Class 10 Science Notes: Chapter 14 Classification of Elements

Chapter 14 Classification of Elements

Class 10 Science Notes

Exercise

1. Choose the correct option for the following questions:

(a) In which group do the elements having electronic configuration $1s^{2}$, $2s^{2}$ $2p^{3}$ lie?

(i) III A

(ii) III B

(iii) V A

(iv) VB

Reason: The period of an element is determined by its highest electron shell, which is n=2 (from $2s^2$ $2p^3$). The group for main-group elements is determined by the total number of valence electrons (electrons in the outermost shell). Here, the valence electrons are 2 (from 2s) + 3 (from 2p) = 5. Elements with 5 valence electrons belong to Group V A (also known as Group 15).

(b) What are the elements between groups IIA and IIIA called in the modern periodic table?

(i) Alkali metals

(ii) Transition metals

(iii) Alkaline earth metals

(iv) Rare Earth metal

Reason: The elements located in the d-block of the periodic table, which are positioned between the s-block (Groups IA and IIA) and the p-block (Groups IIIA to VIIIA), are known as the transition metals.

(i) Fluorine

(ii) Chlorine

(iii) Bromine

(iv) Iodine

Reason: These elements are all halogens (Group VIIA). The reactivity of non-metals is based on their ability to gain electrons (electronegativity). This property decreases as you go down the group. Fluorine (F) is at the top of the group and is the most electronegative element, making it the most reactive non-metal.

(d) Which one of the given metals are the most reactive metals?

(i) Lithium

(ii) Sodium

(iii) Potassium

(iv) Cesium

Reason: These elements are all alkali metals (Group IA). The reactivity of metals is based on their ability to lose electrons (electropositivity). This property increases as you go down the group because the atomic size increases, and the valence electron is held less tightly. Cesium (Cs) is a C.

(e) Which group does the inert gases belong in the modern periodic table?

(i) 0

(ii) IA

(iii) VIIB

(iv) VIIA

Reason: Inert gases (or noble gases) have a complete, stable outer electron shell (a valency of 0). They do not need to react to gain stability. Because of this unique chemical inactivity, they are placed in their own group, which is called Group 0, Group VIIIA, or Group 18.

(f) Which one represents the correct order of increase in chemical reactivity of metals?

(i) $Be < Mg < Ca$

(ii) $Na < Li < K$

(iii) $Mg < Al < Si$

(iv) $C < O < N$

Reason: Beryllium (Be), Magnesium (Mg), and Calcium (Ca) are all Alkaline Earth Metals (Group IIA). For metals, chemical reactivity increases as you go down a group. Ca is below Mg, which is below Be. Therefore, the correct order of increasing reactivity is $Be < Mg < Ca$. Option (ii) is incorrect because the order for alkali metals should be $Li < Na < K$.

(g) The position of elements A, B, C, and D are shown in the periodic table. Which of them can form acidic oxide?

(i) A

(ii) B

(iii) C

(iv) D

Reason: In the periodic table, non-metals form acidic oxides, while metals form basic oxides. The element ‘B’ is located in the upper-right portion of the table, which is where non-metals are found. Therefore, ‘B’ will form an acidic oxide. ‘A’ and ‘C’ are metals (will form basic oxides), and ‘D’ is a transition metal.

2. Give reason:

(a) Classification of elements is necessary.

Reason: Classification is necessary to make the study of the 118 known elements systematic and organized. Instead of studying the properties of each element individually, classification allows us to group elements with similar physical and chemical properties together. This makes it easier to understand their behavior, predict the properties of undiscovered elements, and see the trends in the periodic table.

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

Reason: As you move from top to bottom in a group, a new electron shell (principal energy level) is added for each successive element. This new shell is further from the nucleus. Although the nuclear charge (number of protons) also increases, the effect of adding an entirely new, more distant shell is greater, causing the overall atomic radius (size) to increase.

