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Question Video: Determining Chemical Formula from Successive Ionization Energies Chemistry

The following table shows the data of successive ionization energies of a metal M. What is the likely formula of the sulfide compound formed in the reaction between sulfur and metal M? [A] M₂S₃ [B] MS [C] M₂S [D] MS₂

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Video Transcript

The following table shows the data of successive ionization energies of a metal M. What is the likely formula of the sulfide compound formed in the reaction between sulfur and metal M? (A) M2S3, (B) MS, (C) M2S, or (D) MS2.

The table given lists the successive ionization energies of an unknown metal M from the first ionization energy to the eighth. The first ionization energy can be defined as the amount of energy required to remove the most loosely bound electron completely from an isolated gaseous atom. This definition holds true for the successive ionization energies of the atom changed by which electron is removed. For example, the second ionization energy is the amount of energy required to remove the second most loosely bound electron completely from an isolated gaseous ion.

These successive ionization energies provide a pattern that will allow us to identify the group number in which metal M is found on the periodic table. Group one elements with one valence electron generally have a first ionization energy that is the lowest of their successive ionization energies. This is due to the single valence electron in the outermost energy shell, which is the most loosely bound electron and requires the least amount of energy to remove. There is a noticeably large increase between the first and second ionization energies. This second electron removed is now one energy shell closer to the nucleus and more tightly bound, therefore requiring more energy to remove.

After the second ionization energy, group one elements have a general increase in successive ionization energies with large increases between energy levels. Group two elements see a similar pattern. However, the noticeably large increase occurs between the second and third ionization energies. This is because in a group-two element, the third electron removed is located on an energy shell closer to the nucleus and requires more energy to remove. This pattern would continue with the large increase in ionization energy occurring between the final electron in the valence shell and the first electron removed from a core energy shell.

Let’s have a look at how the successive ionization energies of metal M compare. We can see that the ionization energy generally increases. We can look at the factor by which each consecutive ionization energy increases, indicated by the numbers in pink below the arrows. We can see that the largest increase occurs between the second and third ionization energies. This indicates that metal M is likely in group two of the periodic table. This means that metal M would have two valence electrons and a common ion charge state of two plus.

Sulfur is in group 16 of the periodic table and has six valence electrons, forming a sulfide ion with a charge state of two minus. For metal M and sulfur to form a neutral compound overall, there would be one metal M ion present for every one sulfide ion. Therefore, given the successive ionization energies of metal M, the most likely formula of the sulfide compound formed in the reaction between sulfur and metal M is answer choice (B), MS.

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