Since beryllium is a cation metal and phosphorus is a anion non-metal, they can be combined to create an ionic compound.

In ionic compounds, the cation is always written before the anion in the formula so answer choices with phosphorus as the first element ( and ) are incorrect.

Ionic compounds must have a net charge of zero. Beryllium is an alkaline earth metal and has a charge of , since it is a member of group 2. Phosphorus has a charge of  since it is a member of group 15. To create a molecule with a net charge of zero there must be three beryllium ions for every two phosphorus ions.

 is not a correct answer choice because the net charge of the molecule does no equal zero.

is the balanced ionic formula for beryllium phosphate, but contains oxygen as well as beryllium and phosphorus.

 is an binary ionic compound because it contains one cation species and one anion species, connected by ionic bonds. Compounds with this structure should be named according to the rules for naming binary ionic compounds, which differ from the rules for naming molecular compounds. For these types of compounds the cation (usually a metal) should be listed first and the anion (non-metal) should be listed second with the suffix -ide added to the end. 

Answer choices containing prefixes (mono-, di-, tri- etc) are incorrect because prefixes should only be used when naming molecular compounds.

Answer choices containing roman numerals are incorrect because roman numerals are only used to designate the charge of the cation when the cation is a transition metal, which are generally capable of multiple oxidation states and require the charge designation to differentiate. Magnesium is and alkaline earth metal, not a transition metal, and will always have a charge of positive two.

Using the proper naming conventions, refers to magnesium chloride.

 is a molecular compound, composed entirely of non-metallic elements with covalent bonds. Normally, these compounds follow a specific set of naming rules that would lead to this compound being called "nitrogen trihydride," however this particular compounds is an exception to normal naming rules.

 is an example of a molecular compound that is given a non-systematic common name. The common name for  is ammonia. Ammonium is incorrect, and refers to the complex ion .

Other exceptions to the naming rules include water () and methane ().

is a molecular compound (a compound that contains two non-metals), so we must name it according to the rules for naming molecular compounds. These are different from the rules to name ionic compounds.

In molecular compounds we use prefixes (mono-, di-, tri- etc) at the beginning of each element to indicate the subscript for the element; however, we only use the prefix mono- for the second element in the compound. Mononitrogen dioxide is an incorrect answer choice because the compound name will never begin with mono-. We also need to change the ending of the last element in molecular compounds to -ide. Together, the name becomes "nitrogen dioxide."

Nitrate refers to the complex ion . Nitrogen oxide is simply . Nitrous acid is also known as hydrogen nitrite, and is formed from a hydrogen ion and a nitrite ion with an ionic bond: .

In naming a compound you must look at what is being paired together. In this case a metal is bonding with a non-metal, thus the rules for naming metal compounds must be used. When naming a compound with a metal, the name of the metal goes first followed by the name of the anion. The name of the metal is cobalt and the anion is nitrogen. To name the anion of an atom drop the ending and add -ide, so the anion is named nitride. So far the name of this compound is cobalt nitride. However, cobalt is a metal that can have multiple charges so a Roman numeral is needed in the name. To figure out what Roman numeral is necessary you have to know the charge of the anion. To find the charge of the anion take the group number and subtract 8. Nitrogen is in group 5A so each nitride anion must be . All compounds are neutral unless otherwise indicated so the charge of all ions must total 0.

Each cobalt must have a positive 2 charge so that becomes the Roman numeral necessary to name this compound.

This compound is named cobalt(II) nitride.

The compound  pairs a non-metal element with a non-metal element thus to name this compound prefixes must be used. 

The following are prefixes used for non-metal to non-metal compounds

1- mono

2- di

3- tri

4- tetra

5- penta

6- hexa

7- hepta

8- octa

9- nona

10- deca

In naming a binary non-metal to non-metal compound the proper prefix goes in front of each name. The second element drops the ending and gets an -ide ending. If the second element starts with an o the a on the prefix can be dropped. There are 3 bromines and 8 oxygens, so the name becomes:

tribromine octoxide

You may recognize that this molecule is very similar to the acetate ion, , which results upon deprotonation of acetic acid. Acetic acid is a commonly used weak acid in chemistry.

The compound name is strontium chloride because this is an ionic compound with no transition metal. Remember: ionic compounds do not have prefixes ("di," "tri," or "penta"). When assigning oxidation numbers for the atoms of this molecule, we find that strontium has a charge of  and chlorine has a charge of . Since this is a neutral molecule, two atoms of chlorine are needed to balance one atom of strontium. Since strontium is not a transition metal, it can only have a charge of , so there is no need to specify a charge in Roman numerals, which is the case for transition metal-containing compounds.

 consists of a  ion bonded to a ion, which is a perchlorate ion. Hence, the name of the ion's corresponding acid would be perchloric acid.

In this case, we have an ionic compound. Calcium  is bound ionically to the polyatomic ion, carbonate . Thus, the name of this compound is calcium carbonate.