The noble gases having completely filled electronic configuration neither accept nor lose their electron readily and hence they are chemically inert in nature

If an element have more than are valence or oxidation state, it is said to exhibit variable oxidation state. eg: Fe exhibits oxidation state of 2 and 3 in ferrous and ferric compounds respective. Most of the elements in group 15 which have valence electron 5, show two valencies, 3 and 5. Similarly, transition and inner transition metals also show variable oxidation states

The valence of an atom primarily depends on the number of electrons in the valence shell. As the number of valence electrons remains same for the elements in same group, the maximum valence also remains the same. However, in a period the number of valence electrons increases, hence the valence also increases.

Pauling, assigned arbitrary value of electronegativities for hydrogen and fluorine as 2.2 and 4.0 respectively. Based 'on this the electronegativity values for other elements can be calculated using the following expression (x_A - x_B) = 0.182 \(\sqrt { E } _{ AB }-\left( { E }_{ AA }*E_{ BB } \right) \)
Where E_AB, E_AAand E_BB are the bond dissociation energies of AB, A₂ and B₂ molecules respectively.

The sizes of oxygen and fluorine atoms are comparatively small and they have high electron density. Moreover, the extra electron added to oxygen and fluorine has to be accommodated in the 2p orbital which is relatively compact compared to the 3p orbital of sulphur and chlorine so, oxygen. and fluorine have lower electron affinity than their respective group elements sulphur and chlorine.

Down a group, generally the electron affinity decreases. It is due to increase in atomic size and the shielding effect of inner shell electrons.

Generally electron affinity increases along a period. i.e., the amount of energy released will be more. This is due to an increase in the nuclear charge and decreases in size of atom

It is defined as the amount of energy released (required in the case noble gases) when an electron is added to the valence shell of an isolated neutral gaseous atom in its ground state to form its anion. It is expressed in kJ mol^-1.
A  + e^- \(\rightarrow\)A^- + E_A

Down a group, the number of inner shell electron increases which in turn increases the repulsive force exerted by them on the valence electrons, i.e. the increased shielding effect caused by the inner electrons decreases the attractive force acting on the valence electron by the nucleus. Therefore the ionisation energy decreases

The ionisation energy decreases down a group. Down a group, the valence electron occupies new shells, the distance between the nucleus and the valence electron increases. So, the nucleat forces of attraction on valence electron decreases and hence ionisation energy also decreases. down a group.

It is due to the fact that beryllium with completely filled 2s orbital, is more stable than partially filled valence shell electronic configuration of boron. (2s², 2p¹)

Atomic radius tends to decrease in a period. Along a period, the valence electrons are added to the same shell. The simultaneous addition of protons to the nucleus, increases the nuclear charge, as well as the electrostatic attractive force between the valence electrons and the nucleus. Therefore atomic radius decreases along a period.

The formation of covalent bond involves the overlapping of atomic orbitals and it reduces the expected internuclear distance. Therefore covalent radius is always shorter than the actual atomic radius.

The lanthanides are the elements having the general electronic configuration 4f^1-14 5d^0-1, 6s² and actinides have the general configuration 5f^O-14, 6d^0-2, 7^s2. They are collectively known as 'f' block elements. These elements are metallic in nature and have high melting points. Their compounds are mostly coloured. These elements also show variable oxidation states

These elements also show more than one oxidation state and form ionic, covalent and coordination compounds. They can form interstitial compounds and alloys which can also act as catalysts. These elements have high melting points and are good conductors of heat and electricity

The elements of p-block have high negative electron gain enthalpies. The ionisation energies are higher than that of s-block elements. They form mostly covalent compounds and shows more than one oxidation states in their compounds

These are soft metals and possess low melting and boiling points with low ionisation enthalpies. They are highly reactive and form ionic compounds. They are highly electropositive in nature and most of the elements imparts colour to the flame.

's' block: ns¹ or².
'p' block: ns² np^1-6
'd' block: (n - l)d^1-10 ns^{1 or 2}
'I' block: (n - 2}f^1-14 (n- l)d^Oor2 ns²

The root's for 1 and 9 are un and enn respectively. Hence, the name of the element with Z = 119 is Un + Un + enn + ium = ununennium. Its symbol isuue.
The atomic number of the last number of the 7th period is 118. Thus, this element will belong to 8th period and will be the first element i.e., alkali metal (8 s').
Its electronic configuration :2, 8, 18, 32, 32, 18, 8, 1

Mosley's relationship is given as \(\sqrt { v } \) = a (Z - b) where v is the frequency of X-rays emitted by an element of atomic number Z.
The plot of IV against Z gives a straight line. Using. this relationship, we can determine the atomic number of an unknown (new) element from the frequency of X-ray emitted.

(i) Some elements with similar properties were placed in different groups and those with dissimilar properties were placed in same group.
eg: Tellurium (127.6) was placed in VI group but Iodine (127.0) was placed in VII group.
(U) Similarly elements with higher atomic weights were placed before lower atomic weights based on their properties in contradiction to his periodic law.
eg; ⁵⁹Co₂₇ was placed before^58.7Ni₂₈

There exists a linear correlation between the atomic number and frequency of X-ray emitted by an element. \(\sqrt { v } \) = a(Z- b)
v = frequency of X-ray emitted by the element with atomic number Z, a, b are constants. This relationship is known as Mosley's relationship.

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.