Question

Which one of the following groups represents a collection of isoelectronic species? (At, nos.: Cs-55, Br-35) (a) \(\mathrm{Na}^{*}, \mathrm{Ca}^{2+}, \mathrm{M} \mathrm{g}^{2+}\) (b) \(\mathrm{N}^{3-}, \mathrm{F}, \mathrm{Na}^{+}\) (c) \(\mathrm{Be}, \mathrm{Al}^{3+}, \mathrm{Cl}^{-}\) (d) \(\mathrm{Ca}^{2+}, \mathrm{Cs}^{+}, \mathrm{Br}\)

Short answer

Option (d) represents a collection of isoelectronic species: \text{Ca}^{2+}, \text{Cs}^{+}, and Br all have 36 electrons.

Step-by-step solution

Understanding Isoelectronic Species

Identify that isoelectronic species are atoms, ions, or molecules that have the same number of electrons. To determine if the species given are isoelectronic, count the electrons each species has considering their atomic numbers and charges.

Count Electrons for Each Option

Count the total number of electrons for each species given in all options, using their atomic numbers and ionic charges. A neutral atom has the same number of electrons as the atomic number. The charge indicates how many electrons are lost (positive charge) or gained (negative charge).

Compare Electron Counts

Compare the electron counts for all species within each option to see if they match. The group with the same number of electrons for all species is the collection of isoelectronic species.

Analyze Each Option

Option (a): Na (11 electrons) becomes Na* (10 electrons), Ca (20 electrons) as Ca2+ has 18 electrons, Mg (12 electrons) as Mg2+ also has 10 electrons. They are not isoelectronic. Option (b): N (7 electrons) as N3- has 10 electrons, F (9 electrons) - neutral, has 9 electrons, Na+ (10 electrons). They have different electron counts. Option (c): Be (4 electrons) - neutral, has 4 electrons, Al3+ (13 electrons) has 10 electrons, Cl- (17 electrons) has 18 electrons. They are not isoelectronic. Option (d): Ca2+ (20 electrons) has 18 electrons, Cs+ (55 electrons) has 54 electrons, Br (35 electrons) - neutral, has 35 electrons. Cs+ and Br are isoelectronic, having 36 electrons, but Ca2+ is not.

Key concepts

Atomic Number

The atomic number is one of the fundamental attributes of an element. It is denoted as 'Z' and represents the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the atomic number also indicates the number of electrons in a neutral atom. For instance, carbon has an atomic number of 6, which means that a neutral carbon atom has 6 protons and 6 electrons.
Understanding the atomic number is crucial when discussing isoelectronic species because it serves as the starting point for determining electron count. While the atomic number remains constant for a given element, the number of electrons can change when atoms form ions. This leads to the concept of ionic charge, which greatly affects an element's electron count and thus its isoelectronic nature with other species.

Ionic Charge

The ionic charge is the electric charge an atom carries when it loses or gains electrons to form an ion. When an atom loses one or more electrons, it becomes a positively charged ion, or cation, indicated by a plus sign (+) next to the element symbol and charge number; for example, \( \text{Na}^{+} \) represents a sodium ion with a +1 charge.
Conversely, if an atom gains electrons, it becomes a negatively charged ion, or anion, indicated by a minus sign (-) and the charge number; such as \( \text{Cl}^{-} \), which signifies a chloride ion with a -1 charge. The ionic charge is instrumental in determining the electron count of an ion; positive charges mean electrons have been lost, while negative charges mean electrons have been gained. In terms of isoelectronic species, ions must adjust their electron counts according to their ionic charges to maintain the same number of electrons.

Electron Count

Electron count refers to the total number of electrons around the nucleus of an atom or ion. While neutral atoms have electron counts equal to their atomic numbers, ions have electron counts that are either greater or lesser, depending on their ionic charges.
For example, a \( \text{Ca}^{2+} \) ion, which originates from a calcium atom with an atomic number of 20, will have an electron count of 18 due to the loss of two electrons. Maintaining or comparing electron counts is the essence of determining if species are isoelectronic. For instance, \( \text{O}^{2-} \) and \( \text{Ne} \) are isoelectronic because they both have an electron count of 10 despite having different atomic numbers and ionic charges. It's the equality in electron counts, not atomic numbers or charges, that defines isoelectronic species.