Question

The formulas for the fluorides of the third-period elements are \(\mathrm{NaF}, \mathrm{MgF}_{2}, \mathrm{AlF}_{3}, \mathrm{SiF}_{4}, \mathrm{PF}_{5}, \mathrm{SF}_{6},\) and \(\mathrm{ClF}_{3}\). Classify these compounds as covalent or ionic.

Short answer

NaF, MgF2, and AlF3 are ionic compounds. SiF4, PF5, SF6, and ClF3 are covalent compounds.

Step-by-step solution

Identify the Ionic Compounds

Sodium (Na), Magnesium (Mg), and Aluminium (Al) are metals and they typically form ionic bonds with nonmetals. Hence, NaF, MgF2, and AlF3 are ionic compounds.

Identify the Covalent Compounds

Silicon (Si), Phosphorus (P), Sulfur (S) and Chlorine (Cl) are non-metals and they typically form covalent bonds with other non-metals. Therefore, SiF4, PF5, SF6, and ClF3 are covalent compounds.

Key concepts

Ionic Bonds

Ionic bonds occur when there is a transfer of electrons between atoms. This typically happens between metals and nonmetals. The metal atom donates one or more electrons to become a positively charged ion, known as a cation. Meanwhile, the nonmetal atom gains these electrons, becoming a negatively charged ion, or anion.

• Ionic bonding results in the formation of a crystalline lattice structure. This structure maximizes the attraction between oppositely charged ions and minimizes repulsion between like-charged ions.
• Ionic compounds, such as those involving sodium (\(\mathrm{NaF}\), magnesium (\(\mathrm{MgF_2}\), and aluminum (\(\mathrm{AlF_3}\), tend to have high melting and boiling points due to the strong ionic interactions within their structures.
• These compounds are usually soluble in water, as water can help dissociate the ions, allowing them to conduct electricity when dissolved or melted.

In summary, ionic bonds are strong, require significant energy to break, and are key in forming salts and various crystalline substances.

Covalent Bonds

Covalent bonds form when atoms share pairs of electrons rather than transferring them. This type of bonding is common between nonmetals, allowing them to reach a stable electron configuration with full outer electron shells.

• Compounds like \(\mathrm{SiF_4}\), \(\mathrm{PF_5}\), \(\mathrm{SF_6}\), and \(\mathrm{ClF_3}\) exhibit covalent bonding. Here, silicon, phosphorus, sulfur, and chlorine share their electrons with fluorine to fill their valence shells.
• Covalently bonded molecules typically form discrete molecules, and their intermolecular forces are generally weaker than those in ionic compounds.
• This results in lower melting and boiling points compared to ionic compounds. Many are gases or liquids at room temperature.
• Such compounds do not conduct electricity in any state, as there are no free charges available.

Covalent bonds are fundamental to organic chemistry and play a critical role in forming the myriad molecules essential for life.

Third-Period Elements

The third-period elements in the periodic table are those found in the third row. These elements include sodium (\(\mathrm{Na}\)), magnesium (\(\mathrm{Mg}\)), aluminum (\(\mathrm{Al}\)), silicon (\(\mathrm{Si}\)), phosphorus (\(\mathrm{P}\)), sulfur (\(\mathrm{S}\)), and chlorine (\(\mathrm{Cl}\)).

• As we move from left to right in this period, the properties of the elements exhibit a gradual transition from metallic to nonmetallic. This is evident from the changes in ionization energy, electronegativity, and atomic radii.
• The metallic elements, such as sodium and magnesium, generally form ionic oxides and chlorides, whereas the nonmetallic elements form covalent compounds.
• For example, \(\mathrm{NaF}\) is ionic, while \(\mathrm{ClF_3}\) is covalent, reflecting the shift from metal to nonmetal across the period.

By understanding the electron configurations and bonding tendencies of third-period elements, we gain insights into their chemical reactivity and bonding patterns.