The modem periodic law states that, "the physical and chemical properties of the elements are periodic functions of their atomic numbers."

(i) s-block elements: ns^1-2.where n = 2 -7.
(ii) p-block elements: ns² np^1-6 where n = 2- 6.
(iii) d-block elements: (n -1) d^1-10 ns^0-2where n = 4 - 7.
(iv) f-block elements: (n - 2) f^0-14 (n -1) d^0-1ns² where n = 6 - 7.

Galium and Germanium

Variation down the group: On moving down a group both atomic and ionic radii increase with increasing atomic number. The increase in size is due to introduction of extra energy shells which outweigh the effect of increased nuclear charge.

Lanthanides: [₅₄Xe] 4f^1-14 5d¹ 6s²
Actinides : [₈₆Rn] 5f^0-146d^0-2 7s²

The total number of electrons are less in the cation than the neutral atom while the nuclear charge remains the same. Therefore the effective nuclear charge of the cation is higher than the corresponding neutral atom. Thus the successive ionisation energies, always increase in the following order
IE₁ < IE₂ < IE₃ < .....

(i) The cause of periodicity in properties is the repetition of similar outer electronic configuration after certain regular intervals.
(ii) Eg: Consider elements of group 1s
₃Li - 1s² 2s¹
₁₁Na -1s² 2s²2p⁶ 3s¹
-Due to similar outermost electronic configuration they exhibit similar properties.

Helium is placed in group 18 since it has a completely filled valence shell and so resembles characteristics of noble gases.

Lanthanoids and actinoids are placed in separate rows at the bottom of the periodic table to maintain its structure and to preserve the principle of classification by keeping elements with similar properties in a single column.

Selenium (Se)
Xenon (Xe)

(i) Due to low bond dissociation enthalpy, high electronegativity and large negative electron gain enthalpy, halogen have a strong tendency to accept electrons and thus get reduced.
X²+2e^- ⟶ 2X^-
(ii) The ready acceptance of an electron is due to the strong oxidising nature of halogens.
(iii) F₂ is the strongest oxidising halogen and it oxidises other halide ions in solution or even in the solid phase. In general, a halogen oxidises halide ions of higher atomic number.
(iv) Hence, the oxidising ability of halogens decreases from fluorine to iodine as :
F₂ > Cl₂ > Br₂ > I₂

(i) According to aufbau's principle 5th period has nine orbital (one 5s, five 4d and three 6p) to be filled.
(ii) Nine orbitals can accommodate a maximum of 18 electrons. Hence fifth period of the periodic table should has 18 elements from rubidium (2 = 37) to Xenon (Z = 54).

Merits of Moseley's long form of periodic table:
(i) As this classification is based on atomic number, it relates the position of an element to its electronic configuration.
(ii) The elements having similar electronic configuration fall in a group. They also have similar physical and chemical properties.
(iii) The completion of each period is more logical. In a period as the atomic number increases, the energy shells are gradually filled up until an inert gas configuration is reached.
(iv) The position of zero group is also justified in the table as group 18.
(v) The table completely separates metals and non-metals.
(vi} The table separates two subgroups, lanthanides and actinides, dissimilar elements do not fall together.
(vii) The greatest advantage of this periodic table is that this can be divided into four blocks namely s, p, d and f-block elements.
(viii) This arrangement of elements is easier to remember, understand and reproduce.

(i) According to the recommendation of IUPAC, the groups are numbered from 1 to 181A.
(ii) There are 18 vertical columns which constitute 18 groups or families.
(iii) There are 7 horizontal rows of the periodic table known as periods.
(iv) The first period contains two elements. One present in first group and the other in 18th group.
(v) Second and third periods contain 8 elements in each.
(vi) Fourth and fifth periods are completely filled as they contain 18 elements in each.
(vii) The sixth period contains 32 elements. The seventh is incomplete. Fourteen elements of both sixth and seventh periods are placed in separate panels at the bottom of the table.
(viii) This periodic table is important and useful because we can predict the properties of any element using periodic trend.

(i) Variation of Electronegativity in a period: The electronegativity generally increases across a period from left to right. The atomic radius decreases in a period, as the attraction between the valence electron and the nucleus increases. Hence the tendency to attract shared pair of electrons increases. Therefore, electronegativity also increases in a period.
(ii) Variation of Electronegativity in a group: The electronegativity generally decreases down a group. As we move down a group the atomic radius increases and the nuclear attractive force on the valence electron decreases. Hence, the electronegativity decreases.