The Periodic Table

Reasons for classification of elements are:

1. It helps in studying the elements in an organized manner.
2. It helps in correlating the properties of elements with the fundamental properties of all states of matter.
3. It helps in defining the relationship of one element with another.

Earliest attempts made for classification and grouping of elements were based on factors such as:

• Density
• Malleability
• Ductility
• Metal or Nonmetal

(a) When the atomic mass of A = 7, and that of C is = 39 then according to Dobereiner's triad, atomic mass of B should be mean value of atomic masses of A and C.

$\frac{7 + 39}{2}$ = $\frac{46}{2}$ = 23

Hence, atomic mass of B = 23

(b) Reasons for discarding Dobereiner's triads:

1. Dobereiner failed to arrange all the then known elements in the form of triads.
2. The law did not fully apply even within the same family.

According to Newland's law of octaves, when elements are arranged by increasing atomic mass, the properties of every eighth element starting from any element are a repetition of the properties of the starting element.

Reasons for discarding the Newland's law of Octaves:

1. This classification did not work with heavier elements i.e., those lying beyond Calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves.
2. Newland adjusted two elements Cobalt (Co) and Nickel (Ni) in the same slot, and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
3. Iron, which resembles cobalt and nickel in properties has been placed far away from these elements.

Yes, Dobereiner's triads also exist in the columns of Newland's octaves.

Example: Consider the elements lithium (Li), sodium (Na), and potassium (K) present in second column of Newland's classification of elements. If we start with lithium as the 1st element, then the 8th element from it is sodium, and according to Newlands' law of octaves, the properties of the 8th element, sodium should be similar to those of the 1st element, lithium.

Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newland's law of octaves, the properties of the 8th element, potassium should be similar to those of the 1st element, sodium.

This means that according to Newland's law of octaves, the elements lithium, sodium, and potassium should have similar chemical properties. We also know that lithium, sodium, and potassium form a Dobereiner's triad having similar chemical properties. From this, we conclude that Dobereiner's triads also exist in the columns of Newland's Octaves.

Similar properties of lithium, sodium and potassium were:

1. They have one electron in the outermost shell, so they form unipositive ions.
2. All these are soft metals.
3. They are good reducing agents.
4. These metals impart colour to the flame.

Similar properties of calcium, strontium and barium were:

1. All are metals.
2. Their oxides are alkaline in nature.
3. Each has valency 2.

(a) Mendeleev classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.

(b) As many features of the Mendeleev's periodic table were later used when Modern Periodic Table was made so we can say that Mendeleev's contributions to the concept of a periodic table laid the foundation for the Modern Periodic Table. For example:

1. Mendeleev's periodic table contains vertical column called groups and horizontal rows called periods.
2. They were all in eight groups.
3. All elements in a group had similar properties and same valency.
4. In a period elements gradually changed from metallic to non-metallic character.
5. Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
6. He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
7. He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Mendeleev's periodic law states that the physical and chemical properties of elements are a periodic function of their atomic masses.

(a) Carbon is in group 4. Hydride of any element of group 4 is written as RH₄. Hence, the hydride of carbon will have the formula CH₄.

Silicon is also in Group 4. So, the hydride will be SiH₄.

(b) K is in Group 1 and has the general oxide formula R₂O. So, the oxide will be K₂O.

Al is in Group 3 and has the general oxide formula R₂O₃. So, the oxide will be Al₂O₃.

Ba is in Group 2 and has the general oxide formula RO. So, the oxide will be BaO.

Group zero elements or inert gases were missing from Mendeleev's periodic table as they were not discovered at that time.

Merits of Mendeleev’s classification of elements:

1. Grouping of elements: He generalized the study of the elements then known to a study of mere eight groups.
2. Gaps of undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
3. Prediction of properties of undiscovered elements: He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
4. Incorrect atomic mass corrected: He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time.

For example,

(i) Eka-aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later in 1876.

(ii) Properties of eka-silicon are the properties of germanium.

Through an experiment, Henry Moseley found that number of positive charge present in the nucleus of an atom or atomic number is the most fundamental property of an atom.

According to him, if elements are arranged in order of their increasing atomic number, those with similar properties are repeated after regular intervals, i.e., periodicity in the periodic table occurs based on the atomic numbers (number of protons). Hence, the atomic number is more important to a chemist than its relative atomic mass.

