Question

Refer to the periodic table and predict which of the following ions are isoelectronic with the noble gas krypton: (a) \(\mathrm{K}^{+}\) (b) \(\mathrm{Sr}^{2+}\) (c) \(\mathrm{Cl}^{-}\) (d) \(\mathrm{Se}^{2-}\)

Short answer

\(\mathrm{Sr}^{2+}\) and \(\mathrm{Se}^{2-}\) are isoelectronic with krypton.

Step-by-step solution

Understand Isoelectronic

To determine if ions are isoelectronic with krypton, recall that isoelectronic species have the same number of electrons. Krypton has 36 electrons.

Calculate Electrons in \(\mathrm{K}^{+}\)

Potassium (K) is element 19 with 19 electrons. The ion \(\mathrm{K}^{+}\) loses one electron, having 18 electrons in total.

Calculate Electrons in \(\mathrm{Sr}^{2+}\)

Strontium (Sr) is element 38 with 38 electrons. The ion \(\mathrm{Sr}^{2+}\) loses two electrons, resulting in 36 electrons.

Calculate Electrons in \(\mathrm{Cl}^{-}\)

Chlorine (Cl) is element 17 with 17 electrons. The ion \(\mathrm{Cl}^{-}\) gains one electron, resulting in 18 electrons.

Calculate Electrons in \(\mathrm{Se}^{2-}\)

Selenium (Se) is element 34 with 34 electrons. The ion \(\mathrm{Se}^{2-}\) gains two electrons, resulting in 36 electrons.

Compare with Krypton

Both \(\mathrm{Sr}^{2+}\) and \(\mathrm{Se}^{2-}\) have 36 electrons, making them isoelectronic with krypton.

Key concepts

Periodic Table

The periodic table is like a map of all the chemical elements that we know. Think of it as a big chart that organizes elements based on their atomic numbers, which is the number of protons in an element's nucleus.

• The periodic table arranges elements in rows, called periods, and columns known as groups or families.
• Each element on the table is represented by a unique chemical symbol and has specific properties.

One special feature of the periodic table is how it groups elements: it helps predict an element's characteristics. For example, elements in the same column often behave similarly in chemical reactions. As you move from left to right in a period, you add one more proton and one more electron. This build-up of electrons changes how elements interact with others.
Another interesting fact is that the table's design helps us understand how elements form ions, which are charged atoms formed by losing or gaining electrons in pursuit of stability. You can use the table to quickly find elements and understand their likely electron configurations and chemical behavior.

Noble Gases

Noble gases are a unique group of elements on the periodic table. They are found in Group 18 and are characterized by their extreme lack of chemical reactivity.

• Noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).
• These gases have a full set of electrons in their outer shell, making them very stable.

The outer electron shell configuration of noble gases makes them the model of stability. Because they have complete electron shells, they do not readily form chemical bonds with other elements. This characteristic of having a full electron 'octet' (eight electrons in the outer shell) means they do not seek to gain or lose electrons.
Krypton, one of the noble gases, has a stable electron configuration with 36 electrons. When other ions become isoelectronic with krypton, it means they have modified their electron count to also reach this stable arrangement. Understanding how atoms become isoelectronic with noble gases can help us grasp why certain ions have the charges they do.

Electron Configuration

Electron configuration is like the address or map that tells you how electrons are arranged in an atom. It's important for knowing how elements will interact or bond with each other.

• The configuration is expressed in terms of the principal energy levels or shells indicated by numbers (1, 2, 3, etc.) and subshells (s, p, d, f).
• For example, krypton’s electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\), indicating a full outer shell.

Understanding electron configuration helps predict the behavior of elements in chemical reactions. Atoms with full outer shells, like noble gases, are less reactive, preferring to stay as they are rather than react. Other elements will lose, gain, or share electrons to try to achieve a stable electron configuration similar to that of the nearest noble gas.
For ions like \(\mathrm{K}^{+}\), \(\mathrm{Sr}^{2+}\), \(\mathrm{Cl}^{-}\), and \(\mathrm{Se}^{2-}\), examining their electron configurations allows us to determine if they are isoelectronic with krypton. The ions modify their electron counts to reach a noble gas-like stability, either by losing electrons (becoming positively charged) or gaining electrons (becoming negatively charged). This is why understanding electron configurations is crucial for predicting the formation of stable ions.