Magnesium chloride is an ionic compound while hydrogen chloride is a covalent compound. But both form ions in their aqueous solutions. Explain.

Elements $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ have atomic numbers $9, 20$ and $10$ respectively.
 Stale which one is (i) a non-metal (ii) a metal (iii) chemically inert

Elements $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ have atomic numbers $9, 20$ and $10$ respectively.

Write down the formula of the compound formed by two of the above elements.

Explain the following:

Ionic compounds conduct electricity.

Explain the following:

Ionic compounds have high melting and boiling points while covalent compounds have low melting and boiling points. 

Explain the following:

Ionic compounds dissolve in water whereas covalent compound do not.

Explain the following:

Ionic compounds are usually hard crystals.

Five atoms are labelled from $\mathrm{A}$ to $\mathrm{E}$.

(a) Which one of these atoms:(i) contains $7$ protons (ii) has an electronic configuration $2,7$?

(b) Write down the formula of the compound between $\mathrm{C}$ and $\mathrm{D}$.

(c) Predict which are: (i) metals (ii)non-metals.

What is the difference between ionic compounds and polar covalent compounds?