Reason: As you move from left to right across a period, electrons are being added to the same outermost shell. However, the number of protons in the nucleus also increases. This stronger positive nuclear charge pulls all the electrons (including the valence electrons) more tightly towards the nucleus, resulting in a contraction of the electron cloud and a smaller atomic size.

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

Reason: Hydrogen is placed in Group 1 (IA) with alkali metals primarily because its electronic configuration is $1s^1$, meaning it has only one valence electron, just like all other alkali metals. This allows it to lose one electron to form a $H^+$ ion, similar to how $Na$ forms $Na^+$. However, by its physical properties, it is a non-metal gas. Its unique position is a subject of debate, as it also shares some properties with Group 17 (halogens).

(e) Potassium is more reactive than sodium.

Reason: Both Potassium (K) and Sodium (Na) are in Group IA (alkali metals). Reactivity for metals increases down the group. Potassium is in Period 4, while Sodium is in Period 3. This means K’s valence electron is in a shell further from the nucleus and is held less tightly (lower ionization energy). Because it can lose this electron more easily than Sodium, Potassium is more reactive.

(f) Fluorine is more reactive than chlorine.

Reason: Both Fluorine (F) and Chlorine (Cl) are in Group VIIA (halogens). Reactivity for these non-metals decreases down the group. Fluorine is in Period 2, and Chlorine is in Period 3. Fluorine’s smaller atomic size and high electronegativity mean it has a much stronger attraction for an incoming electron to complete its outer shell. This strong pull makes it more reactive than Chlorine.

(g) The metallic characters of elements decrease and non-metallic characters increase on going from left to right of a period in the modern periodic table.

Reason: Metallic character is the tendency to lose electrons. As you move left to right across a period, the increasing nuclear charge pulls the valence electrons more tightly. This makes it harder for an atom to lose electrons (so metallic character decreases) and easier for it to gain or share electrons (so non-metallic character increases).

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

Reason: Inert gases (like Neon and Argon) have a completely filled outermost electron shell (a stable octet, or duplet for Helium). This stable configuration means they have no tendency to lose, gain, or share electrons. Their valency is 0, and they are chemically inactive or “inert.” They are placed in Group 0 (or 18/VIIIA) to signify this unique stability and zero valency.

(i) A portion of the periodic table is shown below. The atomic size decreases from Li to Ne.

Reason: This trend occurs because Li, Be, B, C, N, O, F, and Ne are all in the same period (Period 2). As you move from Li to Ne, the number of protons in the nucleus increases (from 3 to 10), but the electrons are all being added to the same second shell. The increasing positive charge of the nucleus pulls this second shell in more tightly, causing the atomic size to shrink across the period.

3. Write the differences between:

(a) Group and Period

Feature	Group	Period
Definition	A vertical column in the periodic table.	A horizontal row in the periodic table.
Total Number	There are 18 groups.	There are 7 periods.
Electron Shells	Elements in a group have different numbers of electron shells.	Elements in the same period have the same number of electron shells.
Valence Electrons	Elements in a (main) group have the same number of valence electrons.	The number of valence electrons increases from left to right across a period.
Chemical Properties	Elements in a group share similar chemical properties.	Elements in a period show a gradual change in chemical properties.

(b) Chlorine and Sodium

Feature	Chlorine (Cl)	Sodium (Na)
Element Type	Non-metal	Alkali Metal
Position	Group 17 (VIIA), Period 3	Group 1 (IA), Period 3
Valence Electrons	7 valence electrons.	1 valence electron.
Reactivity	Highly reactive; gains 1 electron to form an anion ($Cl^-$).	Highly reactive; loses 1 electron to form a cation ($Na^+$).
Physical State	A yellowish-green gas at room temperature.	A soft, silvery-white solid at room temperature.
Oxide Nature	Forms acidic oxides (e.g., $Cl_2O_7$).	Forms a strongly basic oxide ($Na_2O$).

