Identify the elements with the following properties and arrange them in increasing order of their reactivity:
$(a)$ An element which is a soft and reactive metal.
$(b)$ The metal which is an important constituent of limestone.
$(c)$ The metal which exists in a liquid state at room temperature.

Which group of elements could be placed in Mendeleev's Periodic Table without disturbing the original order? Give reason.

Describe the basic character of the oxides of third period elements across the period from left to right.

Given below are some elements of the modern periodic table. Atomic numbers of the elements are given in the parentheses: $A(4), B(9), C(14), D(19), E(20)$.
$(a)$ Select the element that has one electron in the outermost shell. Also,write the electronic configuration of this element.
$(b)$ Which two elements amongst these belong to the same group? Give reason for your answer.
$(c)$ Which two elements amongst these belong to the same period? Which one of the two has a bigger atomic radius?

''Elements in Periodic Table show periodicity of properties.'' List any four such properties.

The elements $A, B, C, D$ and $E$ have atomic numbers $9, 11, 17, 12$ and $13$ respectively. Which pair of elements belong to the same group?