Electron Counting: Videos & Practice Problems

Electron counting is a fundamental concept in main group chemistry, particularly when applying the octet rule to assess the reactivity of elements. The octet rule states that atoms are most stable when they have eight electrons in their valence shell, similar to noble gases. When an element has fewer than eight electrons, it can act as a Lewis acid by accepting an electron pair, thereby moving closer to achieving a stable electron configuration.

For example, in the compounds formaldehyde, nitrogen trichloride, and water, the central atoms (carbon, nitrogen, and oxygen, respectively) fulfill the octet rule through various combinations of sigma bonds, pi bonds, and lone pairs. In formaldehyde, carbon forms four bonds, while nitrogen in nitrogen trichloride has three single bonds and a lone pair, and oxygen in water has two lone pairs and two single bonds. Each of these arrangements allows the central atom to achieve a stable electron configuration.

Understanding electron counting is also crucial for analyzing transition metal complexes, as it helps determine their stability and reactivity. The electron count for a transition metal complex can be calculated using the formula:

Electron Count = Valence of Metal (M) - Charge of Metal Complex (Q_m) + X Type Ligands + 2 × L Type Ligands

In this equation, the valence of the transition metal (M) is derived from the sum of its s and d orbital electrons. For instance, nickel has a condensed electron configuration of argon 4s²3d⁸, giving it a total of 10 valence electrons. The charge of the metal complex (Q_m) is considered based on the overall charge of the complex, while X type ligands donate one electron each, and L type ligands donate a lone pair of two electrons.

For example, if we consider a nickel complex with no overall charge and two chlorine ligands (X type) and two phosphine ligands (L type), the calculation would be as follows:

Electron Count = 10 - 0 + 2 + 2 × 2 = 16

This calculation illustrates that the electron count for the transition metal complex is 16. It is essential to remember that when calculating the electron count, the transition metal should be considered in its neutral form, regardless of the charge it carries in the complex.

In summary, mastering electron counting is vital for understanding the behavior of both main group elements and transition metals. The octet rule guides the stability of main group elements, while transition metals can exceed this rule due to their d orbitals. By applying the electron counting formula, one can effectively analyze the stability and reactivity of transition metal complexes.

To determine the electron count of a complex ion, such as one with cobalt connected to six cyanide ligands and an overall charge of -3, follow a systematic approach. First, identify the electron configuration of cobalt, which is represented as Ar 4s²3d⁷. This configuration indicates that cobalt has a total of 9 valence electrons when considering both the 4s and 3d orbitals.

Next, adjust this count based on the overall charge of the complex. Since the complex has a charge of -3, you will add 3 to the initial electron count. Additionally, consider the contribution from the ligands. In this case, cyanide ions (CN^-) are classified as x-type ligands, and there are 6 of them. Since x-type ligands contribute 1 electron each, you add 6 to the total.

The calculation can be summarized as follows:

Electron Count = Valence Electrons of Cobalt + Charge of Complex + Contribution from Ligands

Electron Count = 9 + 3 + 6 = 18 electrons

This method can be applied to any transition metal complex. Remember to treat the transition metal as neutral for the purpose of counting valence electrons, account for the overall charge of the complex, and identify the types of ligands present. By following this formula, you can accurately determine the electron count for various transition metal complexes.

Now, as a practice exercise, try calculating the electron count for another complex on your own. Once you have your answer, compare it with the provided example to check your understanding.