(a) Same group

1. Li, Na, K belong to the same group IA.
2. Be, Ca belong to the same group IIA.

(b) Same period

1. Li and Be belong to Second period.
2. K, Ca belong to Fourth period.
3. Na belongs to Third period.

(a) Gallium (Eka aluminium)

(b) Gold and Platinum

(c) 63

(a) The Modern Periodic Law is stated as, 'the physical and chemical properties of elements are the periodic functions of their atomic number.'

(b) Groups - 18 and Periods - 07

Main characteristics of the last element in each period of the Periodic Table are:

1. They have valency equal to zero.
2. They react under very special condition otherwise they don't react.
3. They have stable gas electronic configuration.

General name of such elements is Inert Gases.

(a) Eighteen vertical columns in the periodic table are called groups.

(b) Seven horizontal rows in the periodic table are called periods.

Number of valence electrons determines which element will be the first and which element will be the last in a period.

(a) The groups are known as:

1. Group 1 — Alkali metals
2. Group 17 — Halogens
3. Group 18 — Noble gases

(b) Two elements of each group are:

1. Group 1 — Lithium, Sodium
2. Group 17 — Fluorine, Chlorine
3. Group 18 — Neon, Argon

(a) 2 elements

(b) 8 elements

(a) In the second period, the number of valence electrons increases from 1 to 8 as we move from left to right. The valency changes as follows:

(b) In the third period, the number of valence electrons increases from 1 to 8 as we move from left to right. The valency changes as follows:

As we move across a period from left to right, the number of shells remains the same and number of valence electrons increases by one. The size of an atom decreases because with increase in number of protons, nuclear pull increases, which decreases the size.

The criteria used by Mendeleev in proposing his periodic table was the atomic masses of elements. He classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.

In Mendeleev's periodic table, Cobalt with atomic mass 58.9 preceded Nickel with atomic mass 58.6. This anomaly was resolved in Modern periodic table by arranging the elements based on their atomic number instead of atomic mass.
Cobalt with atomic number 27 is placed in group 9 and Nickel with atomic number 28 is placed in group 10 in the Modern periodic table.

Since, all isotopes of chlorine i.e., Cl³⁵ and Cl³⁷ have the same number of protons so their atomic number is the same. Hence, they are placed at the same position in the periodic table in group 17, period 3.

The element having three shells [K, L, M] with three electrons in the valence shell is in group 13 [III A] and period 3. The name of the element is Aluminium.

Neon (Ne), It is placed in the zero group and 2nd period.

Atoms of both Helium and Neon have completely filled valence shells which makes them very stable and unreactive.

(a) Greatest metallic character is shown in the elements at the bottom of the group.

(b) Largest atomic size is shown in the elements at the bottom of the group.

Number of valence electrons remains the same as we go down a group.

Example:

Sodium and aluminium both belong to period 3 of the periodic table. They are the first and the third elements of their period belonging to group IA and IIIA, respectively. As they lie towards the left in their period, they exhibit metallic character and being separated by one element exhibit positive valencies of +1 and +3, respectively.
On the other hand, even though chlorine and potassium are also separated by one element, chlorine belongs to period 3 and potassium to period 4 of the periodic table. Chlorine is the rightmost element of period 3 belonging to the Halogens group VIIA. So, it exhibits a negative valency of -1. Potassium is the leftmost element of period 4 belonging to the Alkali metals group IA. So, it exhibits a positive valency of +1.

(a) (i) H and P are noble gases, as they are in group zero and have their octet complete.

(ii) G and O are halogens, as they are in group VIIA and have 7 valence electrons.

(iii) A and I are alkali metals, as they are in group IA and have 1 valence electron.

(iv) D and L have valency 4, as they are in group IVA and have 4 valence electrons.

(b) Valency of A [2, 1] is +1, so it tries to donate an electron and attain a stable state.
Valency of F [2, 6] is -2, so it tries to gain two electrons and attain a stable state. Hence, one F atom takes electron from two A atoms. Therefore, the formula of their compound will be A₂F

(c) Electronic configuration of G is [2, 7] as its atomic number is 9.

(a) Metals — A and B, as they belong to alkali metals group and alkaline earth metals group.

Non-metals — C, as it belongs to halogen group 17.

Noble gases — D and E, as they belong to group 18 or zero group.

(b) A is the most reactive metal as Group 1 (Alkali metals) elements are very reactive and the degree of reactivity further increases down the group. C is the most reactive non-metal as it is in Group 17 (Halogens) and the degree of reactivity of halogens decreases down the group.

