set 14

Answer: (c) 81.25

Explanation:
1. In a triad the middle element has a mass close to the average of the other two.
2. Mass of Br = (35.5 + 127) / 2 = 162.5 / 2 = 81.25.

Teacher's Note:
1. Average of the first and third member gives the middle member.
2. The real value for bromine is about 80, so 81.25 is a close match.

Answer: (a) Arsenic

Explanation:
1. A triad has three elements of the same group in order of atomic mass.
2. P, As, Sb are all Group 15 elements, with arsenic as the middle one.

Teacher's Note:
1. Look for the element in the same group as the two given ones.
2. Other triads are (S, Se, Te), (Li, Na, K) and (Ca, Sr, Ba).

Answer: (d) Newlands

Explanation:
1. Newlands arranged elements by increasing atomic mass and noticed that every eighth element repeated the properties of the first, like musical notes.
2. This was called the law of octaves.

Teacher's Note:
1. Octave means a group of eight, like Sa Re Ga Ma Pa Dha Ni Sa.
2. Dobereiner gave triads and Mendeleev gave the periodic law.

Answer: (a) increases

Explanation:
1. Going down a group the atoms become larger and the outer electrons are held less tightly.
2. So the tendency to lose electrons, which is electropositive character, increases.

Teacher's Note:
1. Metallic and electropositive character both increase down a group.
2. Both decrease across a period from left to right.

Answer: (d) electrons in the outermost shell

Explanation:
1. Elements of one group have the same number of valence electrons.
2. That is why they show similar chemical properties.

Teacher's Note:
1. Same group means same outer electron count, not same protons or neutrons.
2. Period number equals the number of shells.

Answer: (b) seven periods and eighteen groups

Explanation:
1. The long form has 7 horizontal rows called periods and 18 vertical columns called groups.

Teacher's Note:
1. Remember 7 periods and 18 groups.
2. The older Mendeleev table had 8 groups.

Answer: (a) Group 1

Explanation:
1. Li, Na, K, Rb, Cs and Fr, the alkali metals, form Group 1 of the table.

Teacher's Note:
1. Alkali metals have one valence electron.
2. Group 2 elements are the alkaline earth metals.

Answer: (b) 16, 2

Explanation:
1. Z = 8 has configuration 2, 6.
2. Two shells so period 2, and 6 valence electrons so group 10 + 6 = 16.

Teacher's Note:
1. For elements with 3 to 8 valence electrons the group number is 10 plus the valence electrons.
2. The number of shells gives the period.

Answer: (c) N (7)

Explanation:
1. The atom has 5 valence electrons (Group 15, phosphorus).
2. Nitrogen (2, 5) also has 5 valence electrons, so it is chemically similar.

Teacher's Note:
1. Same number of valence electrons gives similar chemical properties.
2. Fluorine has 7, sodium 1 and argon 8.

Answer: (d) number of electron shells

Explanation:
1. Elements of one period have the same number of occupied shells.
2. For example, Li to Ne all have two shells.

Teacher's Note:
1. The period number equals the number of shells.
2. The number of valence electrons changes across a period.

Answer: (d) all of the above

Explanation:
1. Same group means the same number of valence electrons.
2. This gives the same valency and similar chemical properties.

Teacher's Note:
1. Valence electrons decide valency and chemical behaviour.
2. That is why groups are called chemical families.

Answer: (a) 6

Explanation:
1. Group 14 means 4 valence electrons and period 2 means two shells.
2. The configuration is 2, 4, so Z = 6 (carbon).

Teacher's Note:
1. Configuration first, atomic number second.
2. Atomic number 14 is silicon, which lies in period 3.

Answer: (a) C, Si, Sn

Explanation:
1. C, Si and Sn are in Group 14, so each has 4 valence electrons.
2. O, S, Se have 6, B, Al, Ga have 3 and Ne, Ar, Kr have 8.

Teacher's Note:
1. Count the group number minus 10 for elements in groups 13 to 18.
2. Group 14 means 4 valence electrons.

Answer: (d) 3, 20

Explanation:
1. Z = 3 is Li (2, 1) and Z = 20 is Ca (2, 8, 8, 2).
2. In both the last electron enters an s orbital, so both are s-block.

Teacher's Note:
1. s-block elements are in Groups 1 and 2.
2. Z = 12 is also s-block but 6 is not.

Answer: (b) Sixteen

Explanation:
1. Z = 16 is sulphur with configuration 2, 8, 6.
2. Six valence electrons give Group 16.

Teacher's Note:
1. Group number = 10 + valence electrons for p-block.
2. Sulphur is in the oxygen family.

Answer: (c) The atoms lose their electrons more easily

Explanation:
1. Across a period the nuclear charge increases and atoms hold electrons more tightly.
2. So atoms lose electrons less easily, not more easily.

Teacher's Note:
1. Ionisation energy increases across a period.
2. The statement about losing electrons more easily describes a trend down a group.

Answer: (a) Valence electrons

Explanation:
1. Elements of a group have the same number of valence electrons.
2. Size, density and metallic character all change down the group.

Teacher's Note:
1. Valence electrons are the same in a group, while shells increase.
2. This explains the similar chemistry of a group.

Answer: (b) They have the same electronic configuration

Explanation:
1. C (2, 4) and Si (2, 8, 4) are in Group 14 and have the same number of valence electrons.
2. Their complete electronic configurations are different, so statement (b) is wrong.

Teacher's Note:
1. Same valence electrons does not mean same configuration.
2. The number of shells is different.

Answer: (d) Si

Explanation:
1. The formula XCl4 shows that X has a valency of 4, so it is in Group 14.
2. A low melting point solid suggests a covalent chloride, as in \( \ce{SiCl4} \).
3. Si is in Group 14.

Teacher's Note:
1. Ionic chlorides have high melting points, covalent chlorides have low ones.
2. Valency 4 points to Group 14.

Answer: (c) Solid, non-metallic

Explanation:
1. Group 13, period 2 is boron.
2. Boron is a hard solid and a non-metal (a metalloid that behaves mostly as a non-metal).

Teacher's Note:
1. Boron is the first member of Group 13 and differs from the rest of the group.
2. Aluminium and the others below it are metals.