Problem 30 - Chemistry: An Atoms-Focused Approach

Question

Predict the formula and give the name of the ionic compound formed by the following pairs of elements: (a) potassium and bromine; (b) calcium and hydrogen; (c) lithium and nitrogen; (d) aluminum and chlorine.

Step-by-Step Solution

Answer

Question: Predict the formula and name the ionic compound formed for each pair of elements: (a) potassium and bromine, (b) calcium and hydrogen, (c) lithium and nitrogen, (d) aluminum and chlorine. Answer: (a) KBr - Potassium bromide, (b) CaH2 - Calcium hydride, (c) Li3N - Lithium nitride, (d) AlCl3 - Aluminum chloride.

Step 1: Determine the charges of the ions

In order to predict the formula of the ionic compounds, we first need to determine the charges of the cations and anions. The charges of the ions can be found by looking at the group numbers in the periodic table. Elements in group 1 have a +1 charge, group 2 have a +2 charge, and in general, elements in group 3-12 commonly form cations with positive charges based on their oxidation state. Group 17 elements have a -1 charge, group 16 has a -2 charge, and group 15 elements have a -3 charge.

Step 2: Determine the compound formulas

In order to form neutral compounds, we will combine the right amount of each cation and anion so that the positive and negative charges will cancel each other out. (a) potassium (K) and bromine (Br): Potassium, being in Group 1, forms a +1 ion (K+). Bromine, being in Group 17, forms a -1 ion (Br-). Combining one potassium ion and one bromide ion will create a neutral compound: K+ + Br- -> KBr (b) calcium (Ca) and hydrogen (H): Calcium, being in Group 2, forms a +2 ion (Ca2+). Hydrogen, being in Group 1, forms a -1 ion (H-). Combining one calcium ion and two hydrogen ions will create a neutral compound: Ca2+ + 2H- -> CaH2 (c) lithium (Li) and nitrogen (N): Lithium, being in Group 1, forms a +1 ion (Li+). Nitrogen, being in Group 15, forms a -3 ion (N3-). Combining three lithium ions and one nitrogen ion will create a neutral compound: 3Li+ + N3- -> Li3N (d) aluminum (Al) and chlorine (Cl): Aluminum, being in Group 13, forms a +3 ion (Al3+). Chlorine, being in Group 17, forms a -1 ion (Cl-). Combining one aluminum ion and three chloride ions will create a neutral compound: Al3+ + 3Cl- -> AlCl3

Step 3: Name the ionic compounds

Now that we have the formulas for the ionic compounds, we can name them: (a) KBr - Potassium bromide (b) CaH2 - Calcium hydride (c) Li3N - Lithium nitride (d) AlCl3 - Aluminum chloride

Key Concepts

Cation and Anion Charges

In ionic compounds, cations and anions are charged particles that combine to form neutral substances. A cation is positively charged, whereas an anion carries a negative charge.

To predict the charge of an ion, one can often look at the element's position in the periodic table:

• Elements in Group 1, like potassium and lithium, form +1 cations.
• Elements in Group 2, like calcium, form +2 cations.
• Elements in Group 17, such as bromine and chlorine, form -1 anions.
• Elements in Group 15, like nitrogen, form -3 anions.

The charges stem from the atom's tendency to lose or gain electrons to attain a stable electronic configuration, similar to that of the nearest noble gas.

Periodic Table Groups

The periodic table is a helpful tool in predicting the properties of elements, including the formation of ions. Elements are organized into groups or columns which often share similar chemical properties due to having the same number of electrons in their outer shell.

For instance:

• Group 1 elements (alkali metals) readily lose one electron to form +1 cations.
• Group 17 elements (halogens) typically gain one electron to form -1 anions.

Knowing the group of an element can quickly help you anticipate its charge and how it might bond with other elements to form ionic compounds.

Neutral Compound Formation

Neutral compound formation is guided by the principle that the sum of positive and negative charges must be zero. This means that the total positive charge from the cations needs to be balanced by the total negative charge from the anions.

Here are some examples:

• For potassium (K) and bromine (Br), both with single charges, they easily form a 1:1 ratio to create KBr.
• For calcium (Ca) and hydrogen (H), calcium's +2 charge is balanced by two hydrogen ions, each with a -1 charge, forming CaH2.
• With lithium (Li) and nitrogen (N), three lithium ions are needed to balance one nitrogen ion's -3 charge, resulting in Li3N.

By considering the charges and the principle of neutrality, we can deduce the simplest formula for any ionic compound.

Naming Ionic Compounds

Naming ionic compounds follows specific conventions to represent their composition accurately. These compounds consist of two parts: the cation and the anion.

• The cation (metal) retains its elemental name, such as "potassium" for K⁺.
• The anion (non-metal) modifies its name to end in "-ide," such as "bromide" for Br^-.

Putting them together, KBr becomes potassium bromide.
Other examples include:

• CaH2 named as calcium hydride,
• Li3N known as lithium nitride,
• AlCl3 referred to as aluminum chloride.

Following these straightforward naming principles helps communicate the compound's identity clearly and concisely.