(c) Type of ion formed by elements A, B and C will be:

1. Element A will lose the one valence electron and become an electropositive ion [i.e., cation A⁺].
2. Element B will lose two valence electrons and become an electropositive ion [i.e., cation B²⁺].
3. Element C will gain one electron, complete its octet and become an electronegative ion [i.e., anion C^-].

(d) Larger in size

1. E is larger in size as the number of shells increases as we move down a group increasing the size of the atom.
2. B is larger in size as atomic size decreases on moving from left to right across a period due to increase in nuclear charge.

The electronic configuration of $^{35}_{17}\text{T}$ is [2, 8, 7]

(a) Group number of T — VIIA [∵ it has 7 valence electrons]

(b) Period number of T — Third period [∵ it has 3 shells]

(c) Valence electrons — Seven [from electronic configuration]

(d) Valency of T is -1. [∵ it needs to gain one electron to complete its octet]

(e) T is a non-metal [∵ it has a tendency to accept electrons]

(f) Protons = 17, Neutrons = 18

Number of valence electrons

Reason — The arrangement of elements based on their atomic weights in the Modern Periodic Table puts elements with same number of valence electrons in the same group.

32

Reason — The sixth period contains 32 elements (atomic number 55-86). It is the longest period.

5

Reason — Group 15 elements have 5 valence electrons.
Having 3 valence electrons is a property of Group 13 elements. 7 valence electrons belong to Group 17 (halogens). 8 valence electrons indicate a complete outer shell, typical of noble gases (Group 18).

Each element belongs to the same period

Reason — Period 3, known as short period, contains 8 elements i.e., sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorous (P), sulphur (S) and chlorine (Cl).
Each element belongs to different group. Valency of each element is the different. Only Na, Mg, Al are metals. Si is a metalloid. P, S, Cl are non-metals.

Alkaline earth metals

Reason — Group 2 elements, known as alkaline earth metals, have two electrons in their outermost orbit.
Alkali metals have 1 electron in their outermost shell. Halogens have 7 valence electrons. Inert gases have 8 valence electrons (complete shell), except helium (2).

Representative elements

Reason — Representative elements are the s-block and p-block elements. Many of them have incomplete outermost shells, so they tend to gain, lose, or share electrons to complete their octet.

3rd period and VI A group

Reason — The element with atomic number 16 has electronic configuration 2,8,6. It has 3 shells, so it will be in period 3 and 6 valence electrons so it is placed in Group VIA.

One

Reason — As group 1 elements have 1 valence electron, they tend to donate it, attain a stable configuration and get oxidized. Therefore, they are the strongest reducing agents.

Group 6 elements tend to gain electrons, not lose them. Group 7 elements are strong oxidizing agents, not reducing agents. Group 2 elements are weaker reducing agents than alkali metals.

Fluorine

Reason — Reactivity of halogens decreases as we move downwards, hence fluorine is the most reactive non-metal. Reactivity of fluorine is more than oxygen as it has a stronger tendency to take electrons and complete its octet.
Iodine is less reactive because it is larger in size and has weaker attraction for electrons. Chlorine is very reactive, but still less than fluorine.

(I) Mendeleev

(II) N₂O₅

(III) P, Q and R

Reason —
(I) The above table was given by Mendeleev. It was published in a German journal in 1872.

(II) Mendeleev used letter 'R' to represent the element of that particular group. Hence the oxide of nitrogen in the periodic table is R₂O₅. Here 'R' is nitrogen. Hence, oxide of nitrogen is N₂O₅.

(III) Mendeleev's periodic table had anomalous pairs, which did not follow Mendeleev's principles. Major defects in Mendeleev's periodic table were grouping of chemically dissimilar elements and separation of chemically similar elements.

Only Q

Reason — Group 17 has elements in all the three states i.e., solid, liquid, gas. Fluorine is a gas, chlorine is a gas, bromine is liquid and iodine is solid.

Both Q and R

Reason — Modern periodic table can be used to predict nature of chemical bond, formula of compound formed and properties of that compound.

(a) Relative atomic mass of a light element up to calcium is approximately twice its atomic number.

(b) The horizontal rows in a periodic table are called periods.

(c) Going across a period left to right, atomic size decreases.

(d) Moving left to right in the second period, number of valence electrons increases

(e) Moving down in the second group, number of valence electrons remains the same

(a) Helium (He)

(b) Silicon (Si)

(c) 4, 3

(d) Argon (Ar)

(e) Group 18 — Noble gases

(f) Carbon tetrachloride (CCl₄)

(g) Phosphorus and sulphur

(h) Magnesium chloride (MgCl₂)

(i) Lithium (Li) and Magnesium (Mg);
    Beryllium (Be) and Aluminium (Al);
    Boron (B) and Silicon (Si)

(j) Sodium (Na)

(k) Silicon (Si) and phosphorus (P).

