Electronegativity decreases as you go down a group within the periodic table. This is because of the longer distance between the nucleus and the outer valence electron shell within an atom. This decreases the attraction. Thus, the correct order of electronegativities is O > N > P.

According to Dobereiner, when elements of a given triad are arranged on the basis of increasing atomic masses, the atomic mass of the middle element is roughly the mean or average of the atomic masses of the other two elements. Dobereiner gave only three traids - (Li, Na, K); (Cl, Br, I) and (Ca, Sr, Ba).

Mendeleev classified the elements according to their increasing atomic masses in horizontal rows in such a way that the elements with similar chemical properties (i.e. elements with similar formulae of oxides and hydrides) fell in the same vertical column. He preferred only oxygen and hydrogen because oxygen and hydrogen form bond with almost all elements.

Mendeleev's periodic law states that poperties  of elements are periodic functions of their atomic masses. Therefore the elements are arranged in the increasing order of their atomic masses.  However Mendeleev placed cobalt before nickel eventhough it has a slightly higher mass than nickel. 

According to the modern periodic table, elements belonging to group 17 are designated as halogens because they form salts with metals.

Element X, having atomic number 9, contains 9 protons and its valence shell contains 7 electrons. To acquire noble gas configuration, it can accept one electron.

H, F, Cl and Br have some similar properties as they all have valency 1 and exist in nature as diatomic molecules (i.e. in the form of H₂, F₂, Cl₂, Br₂). Cobalt is a transition metal and its properties are different from those of H, F, Cl and Br.

As we move down a group, atomic radii increase. Only option 1 is in the correct order.

In a group, all the elements have the same valency.

The number of shells tells the period and the number of electrons in the last valence shell tells the group.

Elements belonging to the 18^th group of the periodic table have fully filled last valence shell. So, they have zero valency.

The modern periodic table was given by Henery Mosley. 

Atomic radius is defined as the distance between centre of nucleus and outermost shell, which contains electrons in an isolated atom. On moving from left to right in a period, atomic radius decreases. This is because as we move from left to right, the atomic number of each succeeding element increases by 1. This means that the number of protons in the nucleus and the number of electrons in the extra part of the nucleus increase by 1. The electrons are added to the same shell, but the addition of an extra proton in the nucleus increases the nuclear charge, which attracts the electrons closer to the nucleus, thereby decreasing the atomic radius, whereas on moving down a group, atomic radius increases as the number of shells increases. F, Cl and Br belong to group 17 and on moving down the group from F to Br, their atomic size will increase. So, the correct order in terms of decreasing atomic size is: Br > Cl > F. Similarly, Li, Na and K belong to group 1 and on moving down from Li to K, their atomic size will increase. So, their correct order is: K > Na > Li. B, C and N belong to the 2^nd period and on moving across the period from B to N, their atomic size will decrease. So, their correct order is: B > C > N. S, P and Si belong to the 3^rd period and on moving from Si to S, the atomic radius will decrease. So, their correct order in terms of decreasing atomic size is: Si > P > S. Therefore, only option (3) is correct.

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.

In Mendeleev’s periodic table, there are 8 groups, which are further subdivided into 2 parts (e.g. I A, I B) and 7 periods.