Question

Using electron-dot notation, show for each of the following the outer shell electrons for the uncombined atoms and for the molecules or ions that result: (a) \(\mathrm{H}+\mathrm{H} \rightarrow\) Hydrogen molecule (b) \(\mathrm{Br}+\mathrm{Br} \rightarrow\) bromine molecule (c) \(\mathrm{Br}+\mathrm{Cl} \rightarrow\) bromine chloride (d) \(\mathrm{Si}+\mathrm{F} \rightarrow\) silicon fluoride (e) \(\mathrm{Se}+\mathrm{H} \rightarrow\) hydrogen selenide (f) \(\mathrm{Ca}+\mathrm{O} \rightarrow\) calcium oxide

Short answer

(a) Hydrogen molecule: \( \mathrm{H} \cdot \overset{\text{ }|}{\rightarrow} \mathrm{H}\mathrm{H} \) (b) Bromine molecule: \( \begin{array}{|c|c|} \cline{1-1} \cline{2-2} \multicolumn{1}{|c|}{\mathrm{Br}} & \multicolumn{1}{c|}{\mathrm{Br}} \\ \hline \multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} \\ \hline \end{array} \) (c) Bromine chloride: \( \begin{array}{|c|c|}\cline{1-1} \cline{2-2}\multicolumn{1}{|c|}{\mathrm{Br}} & \multicolumn{1}{c|}{\mathrm{Cl}}\\\hline\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{}\\\hline\end{array} \) (d) Silicon fluoride: \(\mathrm{SiF}_4\) (e) Hydrogen selenide: \(\mathrm{H}_2\mathrm{Se}\) (f) Calcium oxide: \(\mathrm{CaO}\)

Step-by-step solution

Draw the uncombined atoms with their valence electrons

For hydrogen, there is only one valence electron. Draw the electron as a single dot around each hydrogen atom: \( \cdot \mathrm{H} \).

Combine the atoms to form a molecule

When both hydrogen atoms combine, they share their single valence electron to form a covalent bond. This can be illustrated as: \( \mathrm{H} \cdot \overset{\text{ }|}{\rightarrow} \mathrm{H}\mathrm{H} \) (b) Bromine Molecule

Draw the uncombined atoms with their valence electrons

Bromine has seven valence electrons. Draw these electrons as seven dots around each bromine atom: \( \begin{array}{|c|} \cline{1-1} \multicolumn{1}{|c|}{\mathrm{Br}} \\ \hline \multicolumn{1}{|c|}{} \\ \hline \end{array} \).

Combine the atoms to form a molecule

When the bromine atoms combine, they share one valence electron each to form a covalent bond. The resulting electron-dot structure is: \( \begin{array}{|c|c|} \cline{1-1} \cline{2-2} \multicolumn{1}{|c|}{\mathrm{Br}} & \multicolumn{1}{c|}{\mathrm{Br}} \\ \hline \multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} \\ \hline \end{array} \). (c) Bromine Chloride

Draw the uncombined atoms with their valence electrons

Br has seven valence electrons, and Cl has seven as well. Draw their valence electrons: \( \begin{array}{|c|}\cline{1-1}\multicolumn{1}{|c|}{\mathrm{Br}}\\\hline\multicolumn{1}{|c|}{}\\\hline\end{array} \), \( \begin{array}{|c|}\cline{1-1}\multicolumn{1}{|c|}{\mathrm{Cl}}\\\hline\multicolumn{1}{|c|}{}\\\hline\end{array}\).

Combine the atoms to form a molecule

When Br and Cl combine, they share one valence electron each to form a covalent bond, resulting in: \( \begin{array}{|c|c|}\cline{1-1} \cline{2-2}\multicolumn{1}{|c|}{\mathrm{Br}} & \multicolumn{1}{c|}{\mathrm{Cl}}\\\hline\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{}\\\hline\end{array} \). (d) Silicon Fluoride

Draw the uncombined atoms with their valence electrons

Si has four valence electrons, while F has seven. Draw their valence electrons: \( \begin{array}{|c|c|c|c|}\cline{1-1}\cline{2-2}\multicolumn{1}{|c|}{\mathrm{Si}}&\multicolumn{1}{c|}{}&\multicolumn{1}{c}{}&\multicolumn{1}{c}{}\\\hline\multicolumn{1}{|c|}{}&\multicolumn{1}{c|}{}&\multicolumn{1}{c}{}&\multicolumn{1}{c}{}\\\hline\end{array} \), \( \begin{array}{|c|}\cline{1-1}\multicolumn{1}{|c|}{\mathrm{F}}\\\hline\multicolumn{1}{|c|}{}\\\hline\end{array} \).