(l) Magnesium (Mg)

(a) Helium

(b) Argon

(c) Neon

(a) Electrolysis of their molten state.

(b) Flame of sodium is golden yellow, and potassium is lilac.

(a) Li < Na < K < Rb < Cs

(b) F > Cl > Br > I

(c) Mg > Na > He

(d) Cl < Mg < Na

1A group and 4th period.

P is a metal because its electronic configuration is 2,8,8,1 and it belongs to the group of alkali metals.

(a) 3, as the electronic configuration of the element is 2,8,3.

(b) +3, as the element will try to donate three valence electrons and attain a stable octet configuration.

(c) Metal, as the element has 3 valence electrons and tries to donate them to attain a stable configuration.

(d) Aluminium (Al), as it is in 3rd period and Group IIIA (13)

(a) Atomic number is 20.

(b) Its electronic configuration is 2, 8, 8, 2.

(c) +2, as the element tries to donate the two valence electrons, gain a stable octet configuration and become an electropositive ion.

(d) Group 2 Period 4

(e) Metal, as the element has 2 valence electrons and tries to donate them to attain a stable configuration.

(f) Reducing agent, as metals are strong reducing agents.

(a) Beryllium (Be), Magnesium (Mg) and Calcium (Ca).

(b) Be + 2HCl ⟶ BeCl₂ + H₂

Mg + 2HCl ⟶ MgCl₂ + H₂

Ca + 2HCl ⟶ CaCl₂ + H₂

(a) Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) are alkali metals. They have 1 electron in their outermost orbit.

(b) (i) Alkali metal like sodium reacts rapidly with oxygen forming metal oxide

4Na + O₂ ⟶ 2Na₂O

(ii) Alkali metal like sodium reacts with water violently and produces its hydroxide and hydrogen

2Na + 2H₂O ⟶ 2NaOH + H₂

(iii) Alkali metals react violently with dil. HCl and dil. H₂SO₄ to produce hydrogen

2Na + 2HCl ⟶ 2NaCl + H₂

(a) Alkaline earth metals are reactive metals, hence they do not occur in free state in nature. Their oxides occur in the earth's crust.

(b) Electronic configuration of beryllium (Be) is [2, 2] and magnesium (Mg) is [2, 8, 2]

(a) They have 8 electrons in their outermost orbits (except He, which has only 2 electrons).

(b) Xenon

(a) As alkali metals are very reactive metals, hence they are kept in inert solvents.

(b) As alkali metals and halogens are very reactive, hence, they do not occur free in nature.

(c) As alkali and alkaline earth metals have 1 and 2 valence electrons respectively; hence, they tend to donate the valence electrons to attain a stable configuration and form electrovalent compounds.

(d) As the outermost orbits of inert gases are completely filled and they are in a stable state hence they do not form compounds.

(a) (i) Group 17 elements (halogens) can react directly with metals to form metal halides (or salts).

(ii) Group 17 elements (halogens) react with non-metals to form halides like chlorides, bromides and iodides.

(b) Group 17 elements are highly reactive as their atoms need only one electron to complete their octet and attain a stable noble gas configuration.

(a) Group 17 elements are called halogens. The name halogen (Greek, halo = sea or salt + gen = producing) means 'salt former'. Group 17 elements form salts, hence, they are called Halogens.

(b) (i) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.

(ii) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.

(iii) The physical state of the elements changes from gas to solid as we move down the group. Fluorine is a gas, chlorine is a gas, bromine is liquid and iodine is solid.

1. Periodic table has been useful in predicting the existence of new elements.
2. It has been useful in the past in correcting the position of elements in relation to their properties.
3. Study of elements and their compounds has become systematic and easier to remember.
4. Position of an element in the periodic table reveals its:
   1. atomic number
   2. electronic configuration
   3. number of valence electrons
   4. properties
5. Nature of chemical bond, formula of compound formed and properties of that compound can all be predicted from the periodic table.
6. Position of an element in the periodic table reveals:
   1. valency of the element
   2. whether the element is a metal or a non-metal — metals occupy the extreme left positions of the periodic table while non-metals are at the extreme right of the periodic table.

