Question

Refer to the periodic table and write the predicted electron configuration for each of the following elements using core notation: (a) Sr (b) \(R u\) (c) Sb (d) \(\mathrm{Cs}\)

Short answer

(a) [Kr] 5s², (b) [Kr] 5s² 4d⁶, (c) [Kr] 5s² 4d¹⁰ 5p³, (d) [Xe] 6s¹

Step-by-step solution

Identify the Core Configuration

For each element, identify the noble gas preceding it. This noble gas will be used as the core configuration to simplify the electron configuration. The noble gases closest to the elements given are: (a) Sr: Krypton (Kr) (b) Ru: Krypton (Kr) (c) Sb: Krypton (Kr) (d) Cs: Xenon (Xe)

Determine Valence Electrons for Sr

Strontium (Sr) is in Period 5, Group 2. After the core of Krypton \(\left[Kr\right]\), Sr continues filling the 5s subshell:(a) Sr: \[ \left[ \text{Kr} \right] 5s^2 \]

Determine Valence Electrons for Ru

Ruthenium (Ru) is in Period 5, Group 8. After the core of Krypton \(\left[Kr\right]\), Ru fills both the 5s subshell and partially fills the 4d subshell:(b) Ru: \[ \left[ \text{Kr} \right] 5s^2 4d^6 \]

Determine Valence Electrons for Sb

Antimony (Sb) is in Period 5, Group 15. After the core of Krypton \(\left[Kr\right]\), Sb fills the 5s, 4d, and partially fills the 5p subshell:(c) Sb: \[ \left[ \text{Kr} \right] 5s^2 4d^{10} 5p^3 \]

Determine Valence Electrons for Cs

Cesium (Cs) is in Period 6, Group 1. It starts filling the 6s subshell right after the core of Xenon \(\left[Xe\right]\):(d) Cs: \[ \left[ \text{Xe} \right] 6s^1 \]

Key concepts

Periodic Table

The periodic table is an essential tool in chemistry. It organizes all known elements in an informative array, based on their atomic number, electronic configuration, and recurring chemical properties. The table is structured with rows called "periods" and columns known as "groups" or "families."
Elements in the same group have similar chemical behavior due to having the same number of electrons in their outer shell. As you move across a period from left to right, each element has one more proton and one more electron than the element before it. This arrangement helps predict not just the properties of an element, but also its electron configuration.
The periodic table also highlights certain element categories: metals, nonmetals, and metalloids. It's important to be familiar with where each type is typically located:

• Metals: Located on the left side and center.
• Nonmetals: Reside mostly on the right.
• Metalloids: Found between metals and nonmetals, sharing properties of both.

Understanding the layout of the periodic table will guide you in predicting the electron configuration of elements.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. They play a crucial role in chemical bonding and determine an element’s chemical properties and reactivity.
Elements in the same group of the periodic table have the same number of valence electrons, which explains why they exhibit similar chemical behaviors. For instance, all alkali metals in Group 1 have one valence electron, which makes them highly reactive, especially with water.
It’s also important to note:

• Valence Electrons and Bonding: These electrons are typically involved in forming bonds with other atoms. Atoms "want" to achieve a full outer shell—often corresponding to having eight electrons, known as the "octet rule."
• Electron Shells: As you move down a group in the periodic table, additional electron shells are added. However, the number of valence electrons in each element of the group remains the same.

Knowing the number and behavior of valence electrons helps in predicting how an element will interact in chemical reactions.

Noble Gas Core Notation

Electron configurations can be made more comprehensible using noble gas core notation. This approach condenses the standard electron configuration by employing the electron configuration of the nearest preceding noble gas as a starting point. The noble gases are stable and inert due to their complete valence electron shell.
The concept works as follows:

• Identify the noble gas from the previous period. For example, for elements in Period 5, Krypton (Kr) is often used.
• Begin the electron configuration with the symbol of this noble gas, placed inside brackets. This represents all the electrons in filled inner shells.
• Append the configuration of the remaining electrons needed to reach the element's total electron count.

By focusing on only the electrons before and after the noble gas core, this notation provides a streamlined way to present configurations. For example, Cesium \( \text{Cs} \) can be written as \[ \left[ \text{Xe} \right] 6s^1 \], where \([ \text{Xe} ]\) accounts for Xenon's electron configuration and highlights Cesium's extra electron in its 6s orbital.