Question

Which ionic compound is expected to form from combining the following pairs of elements? (a) calcium and nitrogen, (b) cesium and bromine, (c) strontium and sulfur, (d) aluminum and selenium.

Short answer

The ionic compounds formed by the given pairs of elements are: a) Calcium nitride: Ca₃N₂ b) Cesium bromide: CsBr c) Strontium sulfide: SrS d) Aluminum selenide: Al₂Se₃

Step-by-step solution

Identify the charges of the ions formed by each element

First, write down the symbols and charges of the ions for each element in the given pairs. You can find the charges using the periodic table; elements in the same group tend to form ions with the same charge. a) Calcium (Ca) and nitrogen (N): Ca^(2+), N^(3-) b) Cesium (Cs) and bromine (Br): Cs^(1+), Br^(-) c) Strontium (Sr) and sulfur (S): Sr^(2+), S^(2-) d) Aluminum (Al) and selenium (Se): Al^(3+), Se^(2-)

Determine the ratio of ions needed to form a neutral compound

To form a neutral ionic compound, the total positive charge must be equal to the total negative charge. Find the least common multiple of the charges of the positive and negative ions of each pair to achieve charge balance. a) Ca^(2+) and N^(3-): least common multiple is 6. b) Cs^(1+) and Br^(-): least common multiple is 1. c) Sr^(2+) and S^(2-): least common multiple is 2. d) Al^(3+) and Se^(2-): least common multiple is 6.

Write the chemical formula for each compound

Now that we have the ratio of ions for each compound, write the chemical formula by using subscripts to indicate the number of ions needed to achieve charge balance. a) Calcium nitride: Ca₃N₂ (3 Ca^(2+) ions with 2 N^(3-) ions) b) Cesium bromide: CsBr (1 Cs^(1+) ion with 1 Br^(-) ion) c) Strontium sulfide: SrS (1 Sr^(2+) ion with 1 S^(2-) ion) d) Aluminum selenide: Al₂Se₃ (2 Al^(3+) ions with 3 Se^(2-) ions) Now we have the chemical formulas for the ionic compounds formed by the given pairs of elements: a) Ca₃N₂ - Calcium nitride b) CsBr - Cesium bromide c) SrS - Strontium sulfide d) Al₂Se₃ - Aluminum selenide

Key concepts

Chemical Formulas

Chemical formulas are shorthand ways to represent chemical compounds. This formula uses element symbols from the periodic table and subscripts to denote the number of each type of atom in a compound. For ionic compounds, the chemical formula is constructed by balancing the charges between the positively charged cations and negatively charged anions.

• The cations and anions are written together without any space between them.
• Subscripts are used to indicate the number of ions needed to balance the charges.

When developing a chemical formula for an ionic compound, it is essential first to understand the charges of the ions. This helps in figuring out the ratio in which these ions combine to form a neutral compound.

Charge Balance

In ionic compounds, the principle of charge balance is critical. This means the total positive charge from cations must equal the total negative charge from anions. Ionic compounds are electrically neutral overall, and this neutrality is achieved by appropriately balancing the charges.

• Determine the charge of each ion in the compound.
• Calculate the minimum number of each ion needed to achieve charge neutrality.

For example, calcium and nitrogen form calcium nitride by balancing the charges: three calcium ions with a charge of +2 each ( 3 Ca^{2+} ions) balance with two nitride ions with a charge of -3 each ( 2 N^{3-} ions), resulting in the neutral ionic compound Ca₃N₂.

Periodic Table

The periodic table is a valuable tool in understanding chemical behavior, including ion formation. Elements are organized in such a way that shows recurring "periodic" trends.

• Elements in the same group (vertical columns) often form ions of the same charge, making it easier to predict ionic compounds.
• Metals on the left side form cations, while non-metals on the right form anions.

Referencing the periodic table, one can quickly ascertain the typical ion charges that common elements form. This understanding is crucial for predicting the ratios in which ions will combine to form neutral compounds. For instance, group 1 elements like cesium typically form +1 ions, while group 17 elements like bromine form -1 ions.

Ion Charges

Ion charges are a fundamental concept for understanding ionic compounds. An ion is an atom or group of atoms that has gained or lost one or more electrons, giving it a net positive or negative charge.

• Metals tend to lose electrons and form positive ions, called cations.
• Non-metals tend to gain electrons and form negative ions, called anions.

The charge of an ion typically depends on its group in the periodic table. For example, calcium from group 2 forms a Ca^{2+} cation by losing two electrons. On the other hand, nitrogen from group 15 forms a N^{3-} anion by gaining three electrons. Understanding these charges allows you to construct the correct chemical formula of the ionic compound by knowing how many of each type of ion are required to balance each other out.