Download The Ionic Bond and more Exercises Chemistry in PDF only on Docsity!
The Ionic Bond
Model 1: A Cation and an Anion Form an Ionic Bond
Recall that an ionic bond is formed between two atoms when the atoms involved undergo a transfer of one (or more) electrons to produce two charged species – a positive ion (or cation ) and a negative ion (or anion ). In many cases, the charge of an ion can be predicted by considering the electronegativity of the atom and its electron configuration. Atoms which have loosely held electrons tend to form positive ions, whereas atoms which can hold additional electrons relatively strongly tend to form negative ions.
Critical Thinking Questions
- a) Determine the core charge and valence shell for Na and Na+. Draw diagrams for each element.
b) Based on your answer to part a), from which species will it be easier to remove an electron: Na and Na+. Explain your reasoning clearly.
c) The ions formed in compounds from Group 1 atoms (the alkali metals, such as Na) are almost exclusively M+^ ions rather than M2+^ ions. Explain this result based on your answers to parts a) and b).
- The ions formed in molecules from Group 2 atoms (the alkaline earth metals, such as Mg) are almost exclusively M2+ ions rather than M3+^ ions. Explain this result in a manner analogous to your analysis from CTQ 1 (critical thinking question).
- a) Determine the core charge and valence shell for F-, and F2-. Draw diagrams for each element.
b) Based on your answer to part a) above, explain why an attempt to add an electron to F-^ does not result in the formation of F2-; that is, the “added” electron does not stay “attached” to the original F- ion.
Model 2: Ionic Bonds and Coulomb’s Law
Recall that in an ionic bond there is no sharing of electrons, but there is a strong attraction between the two ions due to the Coulombic force, as described by equation (1):
Here, q 1 and q 2 are the charges on the ions and the d is the distance between the centers of the two ions. Ionic compounds tend to be solids at room temperature, with high melting points (generally 500-3000°C). In contrast with the discrete molecules formed when covalent bonds are present (for example: H 2 , H 2 O, CH 4 ), ionic compounds tend to exist as huge, three-dimensional networks of ions of opposite charge that are held together by ionic bonds. Figure 1 below shows an example for the ionic compound consisting of sodium ions and chloride ions. The simplest whole-number ration of sodium ions to chloride ions is 1:1 in this structure, so the formula for the compound is written as NaCl.
Figure 1. The three dimensional network structure of NaCl.
Critical Thinking Questions
- In NaCl, what are the charges on the ions? Explain your reasoning.
- In Figure 1, the two different types of ions are represented by spheres of different sizes. Which spheres represent the sodium ions and which spheres represent the chloride ions? Show this by writing “+” charge on sodium and “-“ charge on the chlorides in Figure 1.
- a) What is the charge on the cation formed from Mg? _____________________________
b) What is the charge on the anion formed from O? (called the oxide ion) __________________
c) What is the formula for the ionic compound containing magnesium ions and oxide ions? __________________
Equation 1
Exercises
- A representative group metal (not a transition metal), M, reacts with chlorine and oxygen to form ionic compounds with the formula MCl4 and MO2. Propose a possible identity for the metal, M. Explain your reasoning.
- Predict which ionic compound has the higher melting point in each of the following pairs: (circle compound)
a) NaCl and NaBr b) NaCl and KCl c) MgO and CaO
d) KCl and CaO e) NaCl and MgS f) NaCl and NaNO 3
g) KBr and LiF
- The lattice energy is the amount of energy needed to completely separate (break apart) the ions in one mole of an ionic compound. Indicate which ionic compound is expected to have the larger lattice energy in each of the following pairs: (circle)
a) NaCl and NaBr b) NaCl and KCl c) MgO and CaO
d) KCl and CaO e) NaCl and MgS f) NaCl and NaNO 3
g) KBr and LiF
- Rank the following compounds in order of increasing melting point and explain your reasoning:
CaO CaS KCl K 2 S
- For each of the following, which compound is expected to have the highest melting point? (circle)
a) LiF; LiCl; NaF; NaCl; KI
b) NaF; NaCl; CaS; CaO
c) Na 2 SO 4 ; K 2 SO 4 ; CaSO 4 ; BaSO 4
d) LiF; CaO; BaO; Al 2 O 3
e) H 2 O; NH 3 ; N 2 ; CaSO 4 ; O 2
- Use grammatically correct sentences to describe the differences between covalent and ionic bonding.