Feature	Group IA (Alkali Metals)	Group VIIA (Halogens)
Element Type	Highly reactive metals (except H).	Highly reactive non-metals.
Valence Electrons	1 valence electron.	7 valence electrons.
Ion Formation	Form positive ions (cations) with a +1 charge by losing 1 electron.	Form negative ions (anions) with a -1 charge by gaining 1 electron.
Reactivity Trend	Reactivity increases down the group.	Reactivity decreases down the group.
Examples	Lithium (Li), Sodium (Na)	Fluorine (F), Chlorine (Cl)

(d) Electronegativity and Electropositivity

Feature	Electronegativity	Electropositivity
Definition	The tendency of an atom to attract a shared pair of electrons.	The tendency of an atom to lose electrons and form a positive ion.
Common In	Non-metals.	Metals (also called metallic character).
Trend (Period)	Increases from left to right.	Decreases from left to right.
Trend (Group)	Decreases from top to bottom.	Increases from top to bottom.
Extreme Element	Fluorine (F) is the most electronegative.	Cesium (Cs) is one of the most electropositive.

4. Answer the following questions:

(a) State modern periodic law.

The modern periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic numbers.

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

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

Position: Sodium (Na) is in Group 1 (IA) and Period 3. It is called an alkali metal because it reacts vigorously with water to form a strong base (a soluble base is called an alkali), such as Sodium Hydroxide ($NaOH$), and hydrogen gas.

(d) Write the position of following elements in the periodic table: alkali metals, alkaline earth metals, inert gases, transition metals, lanthanides, and actinides.

Alkali metals: Group 1 (IA)
Alkaline earth metals: Group 2 (IIA)
Inert gases: Group 18 (VIIIA or 0)
Transition metals: Groups 3 through 12 (the central d-block)
Lanthanides: Part of Period 6, placed separately at the bottom (the first row of the f-block).
Actinides: Part of Period 7, placed separately at the bottom, below the lanthanides (the second row of the f-block).

(e) Write the electronic configuration of sulphur and write its position in the modern periodic table.

Sulphur (S) has atomic number 16.
Electronic Configuration: $1s^2$ $2s^2$ $2p^6$ $3s^2$ $3p^4$
Position: Period: The highest shell number is 3, so it is in Period 3. Group: It has 2 + 4 = 6 valence electrons, so it is in Group 16 (VIA).

(f) On which basis do the group and period are separated in the modern periodic table?

Periods are determined by the number of principal electron shells an element’s atoms possess. For example, all elements in Period 3 have 3 electron shells.
Groups are determined by the number of valence electrons (electrons in the outermost shell). For example, all elements in Group 1 have 1 valence electron, and all elements in Group 17 have 7 valence electrons.

(g) In which group does Fluorine, Chlorine, and Bromine lie in the periodic table? Which one is more reactive among them?

Group: They all lie in Group 17 (VIIA), also known as the halogens.
Most Reactive: Fluorine (F) is the most reactive among them.

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

Improvement: The main improvement would be to clarify the position of Hydrogen (H).
Reason: Currently, Hydrogen is placed in Group 1 because it has one valence electron ($1s^1$). However, its properties are completely different from the alkali metals; it is a non-metal gas. It also shares properties with Group 17 (it can gain one electron to form $H^-$) and Group 14 (it is half-filled and forms covalent bonds). Placing it in Group 1 is confusing. A better solution might be to place Hydrogen in its own, separate box at the top-center of the table to show that it is a unique element and does not truly belong to any single group.

Additional Exercises

Multiple Choice Questions:

1. Which of the following is correct about the modern periodic law?

(i) It is based on atomic size.

(ii) It is based on atomic number.

(iii) It is based on valence electron.

(iv) It is based on atomic mass.

Reason: The modern periodic law states that the properties of elements are a periodic function of their atomic numbers, not atomic masses as in Mendeleev’s periodic law.

