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) BF₃, covalent (Boron Trifluoride); (b) KBr, ionic (Potassium Bromide).

Step-by-step solution

Determine Type of Bond for B and F

Boron (B) and Fluorine (F) have different electronegativities but both are nonmetals. Usually, combinations of nonmetals form covalent bonds. Therefore, the compound they form is likely to be covalent.

Identify Empirical Formula for B and F

In covalent compounds, molecules form where each atom achieves a full valence shell. Boron typically forms three bonds, and fluorine forms one bond. Thus, the empirical formula is \(\mathrm{BF_3}\).

Name the Compound for B and F

The compound formed between boron and fluorine is named based on the elements in the formula \(\mathrm{BF_3}\), thus it's called Boron Trifluoride.

Determine Type of Bond for K and Br

Potassium (K) is a metal and Bromine (Br) is a nonmetal. When metals and nonmetals combine, they typically form ionic bonds. Thus, their compound is likely ionic.

Identify Empirical Formula for K and Br

In ionic compounds, the metal (K) loses an electron becoming \(\mathrm{K^+}\) and the nonmetal (Br) gains an electron becoming \(\mathrm{Br^-}\). The empirical formula is \(\mathrm{KBr}\), showing a 1:1 ratio due to charge balance.

Name the Compound for K and Br

The compound formed between potassium and bromine is named after the ion names, resulting in Potassium Bromide.

Key concepts

Ionic Compounds

Ionic compounds form when metals and nonmetals come together. In the process, metals usually lose electrons and become positively charged ions, known as cations. Nonmetals, on the other hand, gain those electrons to become negatively charged ions, called anions. This interchange of electrons results in a strong electrostatic force that holds the ions together, forming a stable ionic bond.
In the case of Potassium (K) and Bromine (Br), the potassium atom loses one electron to become \(\mathrm{K^+}\), while the bromine atom gains an electron to become \(\mathrm{Br^-}\).

• The resulting compound is Potassium Bromide (KBr), and it showcases a perfect 1:1 ratio.
• This ratio allows for a neutral compound overall, balancing positive and negative charges.

These compounds tend to form crystalline structures and usually require high melting and boiling points to break apart the ionic lattice. They are also known for their ability to conduct electricity when dissolved in water, as the ions are free to move and conduct current efficiently.

Covalent Compounds

Covalent compounds are formed by the sharing of electrons between two or more nonmetal atoms. This happens because atoms in a covalent compound often need to achieve a stable electronic configuration, like that of noble gases. In this arrangement, each atom contributes one or more electrons to create a shared pair.
The compound formed by Boron (B) and Fluorine (F) is a great example. Both elements are nonmetals, and they opt for a covalent bonding strategy. In particular:

• Boron usually forms three bonds, corresponding to its three valence electrons.
• Fluorine forms one bond, eager to gain a single electron to complete its outer shell.

Thus, Boron Trifluoride (BF\(_3\)) develops, where Boron shares its three electrons with three fluorine atoms. This molecular strategy allows both elements to satisfy their valence electron requirements, displaying the flexibility and diversity of covalent bonding.

Empirical Formula

An empirical formula represents the simplest whole-number ratio of the elements present in a compound. It doesn't show how the atoms are arranged in the molecule or the number of atoms in a given molecule, but it is an essential way to quickly understand the elemental composition of substances.
For instance, in Boron Trifluoride (BF\(_3\)), the empirical formula is immediately recognized as BF\(_3\), meaning there are three atoms of fluorine for every one atom of boron.

• Provides simplicity by showing the smallest integer ratio.
• Useful for initial classifications and stoichiometry in chemical reactions.

In ionic compounds like Potassium Bromide (KBr), the empirical formula also highlights the balance of positive and negative charges across the ions, maintaining electrical neutrality through \(\mathrm{K^+}\) and \(\mathrm{Br^-}\). Ultimately, this formula is a cornerstone in chemistry to understand, predict, and write reactions more accurately.

Electronegativity

Electronegativity reflects the tendency of an atom to attract electrons in a bond. It plays a vital role in determining what type of bond will form between different elements. Elements with very different electronegativities are more likely to form ionic bonds, as with Potassium (K) and Bromine (Br), because the electron is effectively transferred from one element to another.
In contrast:

• Boron (B) and Fluorine (F) have electronegativities that aren't as drastically different, leading to covalent bonding instead.
• Fluorine has one of the highest electronegativities and strongly attracts electrons, affecting bond polarization.

Electronegativity values help us predict bond strengths and reactivity. They're fundamental for understanding molecule shapes and polar characteristics in compounds.