Question

34\. Which simple ion would each of the following elements be expected to form? Which noble gas has an analogous electron configuration to each of the ions? a. bromine, \(Z=35\) b. cesium, \(Z=55\)

Short answer

a. Bromine forms the Br\(^-\) ion and has an analogous electron configuration to the noble gas krypton (Kr). b. Cesium forms the Cs\(^+\) ion and has an analogous electron configuration to the noble gas xenon (Xe).

Step-by-step solution

Find the valence electrons of the given elements

Bromine (Br) has an atomic number (Z) of 35, which means it has 35 electrons. Its electron configuration is \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\]. Bromine has 7 valence electrons (from 4s and 4p orbitals). Cesium (Cs) has an atomic number (Z) of 55, which means it has 55 electrons. Its electron configuration is \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^1\]. Cesium has 1 valence electron (from the 6s orbital).

Determine the simple ions formed by each element

Bromine (Br) has 7 valence electrons. To achieve a stable electron configuration, it needs to gain 1 electron to fill its 4p orbital. Therefore, bromine forms an anion with a -1 charge: Br\(^-\). Cesium (Cs) has 1 valence electron. To achieve a stable electron configuration, it needs to lose this electron. Therefore, cesium forms a cation with a +1 charge: Cs\(^+\).

Find the noble gases with analogous electron configurations

Bromine forms the Br\(^-\) ion by gaining 1 electron. The electron configuration of Br\(^-\) is \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\]. This configuration is the same as the noble gas krypton (Kr), which has an atomic number (Z) of 36. Cesium forms the Cs\(^+\) ion by losing 1 electron. The electron configuration of Cs\(^+\) is \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6\]. This configuration is the same as the noble gas xenon (Xe), which has an atomic number (Z) of 54. The final answers are: a. Bromine (Br) forms the Br\(^-\) ion and has an analogous electron configuration to the noble gas krypton (Kr). b. Cesium (Cs) forms the Cs\(^+\) ion and has an analogous electron configuration to the noble gas xenon (Xe).

Key concepts

Electron Configuration

Every element has a specific electron configuration, which describes how its electrons are arranged in different energy levels or shells. This arrangement follows the Pauli exclusion principle and is important in determining how an element interacts with others.
Understanding the electron configuration helps us predict the chemical properties of an element and how it might bond with others.

• Electrons fill the lowest energy levels first, moving outward to higher ones as the lower levels are filled.
• S-orbitals can hold a maximum of 2 electrons, while p-orbitals can hold 6.
• The order starts from 1s, moving through 2s, 2p, 3s, and so forth.

For example, bromine with 35 electrons has its electron configuration listed as \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5 \]. Similarly, cesium, with 55 electrons, fills up to \[ 6s^1 \]. Both configurations inform us how these atoms gain or lose electrons to form ions.

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. These are the electrons involved in bonding, as they can be gained, lost, or shared with other atoms. The configuration of these electrons strongly influences an element's chemical properties.
Knowing how many valence electrons an atom has helps predict how it will react with other atoms.

• An atom with a nearly full outer shell, like bromine with 7 valence electrons, tends to gain electrons to achieve a full shell.
• Conversely, an element like cesium, which has 1 valence electron, will tend to lose this electron, achieving a stable, filled inner shell.

In ionic compounds, elements will react in such a way to become more stable, often resembling the electron configuration of noble gases. This is evident as bromine forms the ion Br\(^-\), and cesium forms Cs\(^+\), adjusting their valence electrons accordingly.

Noble Gases

Noble gases are elements found in Group 18 of the periodic table. They are known for having full valence shells, making them exceptionally stable and rarely reactive under normal conditions. This group includes elements like helium, neon, and argon.
The stability of noble gases serves as a goal for other elements; they will gain or lose electrons in reactions to achieve a similar configuration.

• Noble gases like krypton and xenon have electron configurations that other elements mimic when forming ions.
• For instance, once bromine gains an extra electron, it achieves the configuration of krypton, thus forming Br\(^-\).
• Likewise, when cesium loses an electron, it resembles xenon's configuration, resulting in the formation of Cs\(^+\).

Ion formation aiming for a noble gas configuration allows elements to achieve lower energy states, enhancing their stability.