2. Which of the following do [text missing in original] belong to?

(i) S-block elements

(ii) P-block elements

(iii) D-block elements

(iv) F-block elements

Reason: The question seems incomplete, but s-block elements include Groups 1 and 2 (alkali and alkaline earth metals).

3. Which of the following is a highly reactive element?

(i) Beryllium

(ii) Magnesium

(iii) Sodium

(iv) Aluminium

Reason: Sodium is an alkali metal (Group 1) and is highly reactive, while Beryllium and Magnesium are alkaline earth metals (Group 2) and less reactive, and Aluminium is even less reactive.

4. Which one of the given elements is the least reactive?

(i) Fluorine

(ii) Chlorine

(iii) Iodine

(iv) Bromine

Reason: Among halogens, reactivity decreases down the group. Iodine is at the bottom of the group (excluding astatine) and is the least reactive among these options.

5. Which are called the d-block elements?

(i) Halogens

(ii) Representative elements

(iii) Transition elements

(iv) Alkaline earth metals

Reason: The d-block elements are also known as transition elements, located in groups 3-12 of the periodic table.

6. How many groups are there in the modern periodic table?

(i) 7

(ii) 8

(iii) 14

(iv) 18

Reason: The modern periodic table has 18 groups numbered from 1 to 18.

7. Which of the following is the orbital configuration of chlorine?

(i) $1s^{2}$ $2s^{2},$ $2p^{6}$ $3s^{2}$ $3p$

(ii) $1s^{2}$ $2s^{2}$, $2p^{6}.$ $3s^{2}.$, $3p$

(iii) $1s^{2}$, $2s^{2}$, $2p^{6}.$ $3s^{2}$, 3

(iv) $1s^{2}$, $2s^{2}$, $2p^{6},$ $3s^{2}$, $3p^{23}$

Reason: Chlorine has atomic number 17. Its correct electronic configuration is $1s^{2}$ $2s^{2}$ $2p^{6}$ $3s^{2}$ $3p^{5}$. The options appear to have OCR errors, but option (ii) is closest to the correct configuration.

8. Which of the following do inert elements belong to?

(i) group 4

(ii) group 6

(iii) Group 18

(iv) Group 8

Reason: Inert gases (noble gases) belong to Group 18 of the periodic table.

Short Answer Questions:

10. What is the periodic table?

The periodic table is a systematic arrangement of all known chemical elements organized by their atomic number, electron configuration, and recurring chemical properties.

11. Introduce Mendeleev’s periodic law.

Mendeleev’s periodic law states that the properties of elements are a periodic function of their atomic masses. He arranged elements in order of increasing atomic mass and observed that elements with similar properties recur at regular intervals.

12. Introduce modern periodic table.

The modern periodic table is based on the modern periodic law which states that the properties of elements are a periodic function of their atomic numbers. It contains 18 groups and 7 periods, with elements arranged in order of increasing atomic number.

13. Name two alkaline earth metals.

Two alkaline earth metals are Beryllium (Be) and Magnesium (Mg).

14. Define group and period.

A group is a vertical column in the periodic table, while a period is a horizontal row in the periodic table.

15. What are isotopes?

Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to different numbers of neutrons in their nuclei.

16. What factors do the valencies of elements depend upon?

The valency of an element depends on: the number of valence electrons, the desire to achieve a stable electronic configuration (octet rule), and the element’s position in the periodic table.

17. Which important property did Mendeleev use to classify elements in his periodic table?

Mendeleev used atomic mass as the important property to classify elements in his periodic table.

19. In which group, period and block will the element with atomic number 20 be present?

The element with atomic number 20 is Calcium (Ca). It is in Group 2, Period 4, and the s-block.

20. Write the atomic number of the element present in the third period and seventeenth group of the periodic table.

The element in the third period and seventeenth group is Chlorine, with atomic number 17.

22. Use the periodic table to answer the following questions.

(i) Oxygen has six electrons in its outer subshell.
(ii) Sodium would tend to lose one electron.
(iii) Oxygen would tend to gain two electrons.