Merits of Mendeleev’s periodic table:

1. Grouping of elements — He generalized the study of the elements then known to a study of mere eight groups.
2. Gaps of undiscovered elements — Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
3. Prediction of properties of undiscovered elements — He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
4. Incorrect atomic mass corrected — He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Defects of table proposed by Mendeleev:

1. Anomalous pairs — The following elements did not follow Mendeleev's principles:
   1. Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1
   2. Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6
   3. Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9
2. Position of isotopes — According to Mendeleev's periodic table isotopes should be given separate place in the table since they have different atomic mass. But they were not assigned separate places.
3. Grouping of chemically dissimilar elements — Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium etc.) but they have been placed together in the first group.
4. Separation of chemically similar elements — Elements that are chemically similar, such as gold and platinum have been placed in separate groups.
5. Electron arrangement — It does not explain the electron arrangement of elements.
6. Position of hydrogen — Hydrogen was not given a fixed position. It was considered in group IA as well as in group VIIA because it forms both a positive ion, viz. in HCl and a negative ion, viz. in NaH.

1. They include both metals and non-metals. There is a regular gradation from metallic to non-metallic character as one moves from left to right across the period.
2. They form electrovalent as well as covalent compounds with non-metals.
3. Metallic nature increases on moving down any of these seven groups.
4. Metals, which are good conductors of heat and electricity, are present in groups 1 and 2. Non-metals, which are present in groups 16 and 17, are poor conductors of heat and electricity.
5. Some heavier elements, like tin and lead, exhibit variable valencies.

(a) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.

(b) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.

(c) They all form negative ions carrying a single charge [Fluoride ions F^-, Chloride ions Cl^-, Bromide ions Br^-, Iodide ions I^-.]

These elements belong to alkali metals and alkaline earth metals, respectively. Below is the comparison of their characteristics:

(a) John Newland

(b) 18th group of inert elements

(c) This system worked quite well for the lighter elements.

(d) The table shown above relates the properties of the elements to their atomic mass.

(e) Reasons for discarding the law of octaves:

1. This classification did not work for heavier elements i.e., those lying beyond calcium. As more and more of elements were discovered, they could not be fitted in this table.
2. Newland adjusted two elements cobalt (Co) and nickel (Ni) in the same slot and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
3. Iron, which resembles cobalt and nickel in properties, has been placed far away from these elements.

(a) The completed table is given below:

(b)

(c) Sodium belongs to group 1.
Position of hydrogen is not satisfactory. Hydrogen atom is very small in size and therefore many of its properties are different from group 1 elements (alkali metals). It is a gas at room temperature, while other elements of group 1 are solid metals. Secondly, it does not readily form cations, in most compounds, whereas group 1 metals readily form cations and form only ionic bonds. Due to these reasons Hydrogen is not considered while describing the elements of group 1.

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as noble Gases or Inert gases are Group 18 elements. They do not react with other elements they are stable.
R is true as noble gases have complete 8 electrons in their outermost orbits (except He, which has only 2), which is stable electronic configuration.

Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as the atomic masses of Lithium (Li) is 7 , Sodium (Na) is 23 and Potassium (K) is 39. The mean of atomic masses of Lithium and Potassium is

= $\frac{7 + 39}{2}$ = $\frac{46}{2}$ = 23

Therefore, the atomic mass of middle element sodium (23) is the average of the masses of the other two elements. So, R is true.

Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Both A and R are true but R is not the correct explanation of A.

Explanation — Chlorine, a halogen with atomic number 17 has valency -1 so, it is placed in group 17. Chlorine has total three shells- K, L and M. So, it is placed in period 3. Hence, the assertion (A) is true.
The number of shell(s) in an atom signifies in which period it is placed and number of valence electrons indicates its group. Hence reason (R) is true.
But, reason (R) does not explain why chlorine is placed in group 17 and period 3.

Hence, Both A and R are true but R is not the correct explanation of A is the correct option.

Both A and R are true and R is the correct explanation of A.

Explanation — The atomic masses of Fluorine is 19, Chlorine is 35 and Bromine is 80.
The mean of atomic masses of fluorine and bromine is

= $\frac{19 + 80}{2}$ = $\frac{99}{2}$ = 49.5

The obtained mean is not the atomic mass of chlorine. So, they do not constitute a Döbereiner's triad. Hence the assertion (A) is true.
Atomic mass of chlorine is not the average mass of Fluorine and Bromine. Hence the reason (R) is true.
Reason (R) justifies why fluorine, chlorine and bromine are not Döbereiner's triad.

Hence, Both A and R are true and R is the correct explanation of A

A is true but R is false.

