Valency

Can all elements form both electro valency and covalency?

How do we find the valency of the elements in a compound, if three elements are present in a compound eg ${\mathrm{NaAlO}}_2$

Give some elements having valency $2$.

Write the definition of valency.

The valency of oxygen is $3$ in the water molecule.

The valency of oxygen in water is :

What is the general name of the elements having $8$ electrons in the valence shell of their atoms?

Choose from the following list, as to what matches the descriptions below:

$\left[\left(\mathrm{A}\right) {\mathrm{O}}^{2-} \left(\mathrm{B}\right) {\mathrm{O}}_2 \left(\mathrm{C}\right) {{\mathrm{NH}}_4}^{+} \left(\mathrm{D}\right) {\mathrm{OH}}^{-} \left(\mathrm{E}\right) {\mathrm{H}}_2\mathrm{O} \left(\mathrm{F}\right) {\mathrm{NH}}_4\mathrm{Cl} \left(\mathrm{G}\right) \mathrm{HCl} \left(\mathrm{H}\right){\mathrm{Ca}}^{2+} \left(\mathrm{I}\right) \mathrm{MgO} \left(\mathrm{J}\right) {\mathrm{Ca}}^{2+}\right]$

A cation whose electronic configuration resembles argon.

Enter your answer as $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}, \mathrm{E}, \mathrm{F}, \mathrm{G}, \mathrm{H}, \mathrm{I}$ or $\mathrm{J}$

Give reasons for the following : 

Noble gases show least reactivity. 

Find out the valency of the atoms represented by the figure.

Match the column I with column II and select the correct answer by choosing an appropriate option.

The valency shown by an element having atomic number $16$ is

An element $\mathrm{X}$ (atomic no. $=12$) combines with another element $\mathrm{Y}$ whose electronic configuration is $2, 8, 7$. What will be the formula of the compound formed?

How will you find the valency of aluminium, oxygen and fluorine? 

What is the cause of the chemical combination? How is the number of valence electrons related to the combining capacity of the atom?

How do we calculate the valency of ions?

Define valency by taking an example of magnesium $($Atomic number $12)$ and oxygen $($Atomic number $8)$.

The electronic configuration of $\mathrm{O}(\mathrm{Z} = 8)$ is $2, 6$. Why is the valency of oxygen $2$ and not $6$? 
 

Define valency. What conclusions can be drawn about the reactivity of an atom from its valency?