The Periodic Table

Question 1

What are elements. Give a reason why elements need to be properly classified.

Answer

An element is the basic unit of matter. They are pure substances made up of one kind of atoms and cannot be broken down into simpler substances.

Elements need to be properly classified and the best classification is the one which puts together those elements which show maximum resemblance and separates the others.

Question 2

How did the early chemists classify elements. Why was this basis of classification discarded for future classification of elements.

Answer

Early chemists attempted to classify elements on the basis of their properties, metallic and non-metallic nature, character, valencies, etc.

The above method of classification was discarded since the elements behave both as metals and non-metals, show metallic and non-metallic nature and have variable valency.

Question 3

What was Dobereiner's basis of classification of elements. State Dobereiner's Law of Triads with suitable examples.

Answer

Dobereiner arranged elements in increasing order of atomic weights - in groups of three called 'triads'.

Dobereiner's Law of Triads — Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called 'triads' in which the atomic weight of the middle element was found to be generally the average [arithmetic mean] of the atomic weights of the other two elements.

Examples :

Atomic weight of sodium is the average (arithmetic mean) of Lithium and Potassium.

Question 4

Explain why Dobereiner's method of classification of elements did not hold much weightage for future classification.

Answer

Dobereiner's method of classification of elements did not hold much weightage for future classification because —

1. It was a disordered collection of elements not holding true for all elements.
2. Properties of elements grouped into triads were found not to be similar.

Question 5

What was the basis of classification proposed by Newland. State Newland's Law of Octaves with a suitable example.

Answer

Newland arranged elements in increasing order of atomic weights in series of eight.

Newland's Law of Octaves — Elements when arranged in increasing order of their atomic weights show resemblance in physical and chemical properties between the eighth element and the first element [similar to the eighth and first notes on a musical scale]

For example:

The eighth element from Lithium is Sodium and from Fluorine is Chlorine which show similarity in properties.

Question 6

In which way was Newland's Law of Octaves comparable to a musical note.

Answer

Elements when arranged in increasing order of their atomic weights show resemblance in physical and chemical properties between the eighth element and the first element similar to the eighth and first notes on a musical scale.

Question 7

How did Mendeleeff arrange the elements in the periodic table. What was the basis of his classification.

Answer

Mendeleeff arranged elements in increasing order of their atomic weights in the form of a table called 'periodic table'.
This arrangement had elements with similar properties appearing at regular intervals.
This phenomenon is called 'periodicity of properties' and the properties which repeat themselves at regular intervals are called 'periodic properties'.

Question 8

State Mendeleeff's Periodic Law. How did Mendeleeff's arrangement of elements correlate with periodicity of properties of elements.

Answer

Mendeleeff's Periodic Law — The physical and chemical properties of elements are periodic functions of their atomic weights.

Mendeleeff's arrangement had elements with similar properties appearing at regular intervals. This phenomenon is called 'periodicity of properties' and the properties which repeat themselves at regular intervals are called 'periodic properties'. Some examples of periodic properties are atomic radius, ionisation potential, electron affinity, etc.

Question 9

State the contributions made by Mendeleeff towards the periodic table.

Answer

The contributions of Mendeleeff towards the periodic table are:

1. Elements were arranged in increasing order of atomic weights in horizontal rows called 'periods' and vertical columns called 'groups'.
2. Elements which are similar with respect to their chemical properties are grouped together and have atomic weights of nearly the same value.
3. Elements in the same group had the same 'valency' and similar chemical properties.
4. Based on the periodicity of properties a number of gaps were left in the table for undiscovered elements i.e., elements now discovered e.g., Scandium, Gallium and Germanium originally called eka-boron,eka-aluminium and eka-silicon respectively.
5. The properties of undiscovered elements left in the vacant gaps was predicted.
6. Incorrect atomic weights of some of the arranged elements were corrected with the knowledge of the atomic weights of the adjacent elements.

Question 10

What were the defects and anomalies in Mendeleeff's Periodic Table and how are they resolved by Moseley.

