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(1) Newlands’ law of octaves states that when the elements are arranged in the order of their increasing atomic masses, every eighth element has properties similar to those of the first.

(2) Illustration: If the first 21 elements, except inert gases, are arranged in the order of their increasing atomic masses we have octaves as given below:

It is found that Na is the eighth element from Li and both of them have similar properties. Similarly, the elements, in the following pairs show similar 

properties: C and Si, Na and K, Mg and Ca, F and Cl.

Newland arranged the known elements in increasing order of atomic mass. He found that the every eighth element had properties similar to that of first one. He compared it to the octaves found in music and thus it is called Newland’s law of octaves.

(1) To give the proper place in the periodic table, atomic masses of some elements were revised in accordance with their properties. For example, the previously determined atomic mass or beryllium, 14.09, was changed to the correct value 9.4, and beryllium was placed before boron.

(2) Mendeleev had kept some vacant places in the periodic table for elements that were yet to be discovered. Three of these unknown elements were given the names eka-boron, eka-aluminum and ekasilicon from the known neighbors and their atomic masses were indicated as 44, 68 and 72, respectively. Their properties were also predicted.

Later on, these elements were discovered subsequently and were named as scandium (SC), gallium (Ga) and germanium (Ge) respectively. The properties of these elements matched well with those predicted by Mendeleev. Due to this success all were convinced about the importance of Mendeleev’s periodic table.

(3) There was no place reserved for noble gases in Mendeleev’s original periodic table. when noble gases such as helium, neon and argon were discovered, Mendeleev created the ‘zero group’ without disturbing the original periodic table in which the noble gases were placed very well.

(1) Newlands’ law of octaves i.e. applicable to only the first few elements i.e., only up to calcium out of total 56 elements known at that time.

(2) Newlands placed two elements each in some boxes to accommodate all known elements e.g. CO and Ni, Ce and La. He placed some elements with different properties under the same note in the octave. For example, Co and Ni under the note Do along with halogens, while Fe having similarity with CO and Ni away from them along with the nonmetals O and S under the note Ti.

(3) Newland’s octaves did not have provision to accommodate the newly discovered elements.

Two limitations of Newlands’ law of octaves are: 

(i) The law was applicable only upto calcium. 

(ii) In order to fit elements into his table, Newlands adjusted two elements in wrong slot, and grouped unlike elements in same groups.

• Newlands’ law was applicable only till calcium, after Ca, every eighth element did not possess properties similar to first. 

• New elements discovered did not fit into the law. 

• Wrong order of arrangement of elements was done, e.g. Co and Ni do not resemble halogen were found together in same slot, Fe being similar to Co and Ni was separated and kept in different slot.

(a) ‘A’ is a metal because its valence electron is 1, it can readily loose electron to became stable. 

(b) Element ‘C’ belongs to 3rd period it has 3 shells whereas ‘B’ has only 2 shells, it belongs to 2nd period, distance between nucleus and valence electrons is more in C, hence its size is bigger than B. 

(c) ‘B’ belongs to III group, has 3 valence electrons, it can loose 3 electrons to become stable hence its valency is 3.

The repetition of same properties after definite interval is called periodicity in properties.

The Limitations of Newlands’ Law of Oclaves is as follows:

1. It was found that the Law of Octaves was applicable only upto calcium, as after calcium every eighth element did not possess properties similar to that of the first.

2. It was assumed by Newlands’ that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.

3. In order to fit elements into his Table, Newlands adjusted two elements in the same slot, but also put some unlike elements under the same note.

(a) Non-metals

(b) (i) Inoeases

(ii) Decreases

B, C, N, O.

In a period, as the atomic number increases, the size of the atom decreases. This is because of increase in nuclear charge tends to pull electrons, closer to nucleus and reduces the size of the atom.

(i) Elements in th e same group have equal valency.

(ii) The metallic character of elements in a period decreases gradually on moving from left to right

Note: For parts (v) and (vi) examples using symbols of elements may also be accepted.

Detailed Answer:

(i) P has 1 valence electron, Q has 2 valence electrons.

(ii) 'P' is bigger than 'Q'.

(iii) 'P' is more metallic than 'Q'.

(iv) 'P' has more tendency to lose electrons than 'Q'.

(v) P₂O and QO are formulae of their oxides.

(vi) P Cl and Q Cl₂ are formulae of their chlorides.

In the modern periodic table, elements having some electronic configuration are arranged in the same group.

N and P are chemically similar.

(1) In the modern periodic table, there are seven horizontal rows called periods.

(2) In a period, change in valency of an elements varies electronic configuration.

(3) The number of valence electrons is different in these elements. However, the number of shells is the same. In a period, while going from left to right, the atomic number increases by one at a time and the number of valence electrons also increases by one at a time. In a period, there is gradation in properties of elements.

(4) The elements with the same number of shells occupied by electrons belong to the same period. The elements in the second period, namely, Li, Be, B, C, N, O, F and Ne have electrons in the two shells, K and L. The elements in the third period, namely, Na, Mg, Al, Si, P, S, Cl, and Ar have electrons in the three shell6: K, L and M.

(5) The chemical reactivity of an element is determined by the number of valence electrons in it and the shell number of the valence shell. In a period, while going from left to right, the atomic number increases by one at a time as a result atomic radius gradually decreases. Hence, atomic size decreases.

(a) Periods

(b) Increases

(c) Decreases

(d) Increases

(e) Decreases

(i) They can lose electrons easier due to a bigger size, the energy required to remove an electron is less. 

(ii) It is because of greater effective nuclear charge in Mg.

