Matter - Nature and Behaviour : Periodic Classificat...

On moving left to right across a period, the atomic size decreases because the effective nuclear charge increases
 

Newlands found that the properties of the eighth element were similar to the properties of the first element. He compared this with the octaves found in the musical scale. Therefore, this method of classification of elements is called Newlands’ Law of Octaves.

Electronegativity decreases while going down a group.

Newlands, Mendeleev and Dobereiner all tried to correlate the properties of elements with their atomic masses. Newlands, an English scientist, arranged the elements in order of their increasing atomic masses. He started with hydrogen (atomic mass 1 u), the element having the lowest atomic mass and ended with thorium, which was the 56^th element. He found that the properties of the eighth element were similar to the properties of the first element. Dobereiner tried to arrange the elements with similar properties into groups called triads having 3 elements each. According to Dobereiner, when the elements of any particular triad were arranged in order of their increasing atomic masses, the atomic mass of middle element was roughly the mean or average of the atomic masses of the other two elements. Mendeleev, in his periodic table, classified the elements on the basis of their increasing atomic masses and the formula of their oxides and hydrides. Later on, Henry Moseley discovered that atomic number is a more appropriate parameter to determine the properties of an element rather than its atomic mass.

The horizontal rows in the periodic table are called periods.

It fails to accommodate lanthanides and actinides in the main body of periodic table.

On moving down a group, atomic radius increases as the number of shells increases. Li (Z = 3), Be (Z = 4) and B (Z = 5) belong to the 2^nd period. On moving across the period from Li to B, size will decrease. So, their correct order in terms of increasing atomic radius is B < Be < Li. Moreover, N (Z = 7), O (Z = 8) and F (Z = 9) belong to the 2^nd period. On moving from N to F, atomic radius will decrease. So, their correct order is F < O < N. Similarly, Na (Z = 11), Mg (Z = 12) and Al (Z = 13) belong to the 3^rd period. On moving from Na to Al, atomic radius will decrease. So, their correct order is Al < Mg < Na. F (Z = 9), Cl (Z = 17) and Br (Z = 35) belong to the 17^th group and from F to Br, the atomic radius will increase. So, their correct order in terms of increasing atomic radius is F < Cl < Br. Therefore, only option (4) is correct.

The earlier attempts at classification of elements was based on atomic masses. Later on, it was found that atomic number is a more appropriate parameter to determine the properties of an element. Therefore, in the modern periodic table, elements are arranged in the increasing order of their atomic number. 

Newlands, an English scientist, arranged the elements according to their increasing atomic masses. He started with hydrogen (Atomic mass = 1 u), the element having the least atomic mass and ended with thorium, which was the 56^th element.

The noble gases like helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) were not known when Mendeleev gave his periodic table. These gases were discovered later on because they are inert and are present in extremely low concentration in our atmosphere. But, it was the strength of Mendeleev’s periodic table that these gases, when discovered, could be placed in a new group called zero group, without disturbing the existing order. Halogens (F, Cl, Br, I), alkali metals (Li, Na, K, Rb, Cs) and alkaline earth metals (Be, Mg, Ca, Sr, Ba) were already discovered when Mendeleev gave his periodic table. So, they were already occupying specific positions in his periodic table.

According to Döbereiner, when elements of any particular triad were arranged in the increasing order of their atomic masses, the mass of the middle element was roughly the mean or average of the atomic masses of the other two elements. 

Mendeleev left some gaps in his periodic table for some undiscovered elements. Mendeleev boldly predicted that these elements would be discovered later. He even predicted the properties of these unknown elements and named these elements by prefixing a Sanskrit word ‘eka’ to the name of the preceding element in the same group, e.g. eka–boron for scandium, eka-aluminium for gallium and eka-silicon for germanium. Titanium, cesium and iodine were already discovered when Mendeleev gave his periodic table. So, option (1) is the correct answer.

The 1^st period is the shortest period and contains hydrogen and helium.

Atomic size of an element increases as we move from top to bottom in the periodic table.

Atomic size increases down a group because the distance between the outermost electron and the nucleus increases.