Question

For each of the following pairs of elements, state whether the binary compound they form is likely to be ionic or covalent. Write the empirical formula and name of the compound: (a) \(\mathrm{B}\) and \(\mathrm{F}\), (b) \(\mathrm{K}\) and \(\mathrm{Br}\).

Short answer

(a) \(\mathrm{B}\) and \(\mathrm{F}\) form a covalent compound named boron trifluoride (\(\mathrm{BF}_3\)). (b) \(\mathrm{K}\) and \(\mathrm{Br}\) form an ionic compound named potassium bromide (\(\mathrm{KBr}\)).

Step-by-step solution

Determine the type of bond

To determine the type of bond which will be formed by the two elements, look at their positions in the Periodic Table. Non-metals usually make covalent bonds with each other, while metals usually make ionic bonds with non-metals. (a) Both \(\mathrm{B}\) and \(\mathrm{F}\) are non-metals, hence they would form a covalent bond. (b) \(\mathrm{K}\) is a metal and \(\mathrm{Br}\) is a non-metal, hence they would form an ionic bond.

Write the empirical formulas

For covalently bonded elements, the empirical formula usually mirrors the molecule's structure. For ionic compounds, the empirical formula represents the smallest whole number ratio of ions necessary for electric charge balance. (a) \(\mathrm{BF}_3\) - this represents one boron atom sharing electrons with three fluorine atoms. (b) \(\mathrm{KBr}\) - one potassium ion and one bromide ion balance each other out electrically.

Name the compounds

The name of a covalent compound is typically composed of the names of the two atoms, with prefixes indicating the number of atoms if greater than one. For ionic compounds, the name of the positively charged ion comes first followed by the name of the negatively charged ion. (a) \(\mathrm{BF}_3\) - Boron trifluoride. (b) \(\mathrm{KBr}\) - Potassium bromide.

Key concepts

Ionic Compounds

Ionic compounds are formed through the transfer of electrons from one atom to another, resulting in the creation of ions: positively charged cations and negatively charged anions. This kind of bonding typically occurs between metals and non-metals. For example, in the binary compound formed between potassium (K) and bromine (Br), potassium acts as a metal and donates its valence electron to bromine, a non-metal. This transfer leads to the creation of a positively charged potassium ion (\(\mathrm{K}^+\) ) and a negatively charged bromide ion (\(\mathrm{Br}^-\) ).- Ionic compounds are usually solid crystals at room temperature.- They often have high melting and boiling points compared to covalent compounds.- In solution or molten form, they can conduct electricity because of the movement of ions.These characteristics make ionic compounds an essential category of chemical compounds that help to understand the behavior of elements in different contexts.

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms, allowing each atom to achieve a full outer shell of electrons, generally known as achieving a noble gas configuration. This type of bonding occurs mainly between non-metal atoms. The compound formed by boron (B) and fluorine (F) is an excellent example; here, boron shares electrons with fluorine atoms, resulting in the formation of boron trifluoride (\(\mathrm{BF}_3\)).- In covalent bonds, atoms can share one (single bond), two (double bond), or three pairs of electrons (triple bond).- These bonds result in the formation of molecules rather than corresponding charged ions.- Covalent compounds often have lower melting and boiling points compared to ionic compounds.- They can exist in any state of matter at room temperature: gases, liquids, or solids.Understanding covalent bonding helps in grasping the molecular structures and properties of substances encountered in daily life.

Empirical Formula

The empirical formula of a compound is the simplest way to represent the elements present in that compound in their smallest whole number ratio. It does not indicate the actual number of atoms or the arrangement of atoms within the compound but provides an overview of the composition.- For ionic compounds like potassium bromide (\(\mathrm{KBr}\)), the empirical formula is straightforward because each compound is formed from equal numbers of positive and negative ions needed for charge balance.- In covalent compounds such as boron trifluoride (\(\mathrm{BF}_3\)), the empirical formula corresponds to the simplest ratio of the elements, which also reflects the molecular entity in this case.- This simplification allows chemists to quickly understand the basic makeup of a new compound.The empirical formula is a fundamental component of understanding chemical compounds and aids in predicting their properties and behaviors under different conditions.