Answer

Defects in Mendeleeff's Periodic Table :

1. Anomalous pairs of elements — Certain pairs of elements having higher atomic weights have been given positions before the elements having lower atomic weights.
   This defect disappears if elements were arranged according to their atomic numbers.
   e.g., Co [at. wt. 58.9, at. no. 27] was placed before Ni [at. wt. 58.6, at. no. 28].
2. Position of rare earths and actinides — Could be justified only if arranged to their atomic numbers
3. Position of isotopes — Isotopes had to be placed in same position according to atomic numbers

Most of the defects in Mendeleeff's Periodic Table disappear if the basis of classification of elements is changed from atomic weights to atomic numbers.
Moseley arranged the elements in increasing order of atomic number in Seven horizontal rows called 'periods' and Eighteen vertical columns called 'groups'.
This forms the basis of the Modern Periodic Table.

Question 11

How were elements arranged in the long form of the periodic table or the Modern Periodic Table. State the Modern Periodic Law and compare it with Mendeleeff's Periodic Law.

Answer

In the Modern Periodic Table elements are arranged in increasing order of atomic number.

Modern Periodic Law states that the physical and chemical properties of elements are periodic function of their 'atomic numbers'.

Mendeleeff's Periodic Law states that the physical and chemical properties of elements are periodic function of their 'atomic weights'.

The basis of classification in Mendeleeff's Periodic Law was atomic weight of the element whereas in the Modern Periodic Law it is the atomic number of elements. This change in the Modern Periodic Law solves most of the defects in Mendeleeff's Periodic table.

Question 12

What are periods in a periodic table. What is meant by 'period number'. What does it signify.

Answer

Periods in the periodic table are the seven horizontal rows of elements arranged in increasing order of their atomic numbers.

Period number is the number assigned to each period i.e., 1, 2, 3, 4, 5, 6 or 7. It signifies the number of shells present in the atom of the element belonging to that period.

Question 13

Name the period which is the shortest period and state the number of elements present in it. State the number of elements in the second and the third periods of the periodic table.

Answer

Period 1 is the shortest period. It has 2 elements [H and He]

Second period has 8 elements.

Third period also has 8 elements.

Question 14(a)

Name the elements in the correct order of their increasing atomic numbers present in the first, second and third short periods of the periodic table.
State which of the elements are – (a) metallic (b) non-metallic (c) noble gases in each of the periods 2 and 3.

Answer

Period 1 — ¹H, ²He

Period 2 — ³Li, ⁴Be, ⁵B, ⁶C, ⁷N, ⁸O, ⁹F, ¹⁰Ne

Period 3 — ¹¹Na, ¹²Mg, ¹³Al, ¹⁴Si, ¹⁵P, ¹⁶S, ¹⁷Cl, ¹⁸Ar

The metallic, non-metallic and noble gas elements of periods 2 and 3 are listed below:

Metallic — Li, Be, Na, Mg, Al

Non-metallic — B, C, N, O, F, P, S, Cl

Noble gases — Ne, Ar

Question 14(b)

State the property trends in general of elements on moving from left to right in a period of the periodic table.

Answer

Property trends of elements from left to right in a period

(a) Number of electron shell — remains the same.

(b) Valence electrons — increases by one.

(c) Transition from metallic to non-metallic character.

(d) Electronegativity increases.

Question 15

What are groups in a periodic table. State the property trends in general of elements on moving down in a group of the periodic table. State the characteristics which remain similar on moving down in a group of the periodic table.

Answer

Eighteen vertical columns in the periodic table are called groups.

Trends on moving down in a group of the periodic table :

• Number of shells — increases by one
• Metallic character increases
• Electronegativity decrease down a group
• Valence electrons — remains same.
• Chemical properties — remain same [or vary gradually]

Question 16

Write short notes on the following types of elements –

(a) alkali metals

(b) alkaline earth metals

(c) halogens

(d) noble gases

(e) transition and inner transition elements

(f) normal elements

Answer

(a) Alkali metals —

1. The elements of group 1 [IA] from ³Li [Lithium] to ⁸⁷Fr [Francium] are called alkali metals as they react with water forming strong alkalis.
2. They are light metals and are univalent with one valence electron in outer shell. This makes them strong reducing agents as they readily donate the one electron to achieve stable electronic configuration.
3. They are highly reactive electropositive metals, which are soft and can be cut with a knife.
4. They are good conductors of heat & electricity.
5. They form electrovalent compounds with non-metals. e.g. KCl, NaBr.

