Question

What will be the atomic number of the next alkali metal to be discovered? How would you expect the physical properties of the next alkali metal to compare with the properties of the other alkali metals summarized in Table \(20.4 ?\)

Short answer

The next alkali metal to be discovered will have an atomic number of 105. We can expect its physical properties to follow the trends of the alkali metals, meaning it will likely have a lower melting point, lower boiling point, and lower density compared to the other alkali metals. Additionally, it would be more reactive than its predecessors in the alkali metal group.

Step-by-step solution

Understand the alkali metal family

Alkali metals belong to the Group 1 in the periodic table. This group includes elements like lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Since alkali metals are in the same group, they share similar chemical and physical properties. Knowing this, we will identify the next alkali metal in the same group and discuss its possible properties.

Determine the atomic number of the next alkali metal

The atomic numbers of alkali metals increase going down the periodic table. So, to determine the atomic number of the next alkali metal, we can identify the last element in the alkali metal family (francium) and add one to its atomic number (since it is in the same group). The atomic number of francium (Fr) is 87. Therefore, the atomic number of the next alkali metal is \(87 + 1 = 88\). However, element 88 is a member of the alkaline earth metal group, so we must look further down the periodic table to find the next alkali metal. The next alkali metal will be in the following period, so we can calculate its atomic number by adding the difference between the atomic numbers of Cs and Rb to the atomic number of Fr: \(87 + (55 - 37) = 87 + 18 = 105\) Thus, the atomic number of the next alkali metal is 105.

Compare the expected physical properties with existing alkali metals

Alkali metals share similar physical properties, such as low melting points, low densities, and high reactivity. As we go down the group, the melting points, boiling points, and densities generally decrease, while reactivity increases. This trend is due to an increase in atomic size caused by the addition of electron shells. Based on these trends, we can infer that the next alkali metal (with atomic number 105) would have a lower melting point, lower boiling point, and lower density compared to the other alkali metals. In terms of reactivity, this element would be expected to be more reactive than its predecessors in the alkali metal group.

Key concepts

Periodic Table

The Periodic Table is central to the study of chemistry, offering a systematic way of looking at the different elements that make up our world. It is organized in rows, called periods, and columns, known as groups or families. Elements in the same group typically showcase similar chemical behavior because they have the same number of valence electrons, which are the electrons available for bonding.

Alkali metals, found in Group 1 of the Periodic Table, are known for their singular valence electron that contributes to their high reactivity. As we consider the alkali metals, it becomes apparent that their chemical and physical properties align with their position on the table. When looking for the next alkali metal, we refer to the Periodic Table to predict its placement and properties. The alkali metals’ placement indicates a clear pattern within their group, reflecting a straightforward progression in their atomic numbers as we move down the table.

Atomic Number

The atomic number is a fundamental property of an element, representing the number of protons in the nucleus of an atom. This number is not only unique to each element, but also dictates the element's position on the Periodic Table and, alongside electron configuration, contributes to its chemical behavior.

In the context of alkali metals, as we progress down Group 1 on the Periodic Table, each successive element has an atomic number greater by one quantum leap than its predecessor. This leap is not just a count of protons; it has implications on the physical and chemical properties of the element. To find the atomic number of the next alkali metal, we comprehend that after francium (with atomic number 87), we must account for the next quantum leap, leading us to the number 105.

Chemical Properties

The chemical properties of an element are defined by its ability to undergo certain reactions or form specific compounds. Typically, alkali metals are characterized by their eagerness to lose their one valence electron to achieve a stable electron configuration, which results in their iconic high reactivity, especially with water.

Reactivity Trends

Alkali metals display a trend in which reactivity increases as we move down the group due to the increasing atomic size and shielding effect, which make it easier for the atom to lose its valence electron. Predicting the chemical properties of a newly discovered alkali metal would entail an expectation for it to be incredibly reactive, potentially more so than any currently known alkali metal due to its higher atomic number.

Physical Properties

The physical properties of an element are the characteristics that can be observed without changing the substance's chemical identity. For alkali metals, notable physical properties include their softness, low melting points, and low densities.

Melting and Boiling Points

The melting and boiling points of alkali metals tend to decrease as the atomic number increases. This is due to larger atomic sizes and weaker metallic bonds as we descend the group. Consequently, our undiscovered alkali metal with an atomic number of 105 would likely have an even lower melting and boiling point than francium.

Density

Similarly, the densities of alkali metals also tend to decrease moving down the group. So, the next alkali metal is surmised to have a lower density compared to its counterparts, which already exhibit quite low densities.