The given compound is the formula for gaseous hydrogen. Hydrogen atoms have only one valence electron and will form covalent bonds with other hydrogen atoms to fill the s orbital, generating a covalent bond. This tells us that it is a molecular bond, since no ionic bonds are present.

Binary compounds always contain two elements. This compound contains only one element, and cannot be considered binary. Instead, it is referred to as "diatomic," meaning that the molecule is composed of two atoms of the same element. Most diatomic molecules are gaseous, and include , , and (among others). The noble gases will not form diatomic particles.

Naming diatomic molecules is simple, since there is only one element involved. Most commonly,  is simply named hydrogen or gaseous hydrogen. Keep this in mind when working with reactions. If a reaction occurs between nitrogen and oxygen, it is occurring between their diatomic molecules, not the individual atoms themselves.

Binary molecular compounds contain two non-metal elements joined by covalent bonds. Conventions for naming these compounds require that prefixes be used to denote the subscript of each element in the compound, telling the number of atoms in the name. The prefix for one, mono-, is only applied to the second element in the name, assuming that it has no subscript (one atom per molecule). The suffix -ide is attached to the second element in the compound to complete the name.

When determining which element should be first, you must reference the periodic table. Elements located more to the left and toward the bottom of the non-metals will appear first in the compound name. Carbon monoxide is a simple example of the leftmost element coming first, while bromine monochloride is an example of the bottommost element coming first.

The following is a list of prefixes used in naming molecular compounds:

One: mono-

Two: di-

Three: tri-

Four: tetra-

Five: penta-

Six: hexa-

Seven: hepta-

Eight: octa-

Nine: nona-

Ten: deca-

Nitrogen monoxide, also known as nitric oxide or, differs from the other listed compounds in the group because it is a molecular compound (two non-metals). The elements nitrogen and oxygen both have high electronegativities and will create a covalent bond, sharing the electrons between the two nuclei.

All the other answer choices are ionic compounds, containing one cation (metal) and one anion (non-metal). The metals will have relatively low electronegativities, while the non-metals will have relatively high electronegativities. This difference will pull electrons away from the metal (forming a cation) and toward the non-metal (forming an anion). The result is an ionic bond.

In binary ionic compounds, the cation is always written before the anion in the name and formula. This means that metal elements are typically followed by non-metal elements in ionic compounds. Examples are sodium chloride and calcium phosphide.

The prefix mono- is only used in molecular naming conventions, and does not apply to ionic compounds. In naming molecular compounds, this prefix will only be applied if the second compound in the formula has no subscript (only one atom per molecule). An example is bromine monochloride.

Binary ionic compounds contain only two elements, joined by ionic bonds. Polyatomic ions always contain multiple elements, and are often joined by covalent bonds. Ionic compounds that involve polyatomic ions always contain at least three elements and generally have both ionic and covalent bonds. These compounds cannot be considered binary.

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