Science & Technology · Chapter 14

35 solved questions

Chapter 14: Classification of Elements

Every question in this chapter, answered and explained — step-by-step solutions drawn live from the chapter library across 7 question sections.

Very Short Answer Questions10 Short Answer Questions5 Give Reason Questions8 Differentiate Between4 Q1. Describe the characteristics of the modern periodic table.2 Application / Conceptual Questions2 Additional SEE Practice4

Very Short Answer Questions

1SEE Boardqb-q0

State the modern periodic law.

Ans.: The physical and chemical properties of elements are the periodic function of their atomic number.

2SEE Boardqb-q1

Who discovered that the properties of elements depend on atomic number?

Ans.: English scientist Henry Moseley discovered this in 1913 AD.

3SEE Boardqb-q2

How many periods and groups are there in the modern periodic table?

Ans.: There are 7 periods and 18 groups in the modern periodic table.

4SEE Boardqb-q3

What is a valence electron?

Ans.: An electron present in the valence (outermost) shell of an atom is called a valence electron.

5SEE Boardqb-q4

Name the group of alkali metals and their valence shell configuration.

Ans.: Alkali metals belong to group IA, and their valence shell electronic configuration is ns1.

6SEE Boardqb-q5

Name any two metalloids.

Ans.: Silicon (Si) and Germanium (Ge) are metalloids.

7SEE Boardqb-q6

What is Aufbau's Principle?

Ans.: Aufbau's Principle states that electrons are filled in a sub-shell in the increasing order of energy of the sub-shells.

8SEE Boardqb-q7

Which elements are called lanthanides?

Ans.: The 15 elements from Lanthanum (La, atomic number 57) to Lutetium (Lu, atomic number 71) are called lanthanides.

9SEE Boardqb-q8

What is the maximum number of electrons that s, p, d, and f sub-shells can hold?

Ans.: The s, p, d, and f sub-shells can hold a maximum of 2, 6, 10, and 14 electrons respectively.

10SEE Boardqb-q9

Which group of the periodic table contains noble/inert gases?

Ans.: Noble/inert gases are placed in group 0 (18) of the modern periodic table.

Short Answer Questions

1SEE Boardqb-q10

Write the position of sodium in the modern periodic table. Why is it known as an alkali metal?

Ans.: Sodium is placed in group IA, period 3 of the modern periodic table. It is called an alkali metal because it has one electron in its valence shell (ns1), is highly reactive, and forms a strong base (alkali) when dissolved in water.

2SEE Boardqb-q11

Write the electronic configuration of sulphur and state its position in the periodic table.

Ans.: The electronic configuration of sulphur (atomic number 16) is 1s2, 2s2 2p6, 3s2 3p4. Since it has 6 valence electrons and 3 shells, sulphur is placed in group VIA, period 3 of the modern periodic table.

3SEE Boardqb-q12

In which group do fluorine, chlorine, and bromine lie? Which one is more reactive among them?

Ans.: Fluorine, chlorine, and bromine lie in group VIIA (halogens). Among them, fluorine is the most reactive because its atomic size is the smallest, so it attracts electrons most strongly (highest electronegativity) among the three.

4SEE Boardqb-q13

On which basis are groups and periods separated in the modern periodic table?

Ans.: Groups (vertical columns) are formed by keeping elements with the same number of valence electrons (similar properties) together. Periods (horizontal rows) are formed by keeping elements with the same number of shells together, arranged in increasing order of atomic number.

5SEE Boardqb-q14

Write the positions of alkali metals, alkaline earth metals, inert gases, transition metals, lanthanides, and actinides in the periodic table.

Category	Position
Alkali metals	Group IA
Alkaline earth metals	Group IIA
Inert/Noble gases	Group 0 (18)
Transition metals	Groups IIIB to IIB
Lanthanides	Atomic number 57–71, below the main table
Actinides	Atomic number 89–103, below the main table

Give Reason Questions

1SEE Boardqb-q15

Classification of elements is necessary.

Ans.: Since 118 elements exist with widely varying properties, studying each one separately would be very difficult. Classification groups elements with similar properties together, making it easier to study, predict, and compare their physical and chemical behaviour systematically.

2SEE Boardqb-q16

The size of atoms increases on going from top to bottom in a group of the periodic table.

Ans.: As we move down a group, a new shell is added at each step, increasing the distance between the nucleus and the valence electrons. This increases the atomic size, even though the nuclear charge also increases, because the effect of an additional shell dominates.

3SEE Boardqb-q17

The size of atoms decreases on going from left to right of a period in the periodic table.

Ans.: Across a period, the number of shells remains the same, but the number of protons (nuclear charge) and electrons increases. The added electrons go into the same shell, and the stronger nuclear attraction pulls all shell electrons closer, contracting the atom.

4SEE Boardqb-q18

Hydrogen is a non-metal but it is kept with metals in the modern periodic table.

Ans.: Hydrogen has only one electron in its valence shell, giving it a valence configuration of ns1, similar to alkali metals of group IA. Because of this electronic similarity, it is placed with group IA, even though it behaves as a non-metal in most of its chemical properties.

5SEE Boardqb-q19

Potassium is more reactive than sodium.

Ans.: Potassium and sodium are both in group IA, but potassium has more shells than sodium, making its atomic size larger. A larger atomic size means the valence electron is farther from the nucleus and held less tightly, so potassium loses its valence electron more easily, making it more reactive than sodium.

6SEE Boardqb-q20

Fluorine is more reactive than chlorine.

