Question

How many covalent bonds are formed by each atom in the following molecules? Draw molecules using the electron-dot symbols and lines to show the covalent bonds. (a) \(\mathrm{PH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{Se}\) (c) \(\mathrm{HCl}\) (d) \(\mathrm{SiF}_{4}\)

Short answer

PH3 forms 3 bonds, H2Se 2 bonds, HCl 1 bond, and SiF4 4 bonds.

Step-by-step solution

Understanding Covalent Bonds

Covalent bonds are formed by the sharing of electrons between atoms. Each pair of shared electrons constitutes a single covalent bond. By identifying the number of bonds an atom can form, we can understand its typical bonding behavior.

Determining Bonds in PH3

In phosphorus trihydride ( PH₃ ), phosphorus has 5 valence electrons and hydrogen has 1. The phosphorus atom can share one of its electrons with each of the three hydrogens, resulting in three single covalent bonds.

Drawing PH3 Structure

Draw the structure: a central P atom with three lines (bonds) extending to three H atoms. This reflects three covalent bonds between phosphorus and each hydrogen.

Determining Bonds in H2Se

In hydrogen selenide ( H₂Se ), selenium has 6 valence electrons and hydrogen has 1. Selenium shares one electron with each of the two hydrogens, forming two single covalent bonds.

Drawing H2Se Structure

Draw a central Se atom connected by lines to two H atoms, representing two covalent bonds.

Determining Bonds in HCl

In hydrochloric acid ( HCl ), chlorine has 7 valence electrons and hydrogen has 1. The chlorine shares one electron with hydrogen, forming a single covalent bond.

Drawing HCl Structure

Draw the structure: H—Cl, showing one covalent bond between hydrogen and chlorine.

Determining Bonds in SiF4

Silicon tetrafluoride ( SiF₄ ) involves silicon, which has 4 valence electrons, and each fluorine has 7. Silicon shares one electron with each of the four fluorine atoms, forming four single covalent bonds.

Drawing SiF4 Structure

Draw the structure with a central Si atom and four lines extending to four F atoms, representing four covalent bonds.

Key concepts

Covalent Bonds

Covalent bonds are a type of chemical bond essential for molecular structure and stability. They are formed when two atoms share pairs of electrons, allowing each atom to achieve a more stable electron configuration. This sharing of electrons often occurs between nonmetals, facilitating the formation of molecules.

• Each shared electron pair is a single covalent bond.
• Covalent bonds can be single, double, or triple, based on the number of shared electron pairs.
• Single covalent bonds involve one pair of shared electrons, whereas double and triple bonds share two and three pairs, respectively.

The strength of a covalent bond depends on the amount of shared electron pairs and the specific atoms involved. Covalent bonding plays a crucial role in organic chemistry and is foundational to the structure of complex compounds like proteins and DNA.

Electron-Dot Symbols

Electron-dot symbols, also known as Lewis structures, are a simple way to represent the valence electrons of an atom and its bonding capability. They provide a clear visual representation of how atoms share electrons to form covalent bonds. Around the symbol of the element, dots represent electrons. To showcase a molecule, draw lines between symbols for shared electron pairs (covalent bonds) and place any remaining dots to indicate lone pairs:

• In ammonia ( PH₃ ), we can illustrate phosphorus with five dots, three shared as lines with hydrogen's single dot each.
• For hydrogen selenide ( H₂Se ), selenium's electron cloud is depicted with six dots, sharing two with each hydrogen.
• Hydrochloric acid ( HCl ) involves chlorine sharing one of its seven dots with hydrogen's single dot.
• Silicon tetrafluoride ( SiF₄ ) illustrates silicon's four dots, each forming a line with fluorine's single offered electron from its seven surrounding dots.

By drawing electron-dot symbols, we can easily grasp the shared nature of covalent bonds in molecules.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining an element's chemical properties and bonding behavior. These electrons are the ones involved in forming chemical bonds, including covalent bonds.

• For nonmetals in the main groups of the periodic table, the number of valence electrons ranges from 1 to 8, matching their group number (e.g., group 1 elements have 1 valence electron, while group 8 elements have 8).
• Valence electrons are shown as dots in electron-dot symbols, allowing for a quick understanding of potential bonding interactions.
• Atoms tend to gain, lose, or share electrons to fill or empty their outermost shell fully, effectively achieving a stable electronic configuration often resembling the noble gases.

Understanding valence electrons is key to predicting how elements interact and bond, forming diverse and complex molecules seen in nature.