23. The increasing order of reactivity among group 1 elements is Li < Na < K. Explain.

The reactivity increases from lithium to potassium in Group 1 because atomic size increases down the group. Larger atoms have their valence electrons farther from the nucleus and more shielded, making them easier to remove. Thus, potassium loses its valence electron most easily and is the most reactive.

24. Write the general outer electronic configuration of s, p, d- and f- block elements.

s-block: $ns^{1-2}$
p-block: $ns^{2}np^{1-6}$
d-block: $(n-1)d^{1-10} ns^{0-2}$
f-block: $(n-2)f^{1-14}(n-1)d^{0-10}ns^{2}$

25. What are the merits of Mendeleev’s periodic table?

Merits include: predicting undiscovered elements, systematic classification of elements, identifying periodic trends in properties, and leaving gaps for elements yet to be discovered.

26. What are the features of the modern periodic table?

Features include: arrangement by atomic number, organization into periods and groups, classification into s, p, d, and f blocks, clear representation of electronic configurations, and systematic trends in properties.

27. How is the modern periodic table superior to Mendeleev’s table?

The modern periodic table is superior because it is based on atomic number (which resolves anomalies in Mendeleev’s table based on atomic mass), has a fixed position for each element, better explains periodicity of properties, and incorporates the concept of electronic configuration.

28. Why are Li, Na and K placed in the same group of the periodic table?

Li, Na, and K are placed in the same group (Group 1) because they all have one valence electron in their outermost shell, which gives them similar chemical properties and reactivity patterns.

29. What do you mean by p-block elements? Write any five examples of this block.

p-block elements are those in which the last electron enters the p-orbital of the outermost shell. Examples: Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F).

30. Name:

(i) Three elements that have a single electron in their valence shell: Hydrogen (H), Lithium (Li), Sodium (Na)
(ii) Two elements that have seven electrons in their valence shell: Fluorine (F), Chlorine (Cl)
(iii) Three elements with filled valence shell: Helium (He), Neon (Ne), Argon (Ar)

31. Argon is an inert gas because…

Argon is an inert gas because its electron arrangement in the outermost shell is stable (octet configuration) and doesn’t easily react with other elements.

32. The valency of nitrogen is 3 because…

The valency of nitrogen is 3 because it needs to gain three electrons to complete its outermost energy level and achieve a stable octet configuration.

33. Chlorine is an active non-metal because…

Chlorine is an active non-metal because it easily gains one electron to complete its outermost energy level and achieve a stable octet configuration.

34. Fluorine is more reactive than chlorine because…

Fluorine is more reactive than chlorine because it has a smaller atomic size and higher electronegativity, giving it a stronger tendency to gain electrons.

35. Sodium is less reactive than potassium because…

Sodium is less reactive than potassium because sodium has a smaller atomic size, making its valence electron more strongly attracted to the nucleus and harder to remove.

36. The members of group 1A are called alkali metals because…

The members of Group 1A are called alkali metals because when they react with water, they form strong alkaline solutions (hydroxides).

37. Group I elements are very active because…

Group I elements are very active because they have only one valence electron which they can easily lose to achieve stable configuration.

38. Lanthanides and actinides are kept separately in the modern periodic table because…

Lanthanides and actinides are kept separately to maintain the proper structure and periodicity of the table, as placing all these elements in their proper positions would make the table too wide and difficult to use.

39. Transition elements have variable valency because…

Transition elements have variable valency because they can use electrons from both their outermost shell and the penultimate shell in bond formation.

40. Halogens are active non-metals because…

Halogens are active non-metals because they readily gain one electron to achieve a stable noble gas configuration in their outermost energy level.

41. The elements in the same group have similar physical and chemical properties because…

The elements in the same group have similar physical and chemical properties because they have the same number of valence electrons, which primarily determine an element’s chemical behavior.