(i) Alkaline earth metals readily lose their outermost electrons to form +2 ion.
(ii) As we go down the group, their metallic character and reactivity are increased.

1. Alkali metals readily lose their outermost electron to form +1 ion.
2. Alkali metals are soft metals with low melting and boiling points.

The Law of octaves states that, "when elements are arranged in the order of increasing atomic weights, the properties of the eighth element are a repetition of the properties of the first element".

In this system, elements that differed from each other in atomic weight by 16 or multiples of 16 fell very nearly on the same vertical line. Elements lying directly under each other showed a definite similarity. This was the first periodic law.

Mendaleev's periodic law based on the properties of elements are periodic functions of their atomic weights while the modem periodic law based on the properties of elements are periodic function of atomic numbers.

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 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

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

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

It is defined as the relative tendency of an element present in a covalently bonded molecule, to attract the shared pair of electrons towards itself.

Mg + I.E₁ ⟶ Mg⁺ +1e^-     .....(i)
Mg⁺ + I.E₂ ⟶ Mg²⁺ + 1e^-  ....(ii)
Mg²⁺ + I.E₃ ⟶ Mg³⁺ + 1e^-  ....(iii)
(i) The step (iii) which involves the formation of Mg³⁺ requires higher ionisation energy.
(ii) Mg²⁺ consist of 10 electrons (2, 8) attaining the stable noble gas configuration of argon (Z = 10).
(iii) Since the valence orbital is completely filled, more energy will be required to remove electrons.

Ions of different elements having the same number of electrons are called isoelectronic ions.