Periodic Table and Electronic Configuration

How would you explain the fact that first ionisation enthalpy of sodium is lower than that of magnesium but its second ionisation enthalpy is higher than that of magnesium?

Use the periodic table to answer the following question.

Identify an element that would tend to gain two electrons.

Assign the position of the element having an outer electronic configuration in the periodic table.

(n - 1)d² ns² for n = 4

Assign the position of the element having an outer electronic configuration in the periodic table.

(n - 2) f⁷ (n - 1)d¹ ns² for n = 6

Use the periodic table to answer the following question.

Identify an element with five electrons in the outer subshell.

The period number in the long form of the periodic table is equal to ______.

Which of the following sets contain only isoelectronic ions?

(i) \[\ce{Zn^{2+}, Ca^{2+}, Ga^{3+}, Al^{3+}}\]

(ii) \[\ce{K+ , Ca^{2+}, Sc^{3+}, Cl-}\]

(iii) \[\ce{P^{3-}, S^{2-}, Cl- , K+}\]

(iv) \[\ce{Ti^{4+}, Ar, Cr^{3+}, V^{5+}}\]

Write the atomic number of the element present in the third period and a seventeenth group of the periodic table.