In any period of the periodic table, valency of an element with respect to oxygen

In any period, the valency of an element with respect to oxygen

Define a period of the modern periodic table of elements.

With reference of first three periods of the periodic table. Valency of elements in group 17

Name or state the following with reference to the elements of the first three periods of the periodic table. Valency of elements in Group 14 and 15.

With reference to period 3 of the periodic table - State: The number of electrons in the penultimate shell of the element with valency -1.

(a) How does the electropositive character of elements change on going down in a group of the periodic table? (b) State how the valency of elements varies (i) in a group and (ii) in a period, of the periodic table.