Problem 60 - Chemistry: The Central Science in SI Units, Global Edition

Question

The most common charge associated with selenium is \(2-.\) Indicate the chemical formulas you would expect for compounds formed between selenium and (a) barium, (b) lithium, (c) aluminum.

Step-by-Step Solution

Answer

The chemical formulas for compounds formed between selenium and the given elements are: (a) Barium - \(BaSe\), (b) Lithium - \(Li_2Se\), and (c) Aluminum - \(Al_2Se_3\).

Step 1: Determine the charges of the elements

First, identify the charges of each of the elements. Selenium (Se) has a 2- charge. Barium (Ba) is a Group 2 element, so it has a 2+ charge. Lithium (Li) is a Group 1 element (alkali metal), so it has a 1+ charge. Aluminum (Al) is a Group 13 element, and it has a 3+ charge.

Step 2: Combine the elements to form neutral compounds

To form neutral chemical compounds, the charges of the elements must balance each other out. We will combine the elements in such a way that their total positive charge equals their total negative charge. (a) Barium and Selenium: Ba^2+ with Se^2-, the charges are equal, and only one ion of each element is needed. Chemical formula: BaSe (b) Lithium and Selenium: Li^1+ needs 2 Li ions to balance the 2- charge from Se. Chemical formula: Li2Se (c) Aluminum and Selenium: Al^3+ needs 2 Al ions to have a total charge of 6+. Two Se^2- ions have a total charge of 4-. So, 3 Se^2- and 2 Al^3+ would balance the charges. Chemical formula: Al2Se3

Step 3: Final Results: Chemical Formulas

The chemical formulas for the compounds formed between selenium and the other elements are as follows: (a) Barium: BaSe (b) Lithium: Li2Se (c) Aluminum: Al2Se3

Key Concepts

Ionic Compounds

Ionic compounds are made up of ions, which are atoms or molecules that carry a net charge because they have gained or lost electrons. When forming ionic compounds, positive ions (cations) and negative ions (anions) come together. These ions stay close due to strong electrostatic forces, resulting in a neutral compound overall.
Ions can only form ionic compounds when their charges balance out to zero. This means that the total positive charge must equal the total negative charge. In the case of selenium combined with other elements, each compound formed must have a net charge of zero for it to be stable.
For example, when selenium (which forms a 2- charge) combines with barium (which has a 2+ charge), they balance out in a 1:1 ratio, leading to the chemical formula BaSe. These simple formulas help us understand how nature achieves balance at the microscopic level.

Charge Balance

Charge balance is the concept that ensures the total charges in a compound equal zero, which is a fundamental requirement for forming stable ionic compounds. Achieving charge balance involves adding enough of each ion to neutralize the charges between the cations and anions.
For selenium with a 2- charge, different elements have different charges, affecting how many atoms or ions are combined. For example, lithium has a 1+ charge, and so two lithium ions are needed to balance the 2- charge of selenium, forming the compound Li\(_2\)Se.
Aluminum, which has a 3+ charge, needs a different balancing approach, as two aluminum ions have a total charge of 6+. This requires three selenium ions with a total charge of 6- to form the stable compound Al\(_2\)Se\(_3\). Understanding this balance is crucial in writing chemical formulas for ionic compounds.

Periodic Table Groups

The periodic table is organized in such a way that it helps predict the charges of ions formed by elements. Elements are grouped into columns known as "groups," and these groups share common properties, including their typical ionic charges.
For instance, elements in Group 1, such as lithium, tend to form 1+ charged ions. Group 2 elements, such as barium, typically form 2+ charged ions. Aluminum, found in Group 13, often forms 3+ ions. The periodic table thus becomes a crucial tool in predicting how elements will combine to form stable ionic compounds.
By referencing the periodic table, one can quickly identify the common charge associated with an element, aiding in determining the formula of a compound it might form when combined with selenium.