Kossel-Lewis approach, ionic and covalent bonds

Amongst the following, the total number of species NOT having eight electrons around central atom in its outer most shell, is

NH\(_3\), AlCl\(_3\), BeCl\(_2\), CCl\(_4\), PCl\(_5\) :

Magnesium reacts with an element (X) to form an ionic compound. If the ground state electronic configuration of (X) is 1s\(^2\) 2s\(^2\) 2p\(^3\), the simplest formula for this compound is

Iron exhibits bcc structure at room temperature. Above 900°C, it transforms to fcc structure. The ratio of density of iron at room temperature to that at 900°C (assuming molar mass and atomic radii of iron remains constant with temperature) is