(b) Alkaline earth metals —

1. The elements of group 2 [IIA] from ⁴Be [Beryllium] to ⁸⁸Ra [Radium] are called Alkaline earth metals.
2. They are light metals and are divalent with two valence electrons in their outer shell. They are reducing agents as they can donate the two electrons to achieve stable electronic configuration.
3. They have properties similar to alkali metals but are slightly less reactive than alkali metals.
4. They are good conductors of heat & electricity.
5. They form electrovalent compounds with non-metals. e.g. CaCl₂, MgO.

(c) Halogens —

1. The elements of group 17 [VIIA] from ⁹F [Fluorine] to ⁵³I [Iodine] are called Halogens. They are called Halogens as Halogens means salt producer and these elements react with metals to form salts.
2. They are non-metals and are univalent with seven valence electrons in their outer shell. This makes them strong oxidising agents as they readily accept one electron to achieve stable electronic configuration.
3. They are highly reactive, electronegative non-metals.
4. They are bad conductors of heat & electricity.
5. They form covalent compounds with non-metals. e.g. HCl, PCl₃.

(d) Noble gases —

1. The elements of group 18 [0] from ⁹He [Helium] to ⁵³Rn [Radon] are called Noble gases.
2. They are non-metals and have eight valence elections in outer shell [except Helium (He) which has only two]. They have stable electronic configuration hence their valency or combining capacity is Zero.
3. Noble gases do not react with other elements & are therefore inert or unreactive. They are all monatomic gases.

(e) Transition and inner transition elements —

Transition elements —

1. The elements of group 3 to 12 [IB to VIIB & VIII] are called Transition elements.
2. They are all metals.
3. They lie at the centre of the periodic table between metals [highly electropositive] on the left and non-metals [highly electronegative] on the right.

Inner transition elements —

1. Lathanide [Rare earths] and Actinides series are called inner transition elements.
2. They lie at the bottom of the periodic table in two horizontal rows [14 elements each].
3. Lanthanide series : from Cerium ⁵⁸Ce to Lutetium ⁷¹Lu : [Period - 6]
4. Actinide series : from Thorium ⁹⁰Th to Lawrencium ¹⁰³Lr: [Period - 7]
5. They are all metals and chemically very reactive [actinides are radioactive elements].

(f) Normal elements —

1. Elements of groups 1, 2, 13, 14, 15, 16, 17 are called normal elements.
2. Atoms of these elements have all inner shells complete except outermost shell.
3. They consist of metals, metalloids and non-metals.
4. The number of electrons in their outermost shell signifies the group to which they belong.

Question 17

'The periodic table contains elements methodically grouped together'. State the main helpful features of the long form of the periodic table.

Answer

The main helpful features of the long form of the periodic table are —

1. Till date about 118 elements have been discovered and the study of the properties of each element is a difficult and tedious process. The periodic table containing elements methodically grouped makes the study simpler and systematic.
2. The periodic table makes arrangement of elements easy and simple to reproduce as it separates the elements with respect to their nature.
3. Elements in a group have similar or graded properties. Thus, if the properties of one member of a group is known the properties of the other members of the group could be correlated.
4. Periodic properties that reappear at regular intervals are seen across periods.

Question 18

Name or state the following with reference to the elements of the first three periods of the periodic table.

(a) The noble gas having duplet arrangement of electrons.

(b) The noble gas having an electronic configuration 2, 8, 8.

(c) A metalloid in period 2 and in period 3.

(d) The number of electron shells in elements of period 1, period 2 and period 3.

(e) The valency of elements in group 1 [IA].

(f) The group whose elements have zero valency.

(g) An alkaline earth metal in period 3.

(h) The non-metallic element present in period 3 other than sulphur and chlorine.

(i) A non-metal in period 2 having electronic configuration 2, 6.

(j) An electrovalent compound formed between an alkali metal and a halogen.

(k) A covalent compound formed between an element in period 1 and a halogen.

(l) An alkali metal in period 3 which dissolves in water giving a strong alkali.

(m) A metal in period 3 having valency 3.

(n) The bridge elements of period 3 of group 1 [IA], 2 [IIA] and 13 [IIIA].

(o) The periods which contain the inner transition elements.

(p) The formula of the hydroxide of the element having electronic configuration 2, 8, 2.

