Question

The formulas for the chlorides of potassium, calcium, boron, and germanium are, respectively, \(\mathrm{KCl}, \mathrm{CaCl}_{2}, \mathrm{BCl}_{3}\), and \(\mathrm{GeCl}_{4}\). Using the periodic table, predict the chemical formulas for each of the following similar compounds: (a) potassium fluoride (b) calcium fluoride (c) boron bromide (d) germanium iodide

Short answer

(a) KF, (b) CaF₄, (c) BBr₃, (d) GeI₄

Step-by-step solution

Determine Valence Electrons

To predict the chemical formulas, first identify the number of valence electrons for each element based on its group in the periodic table. Potassium (K) is in Group 1, calcium (Ca) in Group 2, boron (B) in Group 13, and germanium (Ge) in Group 14.

Identify Halogens' Valence

Next, identify the valence electrons of the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Each of these elements is in Group 17, meaning each has 7 valence electrons.

Predict Potassium Fluoride Formula

Potassium loses 1 electron, forming K extsuperscript{+}. Fluorine gains 1 electron, forming F extsuperscript{-}. Therefore, the formula for potassium fluoride is KF, given the ratio of ions is 1:1.

Predict Calcium Fluoride Formula

Calcium loses 2 electrons, forming Ca extsuperscript{2+}. Each fluorine gains 1 electron, forming F extsuperscript{−}. Therefore, the formula for calcium fluoride is CaF extsubscript{2}, where two fluorine ions are needed for each calcium ion.

Predict Boron Bromide Formula

Boron forms covalent compounds and will typically form 3 bonds (as in BCl extsubscript{3}). For boron bromide, expect the formula BBr extsubscript{3}.

Predict Germanium Iodide Formula

Germanium can form 4 bonds with halogens (as seen with GeCl extsubscript{4}). Hence, the formula for germanium iodide is GeI extsubscript{4}.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons found in an atom and are primarily involved in chemical bonding. They play a crucial role in determining how an element will chemically interact with other elements. Most atoms seek to achieve a full valence shell, which typically means having eight electrons—a stable, low-energy configuration known as the octet rule.
This desire for stability drives atoms to either transfer or share valence electrons, forming different kinds of chemical bonds. For example:

• Potassium (K) has one valence electron and tends to lose it to achieve a full outer shell, making it highly reactive.
• Calcium (Ca), with two valence electrons, will typically lose both to achieve stability.
• Boron (B) has three valence electrons and often shares these through covalent bonds.
• Germanium (Ge) contains four valence electrons, allowing it to form multiple bonds, usually with nonmetals from Group 17.

Understanding valence electrons helps predict the type of bond formed and thus, the chemical formula of a compound.

Periodic Table

The periodic table is an organized chart of all known chemical elements, arranged by increasing atomic number and grouped by recurring chemical properties. A fundamental tool in chemistry, it provides essential information needed to predict how elements will interact in chemical reactions.
The vertical columns, called groups or families, indicate elements with similar properties and the same number of valence electrons. For example:

• Group 1 contains alkali metals like potassium, each with one valence electron.
• Group 2 hosts alkaline earth metals such as calcium, with two valence electrons.
• Group 13 includes elements like boron, with three valence electrons.
• Group 14, featuring germanium, consists of elements with four valence electrons.
• Group 17, known as halogens, comprises elements like fluorine, chlorine, bromine, and iodine, each with seven valence electrons.

By understanding the layout of the periodic table, students can predict the likely formulas of compounds by identifying how many electrons each element tends to gain or lose.

Ionic and Covalent Bonds

Chemical bonds are mainly classified into ionic and covalent types. Understanding these is crucial for predicting molecular compositions. *Ionic Bonds*: These form when atoms transfer electrons to achieve a full valence shell. Typically, this occurs between metals and nonmetals.

• Potassium fluoride (KF) is a classic example of an ionic bond, where potassium (a metal) loses an electron to become K extsuperscript{+}, and fluorine (a nonmetal) gains an electron to form F extsuperscript{−}. The attraction between these charged ions forms the compound.
• Calcium fluoride (CaF extsubscript{2}) also forms through ionic bonding, with calcium losing two electrons to form Ca extsuperscript{2+} and each fluorine gaining one to form F extsuperscript{−}. Two fluoride ions pair with one calcium ion.

*Covalent Bonds*: These occur when two nonmetals share electrons to fill their outer shells completely.

• Boron bromide (BBr extsubscript{3}) results from covalent bonding, where boron shares its three electrons with three bromine atoms.
• Similarly, germanium iodide (GeI extsubscript{4}) forms through covalent bonds, sharing electrons between germanium and iodine atoms.

Recognizing the type of bond helps in understanding the behavior and structure of the resulting compounds.