Ans.: Fluorine and chlorine are both in group VIIA, but fluorine has a smaller atomic size (fewer shells) than chlorine. Its valence shell is closer to the nucleus, so it attracts an additional electron more strongly, making fluorine more reactive (more electronegative) than chlorine.

7SEE Boardqb-q21

The metallic character of elements decreases and non-metallic character increases on going from left to right of a period.

Ans.: Across a period, atomic size decreases and nuclear attraction on valence electrons increases. This makes it harder for elements to lose electrons (decreasing electropositivity/metallic character) and easier to gain electrons (increasing electronegativity/non-metallic character) as we move left to right.

8SEE Boardqb-q22

Inert gases are kept in group 0 of the modern periodic table.

Ans.: Inert gases have a completely filled valence shell (octet, or duplet for helium), giving them an ns2np6 configuration. Since they have no tendency to gain, lose, or share electrons, they do not react chemically, and are given their own group, labelled 0, separate from all reactive groups.

Differentiate Between

1SEE Boardqb-q23

Differentiate between group and period.

Group	Period
Vertical column of the periodic table.	Horizontal row of the periodic table.
Elements have the same valence electrons/valency.	Elements have the same number of shells.
There are 18 groups.	There are 7 periods.

2SEE Boardqb-q24

Differentiate between chlorine and sodium.

Chlorine	Sodium
A non-metal, in group VIIA.	A metal, in group IA.
Gains 1 electron to complete octet (valency 1).	Loses 1 electron to become stable (valency 1).
Electronic configuration: 1s2, 2s2 2p6, 3s2 3p5	Electronic configuration: 1s2, 2s2 2p6, 3s1
Highly electronegative.	Highly electropositive.

3SEE Boardqb-q25

Differentiate between elements of group IA and VIIA.

Group IA	Group VIIA
Alkali metals; valence configuration ns1.	Halogens (non-metals); valence configuration ns2np5.
Tend to lose 1 electron (electropositive).	Tend to gain 1 electron (electronegative).
Reactivity increases down the group.	Reactivity decreases down the group.

4SEE Boardqb-q26

Differentiate between electronegativity and electropositivity.

Electronegativity	Electropositivity
Tendency of an element to gain electrons.	Tendency of an element to lose electrons.
Forms negative ions (anions).	Forms positive ions (cations).
Increases across a period, decreases down a group.	Decreases across a period, increases down a group.
Property of non-metals.	Property of metals.

Q1. Describe the characteristics of the modern periodic table.

1SEE Boardqb-q27

Explain how the properties of elements change across a period and down a group.

Ans.These trends occur because, across a period, the increasing nuclear charge acts on the same number of shells, pulling electrons in more tightly. Down a group, an additional shell is added at each step, increasing distance from the nucleus and reducing the nucleus's hold on the valence electrons, despite the increased nuclear charge.

Property	Across a Period (Left → Right)	Down a Group (Top → Bottom)
Number of shells	Remains the same	Increases
Valence electrons	Increases	Remains the same
Atomic size	Decreases	Increases
Electropositivity	Decreases	Increases
Electronegativity	Increases	Decreases
Metal reactivity	Decreases	Increases
Non-metal reactivity	Increases	Decreases

2SEE Boardqb-q28

If you are given a chance to make improvements in the modern periodic table, what changes would you make? Explain with reasons.

Ans.(sample, open-ended): One improvement could be giving hydrogen its own separate position instead of placing it strictly under group IA, since hydrogen behaves differently from alkali metals in many reactions (it can also gain an electron like halogens). Another possible improvement is integrating the lanthanide and actinide series directly into the main body of the table instead of placing them separately below, to reflect their true position by atomic number — though this is avoided in practice because it would make the table too wide for convenient printing/display.

Application / Conceptual Questions

1SEE Boardqb-q29

An element X has electronic configuration 1s2, 2s2 2p6, 3s2 3p5. Predict its group, period, and reactivity.

Ans.: Element X has 3 shells and 7 valence electrons, so it lies in period 3, group VIIA (halogens). Its valence configuration ns2np5 means it needs just one more electron to complete its octet, making it highly reactive — it is chlorine (Cl).

2SEE Boardqb-q30

Two elements, P and Q, are in the same period. P has 1 valence electron and Q has 6 valence electrons. Which one is more reactive, and why?

Ans.: Both P and Q can be reactive, but in different ways: P (with 1 valence electron, like an alkali metal) is a highly reactive metal because it easily loses its single valence electron. Q (with 6 valence electrons) is close to completing an octet and is a reactive non-metal. Generally, within the same period, elements at the extremes of valence electron count (near 1 or near 7) tend to be the most reactive, while those near a complete octet (7) are more reactive than those near a half-filled shell (like 4).

Additional SEE Practice

1SEE Boardqb-q31

In which group do elements with electronic configuration 1s2, 2s2 2p3 lie?

Ans.: These elements have 5 valence electrons, so they lie in group VA.

2SEE Boardqb-q32

What are the elements between groups IIIB and IIB in the modern periodic table called?

Ans.: They are called transition metals.

3SEE Boardqb-q33

Arrange Be, Mg, and Ca in increasing order of electropositivity.

Ans.: Be < Mg < Ca, because atomic size (and hence electropositivity) increases down group IIA.

4SEE Boardqb-q34

Which of Na < Li < K represents increasing chemical reactivity of metals — is this order correct?

Ans.: No, this order is incorrect. The correct increasing order of reactivity is Li < Na < K, since reactivity of metals increases down group IA as atomic size increases.

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