Help please

[dr.micl]

Firstly , this is my first post here in this nice forum, i'd like to add an answer for the questions stated above: considering the first qu. i think you are confused in the point of when we choose ions / atoms or molecules, it's quite easy
- Molecule is the large unit which is composed of either ions or atoms , we say ions when the molecule is ionic (contains an ionic bond) e.g. table salt NaCl - MgO - PbBr2 and we say atoms when the molecule is covalent (contains covalent bonds) e.g. NH3 - H2O
You can easily know if the molecule is ionic or covalent if you know that all ionic molecules have metal ion in the beginning , look at NaCl , sodium metal - and also magnesium and lead in the other examples while covalent molecules are formed between non-metals
IF this is useful , please inform,