(q) The valency of the element in period 3 having atomic number 17.

(r) A non-metal in period 2 which is tetravalent.

Answer

(a) The noble gas having duplet arrangement of electrons — Helium

(b) The noble gas having an electronic configuration 2, 8, 8 — Argon

(c) A metalloid in period 2 and in period 3 — Metalloid in period 2 is B (Boron) and in period 3 is Si (Silicon)

(d) The number of electron shells in elements of period 1, period 2 and period 3 — Period 1 —1 shell, Period 2 — 2 shells, Period 3 — 3 shells

(e) The valency of elements in group 1 [IA] — +1

(f) The group whose elements have zero valency — Group 18 or 0 group

(g) An alkaline earth metal in period 3 — Magnesium [Mg]

(h) The non-metallic element present in period 3 other than sulphur and chlorine — Phosphorus [P]

(i) A non-metal in period 2 having electronic configuration 2, 6 — Oxygen [O]

(j) An electrovalent compound formed between an alkali metal and a halogen — Sodium chloride [NaCl]

(k) A covalent compound formed between an element in period 1 and a halogen — Hydrogen chloride [HCl]

(l) An alkali metal in period 3 which dissolves in water giving a strong alkali — Alkali metal is Na and it dissolves in water to give strong alkali - NaOH

(m) A metal in period 3 having valency 3 — Aluminium [Al]

(n) The bridge elements of period 3 of group 1 [IA], 2 [IIA] and 13 [IIIA] — Li & Mg, Be & Al

(o) The periods which contain the inner transition elements — Period 6 and 7

(p) The formula of the hydroxide of the element having electronic configuration 2, 8, 2 —
Since valency of element X is 2+ and valency of OH is 1-

Formula of the compound:

$\text{X}^{2+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{OH}} \\[0.5em]$

So, we get the formula as $\bold{X(OH)}\bold{_2}$

As the element with electronic configuration 2,8,2 is Mg, Hence the compound is Mg(OH)₂

(q) The valency of the element in period 3 having atomic number 17 — -1

(r) A non-metal in period 2 which is tetravalent — Carbon [C]

Question 1(2000)

State the number of elements in Period 1, Period 2, and Period 3 of the Periodic Table.

Answer

Period 1 — 2 Elements
Period 2 — 8 Elements
Period 3 — 8 Elements

Question 2(2000)

Name the elements in Period 1.

Answer

Hydrogen [H] and Helium [He].

Question 3(2000)

What is the common feature of the electronic configuration of the elements at the end of Period 2 and Period 3 ?

Answer

The number of valence shell electrons in the elements at the end of Period 2 and Period 3 is 8.

• Ne = 2, 8
• Ar = 2, 8, 8

Question 4(2000)

If an element is in Group 17 [or Group 7A] is it likely to be metallic or non-metallic in character ?

Answer

It will be non-metallic because being in Group 17 [or Group 7A], it has 7 valence electrons. Thus, its electronegativity is high and it has a tendency to accept electrons.

Question 5(2000)

Supply the missing word from those in brackets: If an element has one electron in it's outermost energy level [shell] then it is likely to be ............... [metallic / non- metallic].

Answer

If an element has one electron in it's outermost energy level [shell] then it is likely to be metallic.

Question 1(2001)

Copy and complete the following sentences choosing the correct word or words from those given in brackets at the end of each sentence :

The similarities in the properties of a group of elements is because they have the same ............... [electronic configurations, number of outer electrons, atomic numbers.]

Answer

The similarities in the properties of a group of elements is because they have the same number of outer electrons.

Question 1(2002)

What is meant by a Group in the Periodic Table.

Answer

The 18 vertical columns in the periodic table are called groups. The group number signifies the number of valence electrons of an element.

Question 2(2002)

How many elements are there in Period 2.

Answer

8 elements

Question 1

Select the correct answer from the words in bracket.

1. He arranged elements in increasing order of atomic numbers. [Dobereiner/ Moseley / Mendeleeff
2. Is a metal in period 2 having electronic configuration 2, 1. [Beryllium / Lithium / Sodium]
3. Is a period having elements from atomic no. 11 to 18. [period – 1/2/3]
4. The most reactive halogen from group 17. [chlorine/fluorine/bromine]
5. Is the group number of the element whose atomic number is 4. [group - 1/group – 2/group – 18]

Answer

1. Moseley
2. Lithium
3. Period - 3
4. Flourine
5. Group - 2

Question 2

Fill in the blanks from the words A to F given below.

