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Chapter 9: Problem 20

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

Expert verified
The bond in \(BF_3\) (boron trifluoride) and \(KBr\) (potassium bromide) is likely to be ionic.

Step by step solution

01

Understand Bonds and Electronegativity

Ionic bonds typically form when the electronegativity difference between a pair of atoms is greater than 1.7. On the other hand, covalent bonds generally form when the difference is less than 1.7.
02

Determining Type of Bond and Writing Empirical Formula for (a) B and F

Boron (B) has an electronegativity of 2.04 and Fluorine (F) has an electronegativity of 3.98. The difference is 1.94, which is greater than 1.7, indicating that the bond is likely ionic. The empirical formula will be \(BF_3\) as boron can form three bonds, and the compound is named boron trifluoride.
03

Determining Type of Bond and Writing Empirical Formula for (b) K and Br

Potassium (K) has an electronegativity of 0.82 and Bromine (Br) has an electronegativity of 2.96. The difference is 2.14, which is greater than 1.7, indicating that the bond is likely ionic. The empirical formula will be \(KBr\) and the compound named potassium bromide.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electronegativity
Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. Understanding electronegativity differences between elements is crucial for predicting the nature of the bond they form.
  • If the electronegativity difference between two elements is greater than 1.7, the bond is typically ionic.
  • If the difference is less than 1.7, the bond is usually covalent.
This concept helps chemists understand how elements will interact with each other, determining the type of compound that will result.
Ionic Compounds
Ionic compounds occur when atoms transfer electrons to achieve stable electron configurations, often forming between metals and non-metals due to their differing electronegativities. In the step-by-step exercise:
  • Potassium (K) and Bromine (Br) form an ionic compound because their electronegativity difference is 2.14, greater than 1.7.
  • This results in the empirical formula of potassium bromide being written as \(KBr\).
The metal atom donates electrons to the non-metal, resulting in a stable and highly structured crystal lattice.
Covalent Compounds
Covalent compounds form when two or more non-metal atoms share electrons to fill their outer shells and achieve stability. This bond type happens when the electronegativity difference between the atoms is less than 1.7. However, in our exercise:
  • Boron (B) and Fluorine (F) form a bond with a difference of 1.94, suggesting a tendency toward ionic, but due to boron's small size and unique chemistry, it still shares some covalent characteristics.
  • Thus creating boron trifluoride \(BF_3\).
Understanding covalent bonding is key to predicting the physical properties of the compound, which are usually softer and have lower melting points than ionic compounds.
Empirical Formula
An empirical formula is the simplest whole-number ratio of atoms present in a compound. Knowing how to determine this form of a formula is essential to help us express the composition of compounds concisely:
  • For boron and fluorine, the empirical formula \(BF_3\) indicates one boron atom combines with three fluorine atoms.
  • Potassium and bromine create a one-to-one ratio, leading to the empirical formula \(KBr\).
This formula provides crucial information about the proportion of elements within a compound, acting as the compound's simplest chemical identity.
Compound Naming
Naming compounds follows systematic rules based on the types of elements involved and their respective ratios:
  • Boron trifluoride \(BF_3\) is named by prioritizing the positive element followed by the negative one, with fluorine taking a prefix indicating three fluorine atoms.
  • Potassium bromide \(KBr\) is named straightforwardly as the metal name comes first, followed by the non-metal with an "-ide" suffix.
Understanding these naming conventions is essential, helping chemists effectively communicate the structure and proportions of compounds across different contexts.

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Most popular questions from this chapter

For each of the following organic molecules draw a Lewis structure in which the carbon atoms are bonded to each other by single bonds: (a) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{C}_{4} \mathrm{H}_{10},\) (c) \(\mathrm{C}_{5} \mathrm{H}_{12}\). For (b) and (c), show only structures in which each \(\mathrm{C}\) atom is bonded to no more than two other \(\mathrm{C}\) atoms.

Nitroglycerin, one of the most commonly used explosives, has the structure The decomposition reaction is $$ \begin{array}{r} 4 \mathrm{C}_{3} \mathrm{H}_{5} \mathrm{~N}_{3} \mathrm{O}_{9}(l) \longrightarrow \\ 12 \mathrm{CO}_{2}(g)+10 \mathrm{H}_{2} \mathrm{O}(g)+6 \mathrm{~N}_{2}(g)+\mathrm{O}_{2}(g) \end{array} $$ The explosive action is the result of the heat released and the large increase in gaseous volume. (a) Calculate the \(\Delta H^{\circ}\) for the decomposition of one mole of nitroglycerin using both standard enthalpy of formation values and bond enthalpies. Assume that the two \(\mathrm{O}\) atoms in the \(\mathrm{NO}_{2}\) groups are attached to \(\mathrm{N}\) with one single bond and one double bond. (b) Calculate the combined volume of the gases at STP. (c) Assuming an initial explosion temperature of \(3000 \mathrm{~K}\), estimate the pressure exerted by the gases using the result from (b). (The standard enthalpy of formation of nitroglycerin is \(-371.1 \mathrm{~kJ} / \mathrm{mol} .\) )

Write a Lewis structure for \(\mathrm{SbCl}_{5}\). Does this molecule obey the octet rule?

The \(\mathrm{N}-\mathrm{O}\) bond distance in nitric oxide is \(115 \mathrm{pm}\) which is intermediate between a triple bond (106 pm) and a double bond (120 pm). (a) Draw two resonance structures for \(\mathrm{NO}\) and comment on their relative importance. (b) Is it possible to draw a resonance structure having a triple bond between the atoms?

Write the Lewis dot symbols of the reactants and products in the following reactions. (First balance the equations.) (a) \(\mathrm{Sr}+\mathrm{Se} \longrightarrow \mathrm{SrSe}\) (b) \(\mathrm{Ca}+\mathrm{H}_{2} \longrightarrow \mathrm{CaH}_{2}\) (c) \(\mathrm{Li}+\mathrm{N}_{2} \longrightarrow \mathrm{Li}_{3} \mathrm{~N}\) (d) \(\mathrm{Al}+\mathrm{S} \longrightarrow \mathrm{Al}_{2} \mathrm{~S}_{3}\)
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