Question

Compare the electron affinity of fluorine to the ionization energy of sodium. Does the process of an electron being "pulled" from the sodium atom to the fluorine atom have a negative or a positive \(\Delta E ?\) Why is NaF a stable compound? Does the overall formation of NaF have a negative or a positive \(\Delta E ?\) How can this be?

Short answer

In summary, the electron affinity of fluorine (-328.0 kJ/mol) and the ionization energy of sodium (+495.8 kJ/mol) result in an endothermic electron transfer process with a positive energy change of +167.8 kJ/mol. However, NaF is a stable compound due to the strong ionic bond between Na+ and F-, which releases energy when formed. This leads to a negative overall energy change, making the formation of NaF an exothermic and favorable process.

Step-by-step solution

Understand electron affinity and ionization energy

Electron affinity is the energy change that occurs when an electron is added to a neutral atom, forming a negative ion. A higher electron affinity indicates that an atom can easily accept an electron. Ionization energy is the energy required to remove an electron from a neutral atom, forming a positive ion. A higher ionization energy implies that it is more challenging to remove an electron from an atom.

Compare electron affinity and ionization energy

The electron affinity of fluorine (F) is -328.0 kJ/mol, while the ionization energy of sodium (Na) is +495.8 kJ/mol. The negative value of electron affinity for fluorine indicates that it can easily gain an electron. The positive value of ionization energy for sodium shows that energy needs to be supplied to remove an electron from it.

Analyze the energy change between sodium and fluorine

When an electron is transferred from sodium to fluorine, the overall energy change for the process can be calculated as follows: \[\Delta E = E_{electron \: affinity} + E_{ionization \: energy}\] Substitute the values for electron affinity and ionization energy: \[\Delta E = (-328.0 \: kJ/mol) + (+495.8 \: kJ/mol) = +167.8 \: kJ/mol\] The positive value of the energy change indicates that the process is endothermic, meaning it requires energy input.

Discuss the stability of NaF and its energy change

NaF is a stable compound because it forms an ionic bond between Na+ and F-. Although the transfer of an electron from Na to F is an endothermic process, the stability of NaF is due to the strong electrostatic attraction between the positive Na+ ion and the negative F- ion, which releases energy when the ionic bond forms. This energy release offsets the energy required for the electron transfer, resulting in a net energy change that is negative, making the formation of NaF exothermic and favorable.

Conclusion

In conclusion, the electron affinity of fluorine is -328.0 kJ/mol, and the ionization energy of sodium is +495.8 kJ/mol. The process of an electron being transferred from sodium to fluorine is endothermic with a positive energy change of +167.8 kJ/mol. NaF is a stable compound due to the strong ionic bond formed between Na+ and F-, and the overall formation of NaF has a negative energy change, making it an exothermic process.

Key concepts

Electron Affinity

Electron affinity is a key concept in understanding how atoms interact when they form compounds. It refers to the energy change when an electron is added to a neutral atom. This process forms a negative ion. The higher the electron affinity, the more readily the atom will accept an electron.

Fluorine, for instance, has a high electron affinity of -328.0 kJ/mol. This negative value means that fluorine releases energy when it gains an electron, making it very eager to attract electrons. This property is crucial because it drives chemical reactions where electron transfer is involved.

• Fluorine's high electron affinity indicates its strong attraction for electrons.
• The negative sign shows energy release during electron addition.

This release of energy makes it a favorable process that contributes to forming stable compounds.

Ionization Energy

Ionization energy is the energy required to remove an electron from a neutral atom, creating a positive ion. A high ionization energy means more energy is needed to remove an electron.

Sodium (Na) has an ionization energy of +495.8 kJ/mol, which is positive, indicating that energy must be supplied to remove its outer electron.

• A positive ionization energy suggests resistance to electron removal.
• Sodium's relatively lower ionization energy compared to other elements makes it reactive, especially with halogens like fluorine.

Understanding ionization energy helps explain why certain elements, like sodium, readily lose electrons to form positive ions in reactions.

Ionic Bonds

When chemical reactions occur between metals and nonmetals, ionic bonds often form. An ionic bond is an attractive force between oppositely charged ions. In the case of NaF, sodium (Na) loses an electron and becomes positively charged ( ^+), while fluorine (F) gains an electron and becomes negatively charged ( ^-).

The strong attraction between Na ^+ and F ^- ions forms a stable ionic bond.

• Ionic bonds result from the complete transfer of electrons from a metal to a nonmetal.
• The electrostatic force between the ions is what holds the compound together.

This bond is crucial in many compounds, providing stability and a lower energy state than the separated ions.

NaF Stability

NaF, or sodium fluoride, is a stable chemical compound due to the strong ionic bond between sodium and fluorine. Despite the fact that forming NaF requires an energy input of +167.8 kJ/mol (endothermic) when moving the electron, the overall process is exothermic, releasing energy.

Here’s why:

• The strong electrostatic attraction between Na ^+ and F ^- ions releases a significant amount of energy.
• This energy release offsets the energy required to move the electron from Na to F.

The net energy change becomes negative, making the formation of NaF favorable and stable. This energy balance explains why NaF exists as a stable, solid compound under standard conditions.