Combine the atoms to form a molecule

Si shares one electron with each of the four F atoms, resulting in a molecule with the structure: \(\mathrm{SiF}_4\). (e) Hydrogen Selenide

Draw the uncombined atoms with their valence electrons

Se has six valence electrons, while H has one. Draw their valence electrons: \(\begin{array}{|c|c|c|}\cline{1-1}\cline{2-2}\multicolumn{1}{|c|}{\mathrm{Se}}&\multicolumn{1}{c|}{}&\multicolumn{1}{c}{}\\\hline\multicolumn{1}{|c|}{}&\multicolumn{1}{c|}{}&\multicolumn{1}{c}{}\\\hline\end{array} \), \(\cdot \mathrm{H} \).

Combine the atoms to form a molecule

Se shares one electron with each of the two H atoms, resulting in a molecule with the structure: \(\mathrm{H}_2\mathrm{Se}\). (f) Calcium Oxide

Draw the uncombined atoms with their valence electrons

Ca has two valence electrons, while O has six. Draw their valence electrons: \( \begin{array}{|c|c|} \cline{1-1} \cline{2-2} \multicolumn{1}{|c|}{\mathrm{Ca}} & \multicolumn{1}{c|}{} \\ \hline \multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} \\ \hline \end{array} \), \( \begin{array}{|c|c|c|} \cline{1-1} \cline{2-2} \multicolumn{1}{|c|}{\mathrm{O}} & \multicolumn{1}{c|}{} & \multicolumn{1}{c}{} \\ \hline \multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c}{} \\ \hline \end{array} \).

Combine the atoms to form a molecule

Ca transfers its two valence electrons to O, resulting in an ionic compound with the formula: \(\mathrm{CaO}\).

Key concepts

Covalent Bond Formation

Covalent bond formation is a fundamental concept in chemistry where atoms share electrons to achieve mutual stability. When two nonmetals bond, they often form a covalent bond by sharing one or more pairs of valence electrons. This sharing allows each atom to resemble the electron configuration of a noble gas, which is typically more stable.

For example, when two hydrogen atoms (each with one valence electron) come together, they share their electrons to form a single covalent bond, resulting in a stable hydrogen molecule (H₂). Similarly, in a bromine molecule (Br₂), two bromine atoms each contribute one of their seven valence electrons to form a covalent bond.

Covalent bonds can form between a variety of atoms, not just the same type. For instance, when bromine (Br) and chlorine (Cl) bond, they share one electron each, forming bromine chloride (BrCl). This sharing reinforces the chemical stability of both atoms, resulting in a stable molecular substance.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are primarily involved in chemical bonding. These electrons determine the chemical properties and reactivity of the element. Understanding valence electrons is crucial for predicting how atoms will interact and bond with one another.

In the electron-dot notation, valence electrons are represented as dots around the symbol of an element. For instance, hydrogen, with one valence electron, is depicted as \( \cdot \mathrm{H} \), while bromine, with seven valence electrons, appears with seven dots. Identifying the number of valence electrons helps chemists understand the potential bonding behavior of an atom.

Atoms tend to form bonds to achieve a complete outer shell, often emulating the stable electron configuration of the nearest noble gas. For example, oxygen has six valence electrons and tends to accept two more to fill its valence shell to achieve a configuration similar to neon.

Molecular Formation

Molecular formation occurs when atoms come together to form a molecule, a group of atoms bonded by covalent bonds. The process involves the sharing of valence electrons between atoms to achieve stable electron configurations.

A molecule like silicon fluoride (SiF₄) is formed when one silicon atom, sharing its four valence electrons with four fluorine atoms, completes its outer shell. Fluorine needs one additional electron to complete its shell, thus bonding with silicon helps fluorine achieve this stability.

Molecules vary greatly in size and complexity, from simple diatomic molecules like hydrogen (H₂) to complex polymers consisting of thousands of atoms. The way these atoms bond and share electrons defines the molecular structure and dictates the chemical properties of the substance.

Ionic Bonding

Ionic bonding is a type of chemical bond where atoms transfer electrons to achieve full valence shells, resulting in ions. This usually occurs between a metal and a non-metal. In ionic bonding, the metal loses electrons to become a positively charged cation, while the non-metal gains those electrons to become a negatively charged anion.

For instance, calcium oxide (CaO) exemplifies ionic bonding. Calcium, a metal, has two valence electrons which it transfers to oxygen, a non-metal. This transfer results in a Ca²⁺ ion and an O²⁻ ion. The electrostatic attraction between these oppositely charged ions holds the ionic compound together.

Unlike covalent bonds, ionic bonds create a lattice structure rather than discrete molecules. The strong forces holding these ions together result in compounds that typically have high melting and boiling points, often making them solid at room temperature.