A: Decreases B: Increases C: Remains same D: Increases by one E: Electropositive F: Electronegative

1. Across a period from left to right in the Modern Periodic Table.
   No. of electron shells ............... ; No. of valence electrons ...............; Electronegativity ...............; Character of elements changes from ............... to ...............
2. Down a group in the Modern Periodic Table.
   No. of electron shells ...............; No. of valence electrons ...............; Electronegativity ...............; Character of elements changes from ............... to ...............

Answer

1. Across a period from left to right in the Modern Periodic Table.
   No. of electron shells remains same; No. of valence electrons increases by one; Electronegativity increases; Character of elements changes from electropositive to electronegative.
2. Down a group in the Modern Periodic Table.
   No. of electron shells increases by one; No. of valence electrons remains same; Electronegativity decreases; Character of elements changes from electronegative to electropositive.

Question 3.1

Give reasons for the following.

Mendeleeff's contributions to the periodic table, laid the foundation for the Modern Periodic Table.

Answer

Mendeleeff's contributions to the periodic table, laid the foundation for the Modern Periodic Table because the best classification is the one which puts together those elements which show maximum resemblance and separates the others and Mendeleeff arranged the elements in increasing order of their atomic weights, in horizontal rows called 'periods' and in vertical columns called 'groups'; such that :

1. the properties repeat at regular interval
2. elements which are similar with respect to their chemical properties are grouped together and have atomic weights of nearly the same value.
3. elements in the same group had same 'valency' and similar chemical properties.
4. the properties of undiscovered elements left in the vacant gaps was predicted.
5. incorrect atomic weights of some of the arranged elements were corrected with the knowledge of the atomic weights of the adjacent elements.

Question 3.2

Give reasons for the following.

Properties of elements are periodic functions of their atomic numbers and not atomic weights.

Answer

The properties of elements are dependent on the electronic configuration of their atoms. As similar electronic configurations repeat after regular intervals of atomic numbers hence properties of elements are periodic functions of their atomic numbers and not atomic weights.
This was seen when Mendeleeff arranged the elements on the basis of their atomic weights. Certain defects were present in Mendeleeff's periodic table which got removed when elements were arranged as per their atomic numbers.

Question 3.3

Give reasons for the following.

A transition from metallic to non-metallic elements is seen on moving from left to right in a period of the periodic table.

Answer

On moving across a period, the tendency to gain electron(s) increases due to an increase in nuclear pull and a decrease in the atomic size. Therefore, non-metallic character increases across a period and there is a transition from metallic to non-metallic elements.

Question 3.4

Give reasons for the following.

Noble gases do not form compounds readily.

Answer

Noble gases have their outermost or valence shell complete i.e., they have 2 electrons in outermost shell e.g., helium [electronic configuration 2] or they have 8 electrons in outermost shell e.g., neon [electronic configuration 2,8] or argon [electronic configuration 2,8,8 ]. So, they have stable electronic configuration. They do not gain, lose or share electrons and are unreactive or inert. Hence, they do not form compounds readily.

Question 3.5

Give reasons for the following.

Group 1 [IA] elements are called alkali metals.

Answer

Elements of Group 1 [IA] are metals and react with water to form alkalis, hence are called alkali metals.

Question 4

State the following.

1. The group to which the element with electronic configuration of 2, 8, 2 belongs.
2. The group from the groups 1[IA], 2[IIA], 16[VIA] and 17[VIIA] whose elements are most electronegative.
3. The group which contains highly electropositive metals including sodium.
4. The group whose elements are unreactive or inert.
5. The group which contains highly reactive electronegative non-metals including chlorine.

Answer

1. Group 2 [ or Group II-A]
2. Group 17 [ or Group VII-A]
3. Group 1 [ or Group I-A]
4. Group 18 [ or Group 0]
5. Group 17 [ or Group VII-A]

Question 5

Match the elements of List-I with their type from List-II.

Answer

Question 6

Complete the table pertaining to the following elements given in Column 1.

Answer