Explanation — Hydrogen resembles the alkali metals of Group IA and the halogens of Group VIIA. This is due to the fact that it has only one electron in its valence shell, hence it behaves like the alkali metals which have one valence electron. At the same, it also behaves like halogens as it has just one electron less than the nearest inert gas. Hence the assertion (A) is true.

Hydrogen has only one electron in its valence shell. Hence reason (R) is false.

Therefore, A is true but R is false is the correct option.

Both A and R are true but R is not the correct explanation of A.

Explanation — Cobalt with atomic mass 58.94 is placed before Nickel with atomic mass 58.68 in Mendeleev's periodic table. They are anomalous pair which do not follow mendeleev's principle. Hence the assertion is true.
Mendeleev's periodic law states that physical and chemical properties of the elements are a periodic function of their atomic masses. Hence the reason (R) is true.
However, reason (R) does not explain why cobalt is placed before nickel in Mendeleev's periodic table.

Hence, Both A and R are true but R is not the correct explanation of A is the correct option.

A is false but R is true.

Explanation — Group 17 elements are known as halogens. Hence the assertion (A) is false.
Halogen name is derived from combination of greek word halo meaning sea or salt and gen meaning producing. Therefore, group 17 elements are salt formers. Hence the reason (R) is true.

Hence, A is false but R is true is the correct option.

(a) The size of atoms generally decreases from sodium to chlorine because, on moving from left to right in a period, the number of protons increases. This increases the nuclear charge, while the electrons are added to the same shell. Hence, the nuclear pull on the electrons increases and the atomic size decreases.

(b) The most metallic element in the third period is sodium (Na).
The most non-metallic element in the third period is chlorine (Cl).

(c) Silicon (Si) acts as a semi-metal or metalloid in the third period, as it shows properties of both metals and non-metals.

(d) Elements in the same group have similar properties because they have the same number of valence electrons in their outermost shell.
Example: Sodium and potassium belong to Group 1. Both have one electron in their outermost shell. Hence, both show valency 1, form unipositive ions and have similar chemical properties.

(e) Argon (Ar) is the noble gas in the third period. It is unreactive because it has a completely filled outermost shell with 8 electrons, i.e. a stable octet.

(f) From sodium to chlorine, the valency first increases from 1 to 4 and then decreases from 3 to 1.

Thus, the valency changes as: 1, 2, 3, 4, 3, 2, 1.

Given elements are:

$^{35}_{17}\text X \quad \text{and} \quad ^{40}_{20}\text Y$

So, atomic number of X = 17 and atomic number of Y = 20.

(a) Group and period of X and Y

For X:

Atomic number = 17
Electronic configuration = 2, 8, 7

Since X has 3 shells, it belongs to Period 3. Since X has 7 valence electrons, it belongs to Group 17.

For Y:

Atomic number = 20
Electronic configuration = 2, 8, 8, 2

Since Y has 4 shells, it belongs to Period 4. Since Y has 2 valence electrons, it belongs to Group 2.

(b) Nature of X and Y

1. X is a non-metal, while Y is a metal.
2. X is an oxidising agent, as it tends to gain electrons.
   Y is a reducing agent, as it tends to lose electrons.

(c) Formation and properties of compound:

Yes, X and Y can combine to form a compound.

Y loses two electrons to form a Y²⁺ ion. Each X atom gains one electron to form an X^- ion.

Hence, X and Y can combine and form an ionic compound.

Three properties of this compound are:

1. The compound has high melting and boiling point.
2. It conducts electricity in molten or aqueous state.
3. It is soluble in water.

(d) Formula of their oxides:

• Oxide of X ⇒ X₂O₇
  (As X is Chlorine [Cl], hence formula of its oxide will be Cl₂O₇)
• Oxide of Y ⇒ YO
  (As Y is Calcium [Ca], hence formula of its oxide will be CaO)

Explanation:

For X:

X has atomic number 17 and electronic configuration 2, 8, 7. It belongs to Group 17. Its valency with respect to oxygen is 7.

Oxygen has valency 2.

$\text{X}^{7} \quad \text{O}^{2}$

By criss-crossing the valencies, the formula of the oxide of X is:

$\text{X}_2\text{O}_7$

For Y:

Y has atomic number 20 and electronic configuration 2, 8, 8, 2. It belongs to Group 2. Hence, its valency is 2.

Oxygen also has valency 2.

$\text{Y}^{2} \quad \text{O}^{2}$

By criss-crossing and simplifying, the formula of the oxide of Y is:

$\text{YO}$

Hence, the formulae of their oxides are:

X₂O₇ and YO