(iii) Fluorine has very small size and just needs one electron to get itself into a stable configuration to that of noble gases. Hence, they react spontaneously with elements to achieve that stable state, contributing to its high reactivity.

There are two elements in the shortest (first period) period.

N (2 ,5 ) ; P (2, 8, 5) ; Nitrogen will be more electronegative because its atom has small size due to which the attraction of its nucleus for the incoming electron is more

(a) Element C is non-metal because it belongs to 17 group and it contains 7 valence electrons so it can gain only one electron. So it is non-metal. 

(b) In non-metals, electronegativity increases from left to right in a period due to increase in effective nuclear charge and decreases from top to bottom in a group due to increment in size. So A is the most active element. 

(c) Since C is non-metal and it has 7 valence electrons. So it can gain only one electron and anion will be formed.

(B) Only I and lll 

(i) E

(ii) B

(iii) C

(iv) B, because atomic radius decreases from left to right due to the increase in the nuclear charge.

(v) Noble gases.

(i) Element E will form covalent compounds by sharing its four valence electrons. 

(ii) Element D is a metal with valency three. 

(iii) Element B is a non-metal with valency three. 

(iv) We know that the size of elements decreases on moving left to right in a period. Therefore, element D is greater than element E. 

(v) The name of the family to which elements C and F belong is the noble gas family (group 18).

Answer: (a) H, He

(d) Period 6

Electronic configuration of ₁₃Al : 2, 8, 3

2^nd period has 8 elements, 5^th Period has 18 elements.

There are 18 vertical columns or groups and seven horizontal rows or periods of the elements in the long form of the periodic table.

Answer: (b) periods

(i) Atomic radius decreases

Reason: Nuclear charge increases which tends to pull the electrons closer to the nucleus.

(ii) Atomic radius increases

Reason: Number of shells increases on going down the group.

(i) Chlorine

(ii) 3^rd period

(iii) 17^th group

(iv) 2,8,7.

(a) Left side.

(b) Right side.

(c) Metalloids.

The answer is (b) (i), (ii) and (iii)

• Elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number hence option i) is wrong . 
• In modern periodic table atomic mass is not a criteria hence option ii) is wrong. 
• Isotopes are given the same position in periodic table hence option iii) is wrong.

The answer is (b) B, C, D

Element with atomic number belong to first period and element B, C and D belongs to second period.

1. Helium 

2. Neon 

3. Argon.

Noble gases such as Helium, Neon and Argon are the three elements whose outermost shells are completely filled.

(i) A and B belong to group 1 and 2 because they form basic oxides. C belongs to group 13 as it has 3 valence electrons. D belongs to group 14 as it forms an almost natural oxide. E and F belong to group 15 and 16 as they form acidic oxides, G belongs to group 17 as it has 7 valence electrons, and H to group 18. They belong to 3^rd period of the periodic table.

(ii) H belongs to noble gas

(iii) A has a largest atomic radius

(iv) E and F are likely to be non-metals

(v) D is likely to be metalloid or semi-metal.

The early attempts to classify elements were made on the basis of atomic masses of the elements. The modern basis is the atomic number. This is better because now it is established that an atomic number of an element is the fundamental property. Further, there are certain anomalies in Mendeleev's periodic table such as the position of isotopes and misplacements of certain elements, e.g. Argon is placed (Atomic mass - 40, atomic number= 18) before potassium in Mendeleev's periodic table (Atomic mass =39, At. no = 19).

(i) Third period
Group-1 ,2,13,14, 15, 16, 17, 18 respectively.
(ii) Nature of compound: Electrovalent/ionic
(iii) Metals-A and B; Non-metals-E, F, G
(iv) G/H
(v) CG₃
(vi) H
(vii) A
(viii) D

(i) A and B, C and D have the same period.

(ii) A and C have one valence electron, so the same group.

(iii) C is more reactive because C is placed below A in the periodic table and reactivity increases down the group.

• Fluorine has the electronic configuration (2, 7).
• It requires only one electron to complete the octet.
• The atomic size of fluorine is the smallest among the halogens. Hence, the nuclear attraction on the outermost electrons is maximum. Hence, fluorine is the most reactive among the halogens.

s-block elements:

1. The groups IA (1) and IIA (2) elements together with hydrogen constitute the sblock.

2. They have one or two electrons in the outermost shell.

3. The elements of the s-block, except hydrogen, are all metals.

p-block elements :

1. The group III A (13) – VII A (17) and the zero group (18) elements constitute the p-block. 

2. They have three to eight electrons in the outermost shell. 

3. The elements of the p-block include a few metals, all metalloids and all nonmetals.

Groups:

1. The vertical columns of elements in the modern periodic table are called groups. 

2. The group number indicates the number of electrons in the outermost shell of an atom of an element belonging to that group.

3. The elements in the same group show similar chemical properties

periods:

1. The horizontal row of elements in the modern periodic table are called periods. 

2. The period number indicates the number of electronic shells present in an atom of an element belonging to that period. 

3. The elements in the same period do not show similar properties, but their chemical properties gradually change from left to right in a period.

I refers to the element Ge, II refers to the element S, III refers to the element Se, IV refers to the element Tl, V refers to the element F. Non-metallic character follows the order: Tl<Ge<Se<S<F

Dobereiner’s law of triads: When elements are arranged in order of increasing atomic masses, groups of three elements (triads), having similar chemical properties are obtained. The atomic mass of the middle elements of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.

For example: Alkali metal group ( Dobereiner’s triad) : Lithium is the 1st element, sodium is the middle element whereas potassium is the 3rd element of the triad.