Question

Which one of the following groupings represents a collection of isoelectronic species? (At. number of \(\mathrm{Cs}=55, \mathrm{Br}=35)\) (a) \(\mathrm{Na}^{+}, \mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}\) (b) \(\mathrm{N}^{3-}, \mathrm{F}^{-}, \mathrm{Na}^{+}\) (c) \(\mathrm{Be}, \mathrm{Al}^{3+}, \mathrm{F}^{-}\) (d) \(\mathrm{Ca}^{2+}, \mathrm{Cs}^{+}, \mathrm{Br}\)

Short answer

The isoelectronic species are in option (b): \( \mathrm{N}^{3-}, \mathrm{F}^{-}, \mathrm{Na}^{+} \).

Step-by-step solution

Define Isoelectronic Species

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. To find isoelectronic species, we must determine the electron count of each option's species.

Calculate Electrons for Option (a)

- \( \mathrm{Na}^{+} \): Sodium has 11 electrons; losing 1 electron gives 10 electrons. - \( \mathrm{Ca}^{2+} \): Calcium has 20 electrons; losing 2 electrons gives 18 electrons. - \( \mathrm{Mg}^{2+} \): Magnesium has 12 electrons; losing 2 electrons gives 10 electrons.

Calculate Electrons for Option (b)

- \( \mathrm{N}^{3-} \): Nitrogen has 7 electrons; gaining 3 electrons gives 10 electrons. - \( \mathrm{F}^{-} \): Fluorine has 9 electrons; gaining 1 electron gives 10 electrons. - \( \mathrm{Na}^{+} \): Previously calculated as 10 electrons.

Calculate Electrons for Option (c)

- \( \mathrm{Be} \): Beryllium has 4 electrons. - \( \mathrm{Al}^{3+} \): Aluminum has 13 electrons; losing 3 electrons gives 10 electrons. - \( \mathrm{F}^{-} \): Previously calculated as 10 electrons.

Calculate Electrons for Option (d)

- \( \mathrm{Ca}^{2+} \): Previously calculated as 18 electrons. - \( \mathrm{Cs}^{+} \): Cesium has 55 electrons; losing 1 electron gives 54 electrons. - \( \mathrm{Br} \): Bromine has 35 electrons.

Identify Isoelectronic Species

From the calculations: - Option (a): 10, 18, 10 electrons (not isoelectronic) - Option (b): All species have 10 electrons (isoelectronic) - Option (c): 4, 10, 10 electrons (not isoelectronic) - Option (d): 18, 54, 35 electrons (not isoelectronic). Thus, option (b) contains isoelectronic species.

Key concepts

Electron Count Calculation

Understanding how to calculate the number of electrons in atoms and ions is essential for solving many chemistry problems. Electrons are subatomic particles that orbit the nucleus of an atom, balancing the positive charge of protons.

To determine if species are isoelectronic, you start by calculating the total number of electrons each species has. This includes both neutral atoms and ions:

• For neutral atoms, the number of electrons is equal to the atomic number.
• For cations (positively charged ions), subtract the charge from the atomic number.
• For anions (negatively charged ions), add the charge to the atomic number.

Let's look at specific examples to illustrate this process. For sodium (\(\mathrm{Na}^+\)), the atomic number is 11. Since it's a cation with a +1 charge, one electron is subtracted, leaving 10 electrons. For nitride (\(\mathrm{N}^{3-}\)), nitrogen normally has 7 electrons, but with a charge of -3, there are 10 electrons in total. The ease with which electron counts can be adjusted allows us to identify isoelectronic species easily.

Atomic Number

The atomic number is a fundamental property of an element. It indicates the number of protons in the nucleus, which in turn determines the number of electrons in a neutral atom. The atomic number is unique to each element and essentially provides the identity of the element.

In a periodic table, elements are arranged in order of increasing atomic number. This arrangement reflects their electron configuration and is crucial for predicting the behavior of elements in chemical reactions.

• The atomic number helps determine the electron arrangement, which dictates how an element interacts with others.
• It also assists in predicting the formation of ions. For example, elements far from completing an octet like sodium (atomic number 11) will often lose electrons to become stable.

When calculating the number of electrons in ions, knowing the atomic number helps adjust for any gained or lost electrons. For beryllium (\(\mathrm{Be}\), atomic number 4), the atomic number tells us there are 4 electrons in its neutral state.

Ions and Electrons

Ions are atoms or molecules that have gained or lost one or more electrons, resulting in a net charge. The process of forming ions is driven by the desire to achieve a more stable electron configuration, often resembling the nearest noble gas configuration.

Understanding how ions form plays a crucial role in grasping chemical reactions and bonding:

• Cations form by losing electrons. This loss results in a positive charge. For instance, calcium (\(\mathrm{Ca}^{2+}\)) loses two electrons to form a stable ion.
• Anions form by gaining electrons, which imparts a negative charge. For example, bromide (\(\mathrm{Br}^-\)) gains an electron to achieve a full outer shell of electrons.
• Isoelectronic species contain the same number of electrons but may have different charges and nuclear structures.

The stability of ions significantly impacts the physical and chemical properties of compounds they form. For instance, ions in a crystal lattice tend to have more stability due to the interactions between their charges, providing the basis for the solid structure we observe in many ionic compounds.