Question

Predict which of the following polyatomic anions has an ionic charge of 1-. (Hint: The total number of valence electrons must be an even number.) (a) thiocyanate ion, SCN?- (b) thiosulfate ion, \(\mathrm{S}_{2} \mathrm{O}_{3}^{\text {?- }}\)

Short answer

The thiocyanate ion (SCN-) has an ionic charge of 1-.

Step-by-step solution

Determine Valence Electrons of SCN-

The thiocyanate ion (SCN-) contains one sulfur (S), one carbon (C), and one nitrogen (N) atom. The valence electrons for each atom are: S = 6, C = 4, N = 5. Sum the electrons: \(6 + 4 + 5 = 15\). Since the ion has a charge of 1-, add one more electron: \(15 + 1 = 16\) valence electrons.

Determine Valence Electrons of S_{2}O_{3}^{?-}

The thiosulfate ion contains two sulfur (S) atoms and three oxygen (O) atoms. The valence electrons for each are: S = 6, O = 6. Calculate the total for neutral atoms: \(2 \times 6 + 3 \times 6 = 12 + 18 = 30\). Since there's a charge of 1-, add one more electron: \(30 + 1 = 31\) valence electrons.

Analyze Total Valence Electrons for Evenness

Compare the total valence electrons: SCN- has 16 (even number) and \(S_{2}O_{3}^{?-}\) has 31 (odd number). According to the hint, the polyatomic anion with an even number of total valence electrons is more likely to have a 1- ionic charge.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in determining how an atom can bond with others. They are especially important for understanding the behavior of polyatomic anions. For each atom in a molecule or ion, we need to consider its valence electrons as these dictate the formation of chemical bonds.

• For sulfur (S), with the atomic number 16, there are 6 valence electrons.
• For carbon (C), with an atomic number of 6, there are 4 valence electrons.
• For nitrogen (N), with an atomic number of 7, there are 5 valence electrons.
• Oxygen (O), atomic number 8, also has 6 valence electrons.

Understanding valence electrons helps predict molecular geometry, reactivity, and the types of bonds that will form, particularly in ions like thiocyanate and thiosulfate.

Ionic Charge

Ionic charge is a property of an ion that defines its overall charge. This is key for understanding why certain ions combine to form neutral compounds or other ionic structures. The ionic charge results from the gain or loss of electrons in an atom or group of atoms. When an ion has gained an electron, it becomes negatively charged, as we see with many polyatomic anions. The thiocyanate ion, for instance, has an ionic charge of 1-, and it has acquired one extra electron compared to its neutral form. Understanding ionic charge helps in predicting the stability of ions and their interaction with other ionic species, which is fundamental in chemical reactions and compound formations. For example:

• The SCN- ion has acquired an extra electron to reach its 1- charge.
• Similarly, adding one electron to thiosulfate, even though it results in an odd number of total valence electrons, still gives it a 1- charge.

Thiocyanate Ion

The thiocyanate ion (\(\mathrm{SCN^{-}}\)) is a polyatomic anion composed of sulfur, carbon, and nitrogen atoms. This ion plays a role in various chemical processes and structures.Key characteristics:

• The formula SCN- indicates the composition of one sulfur, one carbon, and one nitrogen atom.
• This ion has 16 total valence electrons; this is an even number, hinting at a typical formation for a 1- charge.
• Due to its composition, thiocyanate can activate various biological reactions and is known for its significance in the chemistry of organic compounds.

Thiocyanate ions are an excellent example of how valence electrons and ionic charge interact to form stable ionic structures. The even number of electrons helps stabilize the ion with the 1- charge, demonstrating the importance of electron distribution in ionic compounds.

Thiosulfate Ion

The thiosulfate ion (\(\mathrm{S_{2}O_{3}^{2-}}\)) is a well-known polyatomic anion that contains two sulfur atoms and three oxygen atoms. It finds applications in various chemical processes, including photography and gold extraction.In detail:

• The ion has two sulfur atoms and three oxygen atoms.
• Total valence electrons are calculated as follows: \[ 2 \times 6 (\text{for S}) + 3 \times 6 (\text{for O}) = 30 \]The additional electron for the charge means 31 electrons altogether.
• Notice that the total valence electrons sum to an odd number, indicating a less typical situation for stable anions and differentiating it from thiocyanate.

The thiosulfate's ionic charge showcases how electron distribution can change the properties of an ion, as well as its behavior in chemical reactions.