Problem 209 - Chemistry Matter and Change

Question

Given the elements Ar, Cs, Br, and Ra, identify those that form positive ions. Explain your answer. (Chapter 7)

Step-by-Step Solution

Answer

Based on the electron configurations and their positions in the periodic table, Cs (Cesium) and Ra (Radium) are the elements that form positive ions. Cs forms a +1 ion, while Ra forms a +2 ion. This is because they tend to lose electrons and achieve a stable electron configuration, characteristic of metals located on the left side of the periodic table.

Step 1: Locate the elements in the periodic table

First, locate the given elements (Ar, Cs, Br, and Ra) in the periodic table. This will help in understanding their electron configurations and their tendency to form ions.

Step 2: Determine the element's electron configuration

Determine the electron configuration of each element. Electron configurations will provide information about the number of valence electrons and the stability of the element. - Ar (Argon): \[1s^2 2s^2 2p^6 3s^2 3p^6\] - Cs (Cesium): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^1\] - Br (Bromine): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\] - Ra (Radium): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^{10} 6p^6 7s^2\]

Step 3: Identify the tendency of elements to form ions

Determine whether each element would more likely form an ion by losing or gaining electrons based on its electron configuration and position in the periodic table. - Ar is a noble gas with a stable electron configuration. It has a full outer electron shell, so it does not tend to form ions. - Cs is an alkali metal with one electron in its outermost shell. It tends to lose this electron to achieve a stable electron configuration, forming a +1 ion (cation). - Br is a halogen with seven electrons in its outermost shell. It tends to gain one electron to achieve a stable electron configuration, forming a -1 ion (anion). - Ra is an alkaline earth metal with two electrons in its outermost shell. It tends to lose these electrons to achieve a stable electron configuration, forming a +2 ion (cation).

Step 4: Identify the elements that form positive ions

Based on the analysis above, the elements that form positive ions (cations) are: - Cs (Cesium) forms a +1 ion - Ra (Radium) forms a +2 ion

Key Concepts

Periodic Table

The periodic table is a crucial tool in chemistry that organizes all known elements by their atomic number, electron configuration, and recurring chemical properties. Invented by Dmitri Mendeleev in the 19th century, it's structured in rows called periods and columns known as groups. Each element's position in the periodic table provides insights into its properties and behaviors. For example, elements in the same group typically have similar chemical properties, as they have the same number of valence electrons, leading to similarities in their ability to form ions. Understanding how to locate elements like Argon (Ar), Cesium (Cs), Bromine (Br), and Radium (Ra) on the table is foundational in predicting their chemical behavior.

Electron Configuration

Electron configuration describes the distribution of electrons in an atom's orbitals. It's key to understanding an element's chemical reactions and its tendency to form ions. The configuration notation uses a series of numbers and letters. Numbers indicate energy levels, while letters (s, p, d, f) denote orbital types, and superscripts show the number of electrons within those orbitals. For example:

• Argon (Ar): \[1s^2 2s^2 2p^6 3s^2 3p^6\]
• Cesium (Cs): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^1\]
• Bromine (Br): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\]
• Radium (Ra): \[1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^{10} 6p^6 7s^2\]

This configuration helps predict how an element will react, such as whether it will lose or gain electrons to form ions.

Cations

Cations are positively charged ions formed when an atom loses electrons. Elements tend to form cations to achieve a more stable electron configuration, similar to noble gases. Cations are typically metals found on the left side of the periodic table. For instance, Cesium (Cs) loses one electron to form a cation with a +1 charge, whereas Radium (Ra) loses two electrons, resulting in a +2 charge. The process of losing electrons is relatively straightforward because metals like these aim to achieve a stable electron arrangement by emptying their outer electron shell.

Alkali Metals

Alkali metals are located in Group 1 of the periodic table and include elements such as Lithium, Sodium, and Cesium. These metals are characterized by having one electron in their outermost shell. This configuration makes them highly reactive as they readily lose this single electron to form cations with a +1 charge. Due to this property, alkali metals typically form strong cations and are rarely found in a free state in nature. Cesium (Cs) is one such alkali metal that easily forms a cation by losing its outer electron to achieve a more stable configuration.

Alkaline Earth Metals

Alkaline earth metals are part of Group 2 on the periodic table and include elements like Beryllium, Magnesium, and Radium. These elements have two electrons in their outermost shell, which they readily lose to form cations with a +2 charge. Radium (Ra) is an example of an alkaline earth metal that loses two electrons to achieve a stable electron configuration. Such a characteristic makes them less reactive than alkali metals, yet they still play significant roles in forming various compounds. The ability to form +2 cations is inherent in alkaline earth metals due to the stable configuration achieved after the loss of two electrons.