Question

Give three ions that are isoelectronic with krypton. Place these ions in order of increasing size.

Short answer

The three ions that are isoelectronic with krypton are \(Sr^{2+}\), \(Rb^{+}\), and \(Br^-\). They are placed in order of increasing size as follows: \(Sr^{2+} < Rb^{+} < Br^{-}\).

Step-by-step solution

Identify Elements Close to Krypton

First, we need to find elements surrounding Krypton in the periodic table, as they are most likely to have ions isoelectronic to Krypton. Some examples are Rubidium (Rb, atomic number 37), Bromine (Br, atomic number 35), and Strontium (Sr, atomic number 38).

Determine the Ions

Rubidium (Rb) can lose one electron to form Rb+, Bromine (Br) can gain one electron to form Br-, and Strontium (Sr) can lose two electrons to form Sr2+. All of these ions have 36 electrons, which is the same as Krypton, so they are isoelectronic.

Determine the Size Order

Since the number of protons in the nucleus determines the attraction between electrons and the nucleus, larger atomic numbers lead to a stronger attraction and, hence, smaller ion size. Therefore, the order of increasing size would be as follows: \[Sr^{2+} < Rb^{+} < Br^{-}\] These three ions are isoelectronic with Krypton and are placed in order of increasing size: Sr2+ < Rb+ < Br-.

Key concepts

Periodic Table

The periodic table is a systematic arrangement of chemical elements based on their atomic number, electron configurations, and recurring chemical properties. Each element is positioned in the table to showcase trends in chemical behavior and reactivity. When dealing with ions that are isoelectronic with a noble gas like krypton, you specifically look for elements near krypton on the periodic table.

Noble gases are situated in the far right column of the table, characterized by their full outer electron shell, making them stable and largely non-reactive. Elements nearby, such as rubidium (Rb), bromine (Br), and strontium (Sr), can form ions by losing or gaining electrons to achieve a similar electron configuration to krypton.

Understanding the position of elements relative to one another on the periodic table helps in predicting and explaining their behavior, particularly when forming ions that mimic noble gases in terms of electron count.

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom's or ion's orbitals. It follows a specific order dictated by principles like the Pauli exclusion principle and Hund's rule, which define the arrangement energy levels and sublevels.

In the context of isoelectronic species, ions achieve the same number of electrons by either gaining or losing electrons to resemble a noble gas, which offers a stable configuration. For example, krypton has 36 electrons, hence any ions it isoelectronic with must have the same number of electrons.

- **Rubidium (Rb)** loses one electron to become Rb\(+\),
- **Bromine (Br)** gains one electron to form Br\(-\),
- **Strontium (Sr)** loses two electrons to become Sr\(^{2+}\).

Each of these ions will have the electron configuration of krypton: \[1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6.\]
This arrangement is key to understanding isoelectronic properties as they provide stability analogous to noble gases.

Atomic Size

Atomic size, or atomic radius, is determined by the distances between the nucleus and the outermost electrons of an atom or ion. In isoelectronic species, even though they possess the same number of electrons, their sizes may differ based on nuclear charge.

When examining ions isoelectronic to krypton, they differ in the number of protons in their nuclei. The nuclear charge affects how tightly electrons are held: the more protons present, the stronger the pull on electrons, which reduces ionic size. Therefore, among Sr\(^{2+}\), Rb\(+\), and Br\(-\):

- **Sr\(^{2+}\):** Has the smallest size due to a higher positive charge that pulls electrons in closer,
- **Rb\(+\):** With more protons than bromine but less pull compared to strontium ion, has an intermediate size,
- **Br\(-\):** Has extra electrons added around the same number of protons, leading to less pull per electron and therefore, the largest size.

This illustrates why, despite having identical numbers of electrons, these ions display varying sizes: the difference relies on the balance of